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Economic algae are either marine macroalgae (seaweeds) or freshwater and marine microalgae. Commercially interesting microalgae are often either blue-green algae (''Cyanophyta'') or members of the unicellular green algal genera ''Chlorella '' Beij. and ''Dunaliella '' Teodor. Commercial resources of algae are often only minor components of the total standing crop of an aquatic flora. These amounts could be improved by cultivation, to the extent that amounts of material under cultivation could far exceed the natural standing stocks. It has been stated that, as a whole, seaweed resources are not greatly exploited (Michanek, 1975). Only about 3.5 million t of seaweeds are used, out of a total biomass which is probably 100 times larger (Jensen, 1993).

Of the marine algae 107 genera and 493 species have some economic value; mainly macroalgae are used extensively (Tseng, 1981b). The earliest records of the occurrence of ''Porphyra '' C. Agardh and its food value appeared in books in China published in the years 533-544 A.D. One thousand years ago the Chinese already regarded ''Porphyra '' as a delicacy to be presented to the emperor annually (Tseng, 1981a). Several surveys of useful seaweeds have been published (Chapman, 1950, 1970; Hoffmann, 1938; Levring et al., 1969; Sauvageau, 1920; Tressler, 1923). In recent years lists of useful seaweeds have also been made available (Arasaki & Arasaki, 1983; Bonotto, 1979; Tokuda et al., 1987). Few lists also contain vernacular names (e.g. Calumpong & Meñez, 1997; Ganzon-Fortes, 1991; Hatta et al., 1993; Zaneveld, 1955, 1959).

Freshwater microalgae and marine microalgae can be grown in closed systems (including heterotrophic systems), tanks, or shallow ponds for the production of health food ("nutraceuticals"), carotenoids, proteins, fine chemicals, or for medicinal uses. Several microalgae are particularly used for the hatchery cultivation of marine molluscs and prawns (de Pauw et al., 1984; Laing & Ayala, 1990; Yúfera & Lubián, 1990). The difficulty of producing economically large quantities of microalgal feeds is currently one of the major impediments to the further development of the aquaculture industry (Apt & Behrens, 1999; Gladue & Maxey, 1994). Microalgae are a genetically very diverse group with a wide range of physiological and biochemical characteristics. They comprise a large, almost unexplored group of organisms, and thus provide a virtually untapped source of products and possibilities for commercial application (Radmer & Parker, 1994). The blue-green algae ''Arthrospira '' (''Spirulina'') ''platensis '' Gomont and ''A''. (''S''.) ''maxima '' Setch. & N.L. Gardner are at present the richest known sources of plant protein (Jassby, 1988a). In addition, they have the highest vitamin B12 content of any unprocessed plant or animal and relatively high contents of β carotene (Jassby, 1988a; Mshigeni, 1982). In relation to this carotene product, however, the species of ''Arthrospira '' Stizenb. ex Gomont are outstripped by another genus of microalgae: ''Dunaliella '' (Jassby, 1988a; Moulton et al., 1987). Free-living nitrogen-fixing blue-green algae, as well as the endosymbiotic nitrogen-fixing blue-green algae growing in the ''Azolla '' water fern are used as fertilizers for rice fields. These algae function as a "green manure" (Faridah Hanum & van der Maesen, 1997; Metting et al., 1988, 1990; Mshigeni, 1982).

''Algae as vegetables '' Specimens used as vegetables (or as material for medicinal or pharmaceutical use) are usually collected from nature (Hatta et al., 1993). This small-scale use results in offering the fresh specimens for sale at the local market. In South-East Asia especially, coastal inhabitants of the Philippines, Malaysia and Indonesia consume seaweeds. Only occasionally are freshwater macroalgae used as food (Arasaki & Arasaki, 1983).

''Phycocolloids '' Production of phycocolloids is occasionally possible in small-scale ventures. In these cases washed-up specimens collected from the beach can be used, as well as material collected from wild populations or from cultivated stock. Those seaweeds can be sold as raw material, often washed and bleached, to be used for the home production of crude phycocolloids for the preparation of puddings and cakes, or as raw (often untreated) material for animal feed or fertilizer. Often small-scale traders buy dried seaweeds from collectors and farmers for export to other countries or to factories for phycocolloids. The cultivation of algae for phycocolloids takes place in family-owned seaweed farms or in larger complexes owned by cooperatives, exporters or factories.

''Other uses '' Biomass from macroalgae can be prospected to provide environmentally and economically feasible alternatives to fossil fuels and can also be functional in pollution abatement (Gao & McKinley, 1994).

Agar is a hydrophyllic colloid extracted from a number of red algae, which are often collectively designated as agarophytes. Of the total global agar production in 1994, about 35% came from members of the ''Gelidiales '' (e.g. ''Gelidiella '' Feldmann & Hamel, ''Gelidium '' J.V. Lamour. and ''Pterocladia '' J. Agardh); most other agar came from members of the ''Gracilariales '' (e.g. ''Gracilaria '' Grev., including ''Gracilariopsis '' E.Y. Dawson) (Armisén, 1995; Armisén & Galatas, 1987; Chapman, 1970; Indergaard & Østgard, 1991; Lewis et al., 1988; Lobban & Harrison, 1994; Pérez et al., 1992).

Ionized agarose, where molecules are highly sulphated, is dominant in some ''Gracilaria'', producing a flexible and elastic gel which can not, however, meet all specifications for food grade agar (Cosson et al., 1995). Treatment of the ''Gracilaria '' phycocolloid with sodium hydroxide (alkaline hydrolysis), which transforms the agaroid into real agar, makes a product that meets the specifications required (Armisén, 1995). Agar in ''Gracilaria '' has a greater tendency to become hydrolyzed during storage, even under favourable conditions. This is not only caused by agarolytic bacteria; even if these organisms are not present, adequately dried and stored warm-water ''Gracilaria '' may still undergo a reduction in their agar content over a few months in storage. ''Gracilaria '' spp. from colder waters usually have a much greater resistance to hydrolysis but, even so, they are not as resistant to hydrolysis as agar from ''Gelidium '' (Armisén, 1995).

Unique properties of agar are: very strong, stable, brittle, thermo-reversible gel formation in aqueous solution, without the presence of any additives; gelation at temperatures far below the gel melting temperature (= the sol temperature: there the phycocolloid becomes liquid again); resistance to high temperatures (a 1.5% aqueous solution gels between 32 and 43°C 43 °C and does not melt below 85°C85 °C); usability over a wide pH range from 5 to 8; capacity to hold large amounts of soluble solids, flavours and colours; a maximum ash content of 5% (normally maintained between 2.5-4%) (Armisén & Galatas, 1987). The difference between gel and sol temperatures is called hysteresis. This difference can be as high as 50°C and is only displayed by agar (Lewis et al., 1988). While agar is the most commonly accepted term, in French-, Spanish- and Portuguese-speaking countries it is also called "gélose" or "gelosa" and in Japan the dry product is called "kanten".

Carrageenans are formed in the cell walls of some red algae (carrageenophytes), including the tropical genera ''Acanthophora '' J.V. Lamour., ''Betaphycus '' Doty ex P.C. Silva, ''Eucheuma '' J. Agardh, ''Hypnea '' J.V. Lamour. and ''Kappaphycus '' Doty. In ''Eucheuma '' and ''Kappaphycus'', the sporophytes and the gametophytes always contain the same type of carrageenan within each species. This differs to the situation in carrageenophytes from temperate regions, where the life-cycle phases each contain a different type of carrageenan. Although the Malay word "agar-agar" refers to ''Eucheuma'', it is now known that these yield carrageenans rather than agar-type polysaccharides.

*The beta family of carrageenans, a rather new group of carrageenans found in ''Betaphycus gelatinus '' (Esper) Doty ex P.C. Silva, Basson & R.L. Moe (= ''Eucheuma gelatinum '' (Esper) J. Agardh), is competitive with certain functions of agarose in some biotechnology applications (McHugh, 1996).

The unique properties of carrageenan include high-quality, highly viscous, thermo-reversible gel formation; protein reactivity (especially with casein); and it can be used together with guar (''Cyamopsis tetragonoloba '' (L.) Taub.) and locust bean or carob seed (''Ceratonia siliqua '' L.) gums (Anonymous, 1979; Glickman, 1987; Stanley, 1987).

Carrageenans (commercial code E407; for natural grade carrageenan in Europe the code E407a is applied) are the phycocolloids with by far the widest application in food industry (Anonymous, 1998; Bixler, 1996). They are mainly used as stabilizing, thickening, suspending and gelling agents in food such as dietary and baby foods and also in canned pet-foods, syrups, fruit drink powders and frozen concentrations, milk-based products, chocolate, pasta sauce, artificial whipped toppings, imitation coffee creams and pre-cooked, packaged meats. Carrageenans are also used in non-food products: toothpaste, cosmetics, solid gel-type air fresheners and textile paints. A preferred name in the Philippines for the locally produced carrageenan is "natural grade carrageenan" which is sometimes called "Philippines natural grade" or "PNG carrageenan" because of its country of origin. Other commonly used names are "alkaline carrageenan flour" (ACF), "alkaline-modified carrageenan" (AMC), "alkali-modified flour" (AMF), "alternatively refined carrageenan" (ARC), "alkaline-treated cottonii" or "alkali-treated cottonii" or "alkali-treated carrageenophyte" (all as ATC), "natural washed carrageenan" (NWC), "processed ''Eucheuma '' seaweed" (PES), "semi-refined carrageenan" (SRC)and "seaweed flour" (SF) (Anonymous, 1998; McHugh, 1996; Neish, 1990). In a recent survey "SRC" is used as an acronym for the total of alkali-treated chips and semi-processed powder, while "PNG" is exclusively used for the semi-processed powder alone (Anonymous, 1998). Natural grade carrageenan is not universally accepted for classification as carrageenan, although the American Food and Drugs Administration has done so. In the European Union, however, natural grade carrageenan was mainly excluded from the arbitrary definition of carrageenan as having a maximum 2% content of acid-insoluble matter (Bixler, 1996; Luxton, 1993). In the European Union therefore it has received the official separate designation "PES = Processed ''Eucheuma '' seaweed" (Anonymous, 1998; Bixler, 1996).

Algin is often used as the name for the soluble sodium salt of alginic acid, while these and other salts and esters together are called alginates. These products are found in the cell walls of brown algae, including the tropical seaweed genera ''Hormophysa '' Kütz., ''Hydroclathrus '' Bory, ''Sargassum '' C. Agardh and ''Turbinaria '' J.V. Lamour. (Painter, 1983; Trono & Ganzon-Fortes, 1988). Most commercial alginate is produced from alginophytes occurring in temperate or even colder waters. Alginic acid itself is insoluble in water, but it swells when water is added. Alginates are also produced as microbial polysaccharides by certain bacteria (Smidsrød & Christensen, 1991).

Gel formation or binding is an important application of alginates. A solution of 1-2% sodium alginate will stiffen to a gel by addition of calcium ions (50 mM) or other bivalent ions (Ba2+, Pb2+, Sr2+, etc.). The bivalent ions bind the alginate chains together in a three-dimensional gel network in accordance with the "egg box" model (Heyraud et al., 1990; Jensen, 1995). Alginate gels have various strengths, largely dependent on their content of polyguluronic acid blocks (Indergaard & Østgard, 1991). The alginate content of the warm-water seaweeds is usually somewhat lower than that in algae from other waters. In general the alginate of ''Sargassum '' and ''Turbinaria '' is of low viscosity but forms good gels. ''Turbinaria '' thalli usually have a higher alginic acid content (20-22% of the dry weight) than Sargassum (13-18%) (Pérez et al., 1992).

In areas where aquaculture is an important industry, seaweeds could be applied more regularly as a feed for aquatic animals. For example, ''Gracilaria '' can be grown in ponds with milkfish and shrimp. These animals graze on epiphytes on the ''Gracilaria'', and eventually on the red alga itself if they are left in the pond for long enough or their numbers increase out of balance with the seaweed biomass (McHugh & Lanier, 1983). Production of milkfish in fish ponds (in Indonesia: "tambaks") is best when the bottom of the production ponds is covered by a thick mat of blue-green algae such as ''Chroococcus, Gomphosphaeria, Lyngbya, Microcoleus, Oscillatoria, Phormidium '' and ''Spirulina '' spp., as well as by diatoms. This algal periphyton constitutes the main food of cultured milkfish, but filamentous green algae may also be eaten (Bardach et al., 1972). The biological complex of blue-green algae, diatoms, bacteria and various animals, which is typical of well-managed milkfish ponds, is known as "lab-lab" in the Philippines and as "kelekap" in Indonesia (Benitez, 1984; Chong et al., 1984). Red seaweeds of the genus ''Gracilaria '' can also be used as food for milkfish, but the algae are unable to withstand salinities below 5‰ (Bardach et al., 1972). It is also possible to feed molluscs on these ''Gracilaria '' seaweeds obtained from phycoculture. When labour costs are too high to make enough profit from preparing ''Gracilaria '' thalli for agar production, farmers may prefer to use these algae as a direct food for molluscs, as has been the case in Taiwan (Ajisaka & Chiang, 1993).

The blue-green alga ''Arthrospira '' (''Spirulina'') ''platensis'', which occurs in a great variety of inland waters, is renowned as being one of the richest protein sources in the world. The alga contains significant amounts of vitamin B12, other B-complex vitamins, vitamins A and E, β carotene and the essential mineral elements iron, phosphorus, magnesium, zinc and selenium (Mshigeni, 1982; Switzer, 1982; Vonshak, 1997). However, due to the production of certain toxins use of cyanobacteria as food will always entail a degree of hazard, until adequate health standards for production and marketing of cyanobacteria for single-cell protein are adopted and observed (Gorham & Carmichael, 1988).

The potential for using microalgae to combat current problems of famine and malnutrition has been reviewed. Attempts to bring small-scale microalgal production to villages for application in integrated village systems are promising (Jassby, 1988a). ''Arthrospira '' has little value as an energy source, but is interesting as a protein source, especially among malnourished human populations (Van Khuong, 1990). The palatability of small amounts is not the problem, but in larger quantities the strong colour, odour and taste are often not acceptable as a major food source (Jassby, 1988a).

Other freshwater algae, such as the unicellular green algae of the genus ''Chlorella'', also produce a very good protein source. Its protein is especially rich in the amino acids lysine, threonine and tryptophane, which are generally poor in cereal proteins (Lee & Rosenbaum, 1987). However, Chlorella is not accepted as an attractive new food item. It is mainly available in health-food shops and in the form of chlorophyll pills, or added in powder form to various kinds of food products. In this form it increases significantly the level of vitamins and lipids without affecting the palatability of the food (Mshigeni, 1982). Members of ''Dunaliella '' are occasionally used as a protein supplement in bread (Borowitzka & Borowitzka, 1988).

Green algae are mostly less suitable as feed, although ''Chlorella '' and ''Dunaliella '' are often used as feed for fish larvae in commercial fish farming. Several diatoms, however, are more satisfactory in many respects. Nutritional deficiencies in a diet for organisms in aquaculture can be avoided by using mixed algal diets (Volkman et al., 1989). Gross chemical and fatty acid composition of a number of tropical microalgae has been determined (Renaud & Parry, 1994; Renaud et al., 1994; Shamsudin, 1992). The greatest breakthrough in shellfish production would be the production of the right microalgae in the right quantities at the right time (Doty, 1979). Heterotrophic growth may be the solution (Apt & Behrens, 1999; Gladue & Maxey, 1994). The prospects of the green freshwater microalga ''Haematococcus pluvialis '' Flot. for the production of the keto-carotenoid astaxanthin, a natural food and feed colourant often used in the aquaculture industry, are promising (Borowitzka, 1994; Ding et al., 1994).

The earliest information on seaweed utilization for medicinal purposes originate from the Chinese "Materia Medica" of Shên-nung, dating from 2700 B.C. (Hoppe, 1979). In many areas numerous algae are used as medicaments, especially in coastal countries. They are used in folk medicine against goitre, nephritic diseases, helminths, catarrh, etc. Several algal species contain substances of pharmaceutical interest of which the active compounds are not yet characterized. In others, however, the active principle compound has been identified like a mixture of kainic and allokainic acids responsible for vermifugal properties in the red alga ''Digenea simplex '' (Wulfen) C. Agardh (Arasaki & Arasaki, 1983; Chapman, 1970; Michanek, 1979). Substances with anticoagulant properties, with cytotoxic activity (anticancer or antineoplastic activity), antibiotic, antifungal, antiviral or antioxidant activity are known to occur in some seaweeds, as are haemagglutinins (Chapman, 1970; Fujimoto, 1990; König, 1992; Mabugay et al., 1994; Matsukawa et al., 1997; Michanek, 1979; Neushul, 1990; Pesando, 1990; Reichelt & Borowitzka, 1984; Santos & Guevara, 1988; Shiomi & Hori, 1990; Wong et al., 1994). Antibiotic activity is not present at all times and even samples of the same species, when collected from different localities, may give different results (Padmakumar & Ayyakkannu, 1997).

Fucoidan, a polymer of fucan sulphate, found in members of the ''Phaeophyta'', contains a fairly high percentage of sulphate ester. This substance is known to have the same anticoagulant effect as heparin and a 1% aqueous solution of fucoidan obtained from a ''Sargassum '' sp. has shown greater antithrombic activity than the same concentration of heparin (Arasaki & Arasaki, 1983). Fucan sulphate, however, can not yet be stored satisfactorily; even deep-frozen material is not stable (Nishino & Nagumo, 1991).

Compounds that are toxic for the freshwater snail ''Biomphalaria glabrata '' (vector for schistosomiasis) have been found in marine algae, whereas some anti-malarial activity has been detected in compounds isolated from the brown algal genus ''Dictyota '' J.V. Lamour. and other seaweeds (König, 1992; Subramonia Thangam & Kathiresan, 1991).

A bulk use of seaweeds as a food material with obvious therapeutic benefits (nutraceuticals) is found in the brown algae which contain iodine which combats endemic goitre. Products made from ''Laminaria japonica '' Aresch. are widely used for such purposes in China (Michanek, 1981). Endemic goitre may also possibly be eradicated by public health information and use of seaweed food, especially in Indonesia and Malaysia (Michanek, 1979). Excess intake of iodine may, however, also produce fatal effects in both humans and animals. Goitre (thyrotoxicosis) has been demonstrated as a consequence of the high intake of kelp products by children in Japan and in adults in Australia and Finland (Liewendahl, 1972; Wheeler et al., 1982; Zuzuki, 1965).

Until recently, only very few macroalgae had been recorded as more or less un-wholesome, especially some ''Caulerpa '' spp. and ''Turbinaria ornata '' (Turner) J. Agardh (Russell, 1984). ''Caulerpa '' spp. have occasionally been mentioned as producing neurotoxins (Lemée et al., 1993; Paul & Fenical, 1986; Ribera et al., 1996; Schantz, 1970). ''Turbinaria '' is recorded as creating gastro-intestinal distress in humans (Russell, 1984). In comparison with the more than 1200 toxic marine organisms that were included in Russell's review of marine poisonous organisms, one must conclude that the macroalgae, as a group, are more or less non-toxic (Indergaard & Minsaas, 1991). A well-documented case where people died after eating the red alga ''Gracilaria tsudai '' (I.A. Abbott & I. Meneses) I.A. Abbott on Guam, however, indicates that this alga may form relatively high levels of previously undescribed toxic compounds at some stages of its life cycle (Yasumoto, 1993). The occurrence of toxic substances seems not related to the neurotoxins causing the marine food poisoning known as "ciguatera", which is associated with toxins formed by the dinoflagellate alga ''Gambierdiscus toxicus '' Adachi & Fukuyo and accumulated in fish and molluscs (Stadler, 1993; Steele, 1993). The latter dinoflagellate, however, may contaminate macroalgae, to which it strongly adheres (Nakahara et al., 1996; Saint-Martin et al., 1988). Around Ambon (Indonesia) these dinoflagellates occur attached to ''Sargassum, Turbinaria '' and ''Halimeda '' spp. (Sidabutar, 1996).

There are several blue-green algae (''Cyanobacteria'') that are known to be poisonous, especially the genus ''Lyngbya '' C. Agardh ex Gomont (Madlener, 1977). In the microalgae many highly toxic species occur which have not, however, been reported, as having harmful effects on macroalgal cultivation (Correales & MacLean, 1995). Most of these microalgae are not included in the present book of plant resources, except some blue-green freshwater algae that are useful because of their nitrogen-producing capacities. Not all blue-green algae are toxic, however. ''Arthrospira platensis'', which is often used as food or a food supplement for humans, is completely non-toxic. A list has been published of experiments on potential therapeutic applications of ''Arthrospira '' (as ''Spirulina '' Turpin ex Gomont) as well as a review of public health aspects of microalgal products (Jassby, 1988a, 1988b). The status of microalgae as sources for pharmaceutical and other biologically active molecules has been reviewed (Borowitzka, 1995). Experiments to prepare mosquitocidal cyanobacteria provide an interesting prospect (Stevens et al., 1994).

Manual harvesting of beach-cast algae, mainly members of the Phaeophyta, has been carried out since ancient times for spreading on fields as a fertilizer and for soil conditioning, especially in maritime parts of Europe. These and other stranded algae are often the result of proliferation, due to an abundant presence of nutrients, favourable meteorological conditions and accumulation in confined areas (Morand et al., 1991). In the Philippines, in the coastal area of Ilocos Norte (north-western Luzon) the use of brown seaweeds of the genera ''Hormophysa, Padina '' Adans., ''Sargassum '' and ''Turbinaria '' as a fertilizer and soil conditioner is well documented (Fortes et al., 1993; Tungpalan, 1983). A product based on composted ''Ascophyllum nodosum '' (L.) Le Jol., a North Atlantic brown alga, has been used successfully in landscaping and reclamation projects. The method used to apply the soil conditioner will depend on the nature of the site. Where topsoil is available, the composted algae are mixed with the soil at a rate of 1.5 kg/m3. However, in many cases no topsoil is available and the seaweed product must be applied to subsoils. If the site is relatively flat, the soil conditioner is worked into the top 5 cm of subsoil at a rate of 75 g/m3, and then appropriate fertilizers and seeds are added using ordinary horticultural techniques. Often, the area to be treated includes steep slopes which are impossible to cultivate using conventional equipment and thus more liable to suffer soil loss due to run-off than flat sites. In tropical regions, heavy rains often make the problem of erosion due to run-off more acute than in the temperate zones. Spraying with a mixture containing composted ''Ascophyllum'', together with clay, fertilizer, seed, a mulch (either cellulose pulp or peat) and water has given satisfactory results, even on bare rock and in tropical countries (Blunden, 1991). Processed seaweed products for crop use are of three kinds:

 Several crustose, calcareous red algae of the family ''Corallinaceae '' are used as fertilizers and soil-conditioning agents, primarily on acid soils. 

The use of cyanobacterial bio-fertilizers, especially for growing rice, is promising (Kannaiyan et al., 1997; Venkataraman, 1994). In terms of ultimate nitrogen input in the paddy, algalization is feasible at about one-third of the cost of a chemical fertilizer. Bio-fertilization by blue-green algae can also be done indirectly by using the heterosporous floating aquatic ferns of the genus ''Azolla''. Specialized leaf cavities in the water fern house the cyanobacterium ''Anabaena azollae '' Strasb. ex Wittr. (Faridah Hanum & van der Maesen, 1997; Metting, 1988; Metting et al., 1988, 1990). Among the known plant-cyanobacteria symbioses, only the ''Azolla-Anabaena '' associations have significant potential as alternative nitrogen source in agriculture, since the symbiont is capable of fixing atmospheric nitrogen at high rates. The utilization of these inexpensive bio-fertilizers has several advantages over chemical fertilizers: they make use of freely available solar energy, atmospheric nitrogen and water, thus utilizing renewable resources. In addition, they are non-polluting and, besides supplying nitrogen to crops, they also supply other nutrients such as vitamins and growth substances and improve the general fertility of the soil by improving the soil structure and increasing the organic matter. The benefits brought about by green manure such as ''Azolla '' are long-term, increasing grain yield during several successive crops of rice. Moreover, in low-potassium environments the application has a greater ability to accumulate potassium than does rice. Thus, when the fern decomposes, it acts indirectly as a potassium fertilizer.

The following taxa of seaweeds were cultivated in South-East Asia around 1973 (FAO, 1974): ''Caulerpa racemosa '' (Forsk.) J. Agardh, ''Chaetomorpha antennina '' (Bory) Kütz., ''C. crassa '' (C. Agardh) Kütz., ''Cladophora '' spp., ''Enteromorpha compressa '' (L.) Nees, ''Eucheuma edule '' J. Agardh (probably partly ''Betaphycus gelatinus '' and partly ''E. serra '' (J. Agardh) J. Agardh), ''E. denticulatum '' (Burm.f.) Collins & Herv. (as ''E. spinosum '' J. Agardh).

Other seaweeds have been added recently as cultivated organisms: ''Caulerpa lentillifera '' J. Agardh (this is probably the correct identification of most cultivated ''Caulerpa racemosa''), ''Enteromorpha clathrata '' (Roth) Grev., ''E. intestinalis '' (L.) Nees, ''Gracilaria '' spp., ''Kappaphycus alvarezii '' (Doty) Doty ex P.C. Silva, ''K. striatus '' (F. Schmitz) Doty ex P.C. Silva.

Few quantitative data are available on the production of microalgae (Zhu & Lee, 1997). Microalgal production in 1991 was limited to approximately 2000 t, and was used primarily as health food and for the extraction of β carotene (Benemann, 1992). In 1984 the ten largest commercial ''Spirulina '' farms produced just over 700 t of food-grade Spirulina powder, the Siam Algae Company in Thailand being the leader in terms of productivity (Jassby, 1988a). Since then other producing plants have taken over the lead position (Belay et al., 1994; Venkataraman, 1989; Vonshak, 1997). The production costs, however, are rather high (Belay et al., 1994; Vonshak, 1997).

In 1980 about 36 000 t (dry weight) agarophytes were harvested around the world, including 18 100 t from Asia (1470 t from the Philippines), and used to produce 7000 t of agar (3500 t from Asia) in the same year. Indonesia and Thailand were already known to be producing and exporting agar-bearing seaweeds, but no data were available (McHugh & Lanier, 1983; Soegiarto & Sulistijo, 1986). In 1984 150 t of agar were produced in Indonesia, probably mainly from ''Gracilaria '' (Armisén & Galatas, 1987), rising to 450 t in 1993 (Armisén, 1995). 

The yield of carrageenan in percentage of dry weight of the seaweeds varies from 14-27% in Eucheuma gelatinum in China to 58-65% in ''Kappaphycus alvarezii '' in the Philippines (Pérez et al., 1992). A 39% yield of carrageenan for conventionally dried ''Kappaphycus alvarezii '' has been mentioned, which can be increased to a yield of 60% after first washing the dried algae with freshwater (Bixler, 1996).

The total amount of seaweeds used as food for direct human consumption in the world was 385 000 t (dry weight) in 1980 (McHugh & Lanier, 1983). In 1989 the production of ''Hizikia, Laminaria, Porphyra '' and ''Undaria '' spp. combined was 454 800 t (dry weight) (McHugh, 1991). For 1990 it was 200 000 t (dry weight) for ''Laminaria, Porphyra '' and ''Undaria '' spp. together (Jensen, 1993). Except for ''Porphyra'', none of these algal genera occur in South-East Asia.

Dried seaweeds are often exported, especially for the production of phycocolloids. In South-East Asia these seaweeds are usually exported as raw material, although in particular the production of (semi-refined) carrageenan and salted ''Caulerpa lentillifera '' is growing.

Of the microalgae produced in South-East Asia (''Arthrospira '' mainly in Thailand but also in Vietnam), most is exported in the form of dried algal powder.  <center>''''''</center> {| class="wikitable"|+Table 5. Import and export values of dry seaweeds during the period 1995-1997<br>for Indonesia, Malaysia, Singapore, the Philippines and Thailand.!colspan="2"|Country!!colspan="3"|Import!!colspan="3"|Export|-!colspan="2"| !!Quantity<br>(t)!!Value<br>(10³ US$)!!Unit price<br>(US$/t)!!Quantity<br>(t)!!Value<br>(10³ US$)!!Unit price<br>(US$/t)|-|rowspan="3"|'''Indonesia'''||1995||50||213||24 620||24 957||16 262||651|-|1996||30||168||5 600||22 310||18 962||850|-|1997||131||492||3 755||12 698||10 521||829|-|rowspan="3"|'''Malaysia'''||1995||1||1 005||1 005 000||3||320||106 666|-|1996||1 263||1 540||1 219||477||727||1 524|-|1997||353||2 588||7 331||1 299||597||460|-|rowspan="3"|'''Singapore'''||1995||4141||4 686||11 318||2621||1 790||6 832|-|1996||8741||8 160||9 336||4941||3 274||6 627|-|1997||6121||5 710¹||9 330||331¹||2 025¹||6 117|-|rowspan="3"|'''The Philippines'''²||1995|| || || ||28 920||39 106||1 352|-|1996|| || || ||26 406||41 974||1 589|-|1997|| || || ||n.a.||54 992||n.a.|-|rowspan="3"|'''Thailand'''||1995||298||5 932||19 906||110||1 649||14 990|-|1996||477||8 364||17 535||77||1 298||16 857|-|1997||383||3 697||9 653||55||797||14 491|}¹ FAO estimate (1999b); ² only carrageenophytes; n.a. = not available.<br>Sources: FAO, 1999a, 1999b, 1999c; Trono, 1999.  <center>''''''</center> {| class="wikitable"|+Table 6. Import and export values of agar during the period 1995-1997<br>for Indonesia and Singapore.!colspan="2"|Country!!colspan="3"|Import!!colspan="3"|Export|-!colspan="2"| !!Quantity<br>(t)!!Value<br>(10³ US$)!!Unit price<br>(US$/t)!!Quantity<br>(t)!!Value<br>(10³ US$)!!Unit price<br>(US$/t)|-|rowspan="3"|'''Indonesia'''||1995||496||4 711||9 498||931||2 942||3 160|-|1996||557||3 783||6 792||981||4 974||5 070|-|1997||754||6 640||8 806||637||3 327||5 223|-|rowspan="3"|'''Singapore'''||1995||369||6 168||16 715||126||1 714||13 603|-|1996||485||7 180||14 804||114||616||5 404|-|1997||319||4 897||15 351||32||369||11 531|}Source: FAO, 1999b.

''Marine vegetables '' Varieties of red, brown and green seaweeds are eaten by coastal inhabitants as a salad or as cooked vegetables (Hatta et al., 1993). At least 61 species in 27 genera of marine macroalgae are consumed as food and at least 21 species are used as herbal medicine (Istini et al., 1998).

''Agarophytes '' Much of the final product of the agarophytes is in the form of agar strips, which are used in food preparation. In 1984, 62 t (probably dry weight) of "''Gelidium '' seaweeds" from Indonesia were imported to Japan, together with 69 t of "other agarophytes", probably mainly Gracilaria (Armisén & Galatas, 1987). However, ''Gelidium '' production (potential: 4500 t wet weight) is still only done by gathering from natural stocks (Mintardjo, 1990). Less than 1000 t (dry weight) of agarophytes are exported annually, of which in 1991 603 t of dried Gracilaria to Japan and 59 t of ''Gelidium '' to New Zealand and Italy.

About 90% of this production was sold on domestic markets, the rest exported (McHugh, 1996). However, that is not in agreement with other published data (FAO, 1999b). Indonesia still imports agar (FAO, 1999b; Istini et al., 1998; McHugh, 1996) (Table 6). The combined capacity of the Indonesian agar production plants is 1200 t agar/year, limited by the quantity and quality of cultivated ''Gracilaria '' available. The Indonesian production levels of agar require 7200 t (dry weight) of agarophytes per year. However, only 5500 t of dried ''Gracilaria '' is produced by phycoculture, and thus demand exceeds the present supply (McHugh, 1996). The potential amount of agarophytes available from wild crops is estimated at 28 000 t (wet weight) (McHugh, 1996).

''Carrageenophytes '' It was not until ''Eucheuma '' spp. were recognized as valuable carrageenophytes by the western carrageenan industries that large-scale export became established (Adnam & Porse, 1987; Rachmat et al., 1986; Stanley, 1987). The main species of carrageenophytes for phycoculture in Indonesia are ''Eucheuma denticulatum '' and ''Kappaphycus alvarezii''. In the 1980s, inadequate drying and post-harvest contamination with sand resulted in poor export quality for foreign processors (Doty, 1986; Luxton, 1993; McHugh & Lanier, 1983). On the other hand, consumption by Indonesian processors and sustained demand by the Chinese food market for ''Eucheuma denticulatum '' (1500-2000 t annually) ensured a predictable base of farm production.

In 1990 between 6000 and 6500 t (dry weight) of ''Eucheuma denticulatum '' and between 4000 and 5000 t (dry weight) of ''Kappaphycus alvarezii '' were exported, to which can be added 2100 t (dry weight) of the latter species consumed by local processors (Luxton, 1993). In 1993 15 000-21 000 t (dry weight) of carrageenophytes (iota and kappa types) were exported from Indonesia for food grade carrageenan production (Bixler, 1996; McHugh, 1996). Data for 1994 give an estimated production of 26 294 t dried seaweeds, including "''Eucheuma''", ''Gracilaria, Gelidiaceae '' and others, of which 16 100 t was exported (Istini, 1998). In 1995 the dry weight production of ''Eucheuma denticulatum '' and ''Kappaphycus alvarezii '' together was estimated at 20 000 t (Trono, 1998). In 1994 6 carrageenan-processing companies in Indonesia collectively produced 2300 t semi-refined carrageenan (SRC) and one company produced 120 t refined carrageenan (McHugh, 1996). The semi-refined carrageenan produced locally is not yet a final product, however, and has to be exported for final processing. None of this domestically produced, semi-refined carrageenan is consumed in the country itself and Indonesia must also import to meet its requirements for refined carrageenan (150-170 t annually), because its total produce is exported to Japan. For further information on carrageenophytes ("''Eucheuma''"), see Table 7.  <center>''''''</center> {| class="wikitable"|+Table 7. Production of carrageenophytes in Indonesia and the Philippines<br>for the period 1990-1997.! !! 1990-1992!! 1993!! 1994!! 1995!! 1996!! 1997|-|'''Indonesia'''|-| quantity, wet (t)||95 000¹||110 000¹||102 000||102 000||148 000||157 000|-| value (10³ US$)||8 150||12 100||11 220||11 220||14 800||15 700|-| unit price (US$/t (wet)||87||110||110||110||100||100|-| from wild populations, wet (t)||11 284||8 395||8 438||9 575||13 543||15 000¹|-|'''The Philippines'''|-| quantity, wet (t)||289 664||376 379||454 708||545 407||611 561||602 215|-| quantity, dry (t)||38 677||47 844||50 614||58 324||117 511²||no data|-| value (quantity wet)(10³ US$)||40 017¹||37 384||41 984||49 158||58 716||50 910|-| unit price (US$/t (wet))||138||99||92||90||96||84|-| from wild populations, wet (t)||1 265||1 144||1 062||919||884||494|}¹ FAO estimate; ² equivalent, calculated from produced carrageenan together with exported dry seaweed.<br>Sources: FAO, 1999a, 1999c.

''Alginophytes '' Alginate has been produced by a factory in Bandung (West Java) since 1992. Production in 1992 was 300 t alginate which required 3000 t of dried ''Sargassum '' seaweed (Istini et al., 1998). Other data, however, estimate production at 100 t alginate per year and required 1000 t of dried Sargassum (McHugh, 1996). Nevertheless, Indonesian alginate imports rose from about 3700 t in 1987 to 5100 t in 1991. The estimate for 1994 was 4000 t. The main uses were in the food, brewing, pharmaceutical and textile industries (McHugh, 1996).

''The farms '' Farming of ''Eucheuma denticulatum '' (market name: "spinosum"), ''Kappaphycus alvarezii '' (market name "cottonii") and ''Gracilaria '' is a promising activity. In 1988 22 600 ha were identified as potentially suitable sites for seaweed culture, of which 17 700 ha was considered suitable for ''Eucheuma '' culture. Of these potential sites, about 900 ha are actually used for seaweed cultivation (Mintardjo, 1990). Later figures of the estimated potential exploitable area for ''Eucheuma '' culture, however, are approximately 9000-10 000 ha, with a production capacity of 450 000 t (dry weight) per year (Istini et al., 1998). ''Gracilaria '' is mainly farmed in South Sulawesi, where about 2000 farmers produce approximately 5500 t dried seaweed per year.

''Microalgae '' PT Sun ''Chlorella '' Indonesia Manufacturing Company in East Java, a joint venture with Japanese companies, has 16 circular culture ponds, each capable of producing 36 t of wet ''Chlorella''. The enterprise has a total production of 150 t ''Chlorella '' powder a year. Production started in 1995 with 6 t/year. All of the production is exported to Japan for processing and packaging.

Marine vegetables Coastal inhabitants eat green, brown and red seaweeds, sometimes as salad, or cooked as vegetables (McHugh & Lanier, 1983). Seaweeds such as ''Laminaria '' ("kombu"), ''Porphyra '' ("nori") and ''Undaria '' spp. ("wakame") are imported, mainly from Japan, Korea and China.

''Agarophytes '' In 1993 and 1994 about 6 t (dry weight) of agar from ''Gracilaria '' was produced by a small-scale, Thai-owned processing factory in Selangor (Peninsular Malaysia) for local consumption in jelly-type sweetmeats. In 1987 about 240 t of agar, worth US$ 2 600 000, was imported, mainly from Korea and Japan (McHugh, 1996). Since trials for ''Gracilaria '' cultivation were successful, and because there is a large market for domestically produced agar strip, the country seemed to be a good choice for future development of agar strip production facilities for the region (McHugh, 1996).

''Carrageenophytes '' Jellies are also made from carrageenan extracted from "''Eucheuma''" by local coastal populations. Refined carrageenan, however, in quantities of about 150 t, with a value of US$ 1 600 000 are imported annually. These carrageenans are used in industry and this market is expected to grow at 5% per year (McHugh, 1996).

''Alginophytes '' The domestic market for alginates in 1978 was thought to be sufficiently large to support a small alginate processor. There was, however, no information available on the extent of ''Sargassum '' and ''Turbinaria '' beds in the area, nor of the quality of any alginate extracted from them (McHugh & Lanier, 1983). Some Sargassum spp. from Sabah (East Malaysia), however, have a high content of guluronic acid (Wedlock et al., 1986). Alginates of these algae are supposed to form strong gels, sought after for special applications (McHugh, 1987). Recent annual import quantities of alginate for Malaysia are estimated to be 60 t (McHugh, 1996).

''The farms '' Farming of ''Gracilaria changii '' (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia is promising and is in an experimental stage in Peninsular Malaysia (Phang, 1998). The seaweeds are cultivated in an integrated polyculture system with shrimps (''Panaeus monodon '' and ''Lates calcifer''). In Sabah (East Malaysia) small-scale ''Kappaphycus '' culture takes place, which has resulted in the export 80 t (dry weight) of cultured ''Kappaphycus alvarezii '' (as ''Eucheuma'') to Denmark in 1990 (Choo, 1990). More recently the crop (500-1800 t dry weight per year) is exported to the Philippines (FAO, 1999b; McHugh, 1996; Phang, 1998).

''Microalgae '' Some microalgae are cultured to be used as feed for larval stages of organisms grown in aquaculture or for use in integrated systems for wastewater treatment (Phang, 1987; Shamsudin, 1992).

''Marine vegetables '' Over 40 species of seaweeds are gathered and directly utilized as food in the coastal areas of the Philippines. Production is seasonal and in small quantities and detailed information is lacking (Trono, 1998). There is, however, one exception: ''Caulerpa racemosa '' (and more recently ''C. lentillifera'') has been produced in phycoculture ponds since as early as 1950. More than 400 ha of ponds are used for the cultivation of ''C. lentillifera '' in Mactan Island, Cebu. These algae are either sold fresh or exported as a brine-cured product to Japan. More than 20 000 t (wet weight) of these green algae are produced (FAO, 1996, 1999c). See also Table 2.

''Agarophytes '' Of the agarophytes, mainly ''Gracilaria '' is available and utilized, although other algae are used as agarophytes as well (Montaño & Pagba, 1996). Domestically produced agar strips ("gulaman bars") are sold in 5 g pieces; there is a total agar production of about 30 t per year. Only relatively small amounts (less than 10 t/year) of agar are imported, mainly for applications in the biotechnology and pharmaceutical industries.Farming of Gracilaria is promising (FAO, 1996, 1997) and more than a dozen species are presently grown in culture, which produced 10 t (wet weight in 1994). However, only 4 species are in commercial production: i.e. Gracilaria heteroclada C.F. Zhang & B.M. Xia (= Gracilariopsis heteroclada C.F. Zhang & B.M. Xia), G. firma C.F. Zhang & B.M. Xia and a still unidentified Gracilaria sp. are partly produced through cultivation, while G. tenuistipitata C.F. Zhang & B.M. Xia is produced mainly from natural stocks (Trono, 1998). The total amount of wild Gracilaria harvested annually is estimated at 200 t (dry weight). Apart from Gracilaria only Gelidiella acerosa is suitable for agar production, and is almost exclusively harvested through the gathering of local stocks (Trono, 1998).

Farming of Gracilaria is promising (FAO, 1996, 1997) and more than a dozen species are presently grown in culture, which produced 10 t (wet weight in 1994). However, only 4 species are in commercial production: i.e. ''Gracilaria heteroclada'' C.F. Zhang & B.M. Xia (= ''Gracilariopsis heteroclada'' C.F. Zhang & B.M. Xia), G. firma C.F. Zhang & B.M. Xia and a still unidentified ''Gracilaria'' sp. are partly produced through cultivation, while ''G. tenuistipitata'' C.F. Zhang & B.M. Xia is produced mainly from natural stocks (Trono, 1998). The total amount of wild ''Gracilaria'' harvested annually is estimated at 200 t (dry weight). Apart from ''Gracilaria'' only ''Gelidiella acerosa'' is suitable for agar production, and is almost exclusively harvested through the gathering of local stocks (Trono, 1998). ''Carrageenophytes '' The phycoculture of ''Eucheuma '' in the Philippines was pioneered and developed in the period 1968-1971. This greatly promoted the carrageenan industry (Anonymous, 1998; Laite & Ricohermoso, 1980; Lim & Porse, 1981; Ricohermoso & Deveau, 1979; Stanley, 1987). Annual production increased from 300 t dry wild seaweed in 1970 to 13 500 t dry weight seaweed (mainly from phycoculture) during the period 1978-1980 to 117 511 t dried carrageenophytes or an equivalent of 822 500 t wet weight in 1996 (Anonymous, 1998; McHugh & Lanier, 1983). Of the 15 000 t dry seaweed produced in the Philippines in 1980, only 700 t of carrageenans were produced in that country, and most of the seaweeds were exported as raw material. In 1990 1295 t (wet weight) of carrageenophytes were still collected from wild stocks. That amount declined to 494 t (wet weight) in 1997, as compared to 291 176 (1990) and 627 105 (1997) t (wet weight) produced by phycoculture (FAO, 1999a). Data from different sources are not always comparable, however (Dawes et al., 1990; Llana, 1990; McHugh, 1990; Trono, 1990, 1998). See also Tables 2, 5 and 7.

The principal species grown in the Philippines is ''Kappaphycus alvarezii '' ("cottonii"), with smaller and irregular amounts of ''Eucheuma denticulatum '' ("spinosum"). The seaweeds are either exported to carrageenan producers, or semi-refined carrageenan ("natural grade carrageenan") is produced locally. At present 10 different companies (mostly members of the Seaweed Industry Association of the Philippines, SIAP) export semi-refined carrageenan, while 3 export pure carrageenan (Anonymous, 1998). In 1990 these companies provided employment for more than 10 000 people (Trono, 1998). By 1987, more than 50% of all Philippine ''Eucheuma/Kappaphycus '' harvests were utilized by local processors in the manufacture of carrageenan products, which then generated about 60% of the country’s foreign exchange earnings. Seaweeds and their products form the third most important fishery export of the Philippines (Trono, 1999). Large quantities of semi-refined carrageenan were produced in 1995 and 1996, while 14 493 and 18 292 t respectively of semi-refined carrageenan were exported, as well as 2375 and 2252 t of refined carrageenan. The local market, however, is only small (McHugh, 1996; Trono, 1999).

In 1991 the United States Food and Drug Administration (USFDA) classified "PNG" as carrageenan. In the European Union, however, the product is known as "processed ''Eucheuma '' seaweed" (PES) and is accepted as a food additive (INS - E407a). Another designation, "Alternatively Refined Carrageenan" (ARC) is also used (Anonymous, 1998).

''Alginophytes '' ''Sargassum '' is mainly collected sun-dried and shipped to Japan to be used as fertilizer or in powder form as a binder of heavy metals in sewage water treatment. However, the bulk of the collected ''Sargassum '' biomass is presently processed into seaweed meal and utilized in the production of animal feed, whereas a part (5000 t in 1987) is still exported to Japan (Trono, 1998). The harvest of local stock is presently limited to northern Mindanao and Visayas. There is no local alginate production in the Philippines (McHugh, 1996).

''The farms '' Over 10 000 family-owned and commercial seaweed farms were in operation around 1991, with over 170 000 people employed (Dawes et al., 1993). For 1989 the total number of "fisher folks" directly involved in the farming of "''Eucheuma''" was about 70 000 people (Trono, 1990). In 1998 about 80 000 farmers were involved in seaweed cultivation and, in addition, more than 300 000 people engaged in activities related to the seaweed industry (Trono, 1998). In 1997 10 000-15 000 ha of seaweed farms were located in the shallow coastal waters of the Philippines (Anonymous, 1998). Farm sites are mainly centered in south-western Mindanao (Zamboanga), the Sulu Archipelago, Tawi-tawi and southern Palawan (Trono, 1998).

''Microalgae '' There is no large-scale commercial production of microalgae. A company is promoting a novel concept for the contract growing of ''Arthrospira '' (''Spirulina''). The company provides training and materials such as ''Spirulina '' inoculum and chemicals for culture medium are sold at cost price to the contract producers. The company buys back the dried microalgae which are produced (Lee, 1997).

Studies of the seaweed flora of Myanmar are still incomplete. Up to now 307 species of seaweeds in 122 genera have been recorded (Soe-Htun, 1998). Of these algae, experimental cultivation of ''Catenella nipae '' Zanardini, ''Gracilaria salicornia '' (as ''G. crassa'') and ''G. edulis '' (S.G. Gmelin) P.C. Silva is being undertaken at Setse Aquaculture Research Centre, on the Tanintharyi coast.

''Marine vegetables '' Seaweeds are not generally popular as vegetables. Nevertheless, Catenella nipae is available as a sea vegetable from the market in Yangon. Edible seaweeds in a dried form are sold on the domestic Burmese market, these include ''Catenella, Enteromorpha '' and ''Hypnea '' spp. Coastal people use ''Catenella '' Grev., ''Gracilaria, Halymenia '' C. Agardh, ''Hypnea, Sargassum '' and ''Solieria '' J. Agardh in salads (Soe-Htun, 1998).

''Agarophytes '' There is no internal agar industry, although agar powder imported from neighbouring countries is very popular among the people. Locally farmed ''Gracilaria edulis '' did not attract enough demand to warrant continued production. If a local agar-processing industry could be initiated, both the domestic demand and the potential to farm ''Gracilaria '' are positive factors (Soe-Htun, 1998).

''Carrageenophytes '' Carrageenan is mainly obtained from ''Hypnea'', of which there is a standing stock of 1500 t (dry weight). About 25 small factories produce strips of carrageenan for the domestic market. However, this product is not very popular since people prefer imported agar powder for making jelly desserts (Soe-Htun, 1998).

''Microalgae '' About 30 t of ''Arthrospira '' (''Spirulina'') are commercially harvested from volcanic lakes (Twyn Taun and others) near Butalin in Central Burma. Spirulina flakes are first sun-dried, then ground into fine powder and finally punched into tablets (Lee, 1997).

''Marine vegetables '' Especially ''Caulerpa racemosa '' and ''Porphyra '' are used as a vegetable. The total crop of ''Porphyra '' is less than 100 kg (dry weight) per year, while no data are available for ''Caulerpa''. Imports in 1989 of 78 t of dried and preserved seaweeds consisted mainly of ''Laminaria '' ("kombu"), ''Porphyra '' ("nori") and ''Undaria '' spp. ("wakame"), which are used as food. These imports mainly came from Japan, China and Korea.

''Agarophytes '' In 1985 production of 4233 t of seaweeds was recorded (FAO, 1995). Seaweed production data for later years are lacking (FAO, 1995, 1996, 1997). Up till 1989 about 100 t (dry weight) of seaweeds were annually exported from Thailand, including 30-50 t of ''Gracilaria'', some of which was obtained from phycoculture (Saraya & Srimanobhas, 1990). Pond culture of ''Gracilaria fisheri '' (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia (also by monoline culture) and ''G. tenuistipitata '' is currently successful. The combined production of ''Gracilaria '' from natural stocks and cultivation in ponds ranged between 50-400 t/year in Pattani Province, while data on the production from the monoline cultures in Songkhla Lake are not yet available (Lewmanomont, 1998; McHugh, 1996). Most of the material is exported. In some years, Thailand imported rather large quantities of agar (1989: 275 t) for different industrial uses and re-exported much smaller amounts (e.g. 1989: 3 t) as repacked flavour agars (Saraya & Srimanobhas, 1990).

''Carrageenophytes '' Annual imports for carrageenan are about 1100 t/year. The large pet-food industry in Thailand uses about 780 t of semi-refined carrageenan annually, valued at US$ 2 500 000. Tuna processing, the jelly and confectionery industry and toothpaste manufacturers each use more than 100 t of imported, refined carrageenan per year. To cope with its annual demand for approximately 780 t of carrageenan, Thailand could support a semi-refined carrageenan facility if it imports the raw seaweed material from neighbouring countries or established its own "''Eucheuma''" cultivation industry (McHugh, 1996).

''Alginophytes '' The brown seaweed ''Sargassum '' is the most common genus of marine algae in Thailand. When these algae are used, it is mainly for fresh consumption or as a herbal medicine. The import of 316 t of alginate for different industrial uses is documented for 1989, for 1994 the figure was 400 t, the latter with a value of US$ 4 000 000.

''Microalgae '' The two major producers of ''Arthrospira '' (''Spirulina'') produce 150 t and 20 t ''Spirulina '' powder per year respectively, mainly for human consumption as health food (Lee, 1997).

''Agarophytes '' In 1984 a small agar production facility existed, but production data are not available (Armisén & Galatas, 1987). The main commercial seaweeds are about 15 ''Gracilaria '' spp., with a total production from natural stocks of about 9300 t wet weight (= about 800 t dry weight) in 1990. These algae were harvested from the total estimated available wild biomass of 30 000 t (wet weight). At present, the main species cultivated are ''G. vermiculophylla '' (Ohmi) Papenf. (as ''G. asiatica '' C.F. Zhang & B.M. Xia), ''G. tenuistipitata '' and ''Gracilariopsis heteroclada''. Of these gracilarioid algae, together with some ''Gelidiella acerosa '' (Forssk.) Feldmann & Hamel collected from natural populations, 100-300 t (dry weight) were used for agar-agar processing for foodstuffs, resulting in a production of 10 t food quality agar in 1987, 20 t in 1989 and 80-100 t in 1996 and 1997 (Huynh & Nguyen, 1998). ''Gracilaria '' production has become considerable, and was estimated in 1997 at 1500-2000 t (dry weight). A large part of this material is being exported to Russia, Japan and China (FAO, 1996; 1999a; Huynh & Nguyen, 1998). See also Table 2.

''Carrageenophytes '' Both ''Kappaphycus cottonii/alvarezii '' and ''Betaphycus gelatinus '' are grown in phycoculture, each with an annual production of about 10 t (dry weight). These algae are, up to now, mainly used for food purposes. Cultivation of ''K. alvarezii '' started recently and is expected to become economically much more important in the coming years (Huynh & Nguyen, 1998).

''Alginophytes '' The brown seaweed genus ''Sargassum '' is the largest natural seaweed resource of Vietnam. The annual production in natural ''Sargassum '' beds is estimated to be over 5000 t (dry weight), but the total amount harvested is only 300-500 t/year (dry weight). Annual production of alginate paste and powder from ''Sargassum '' is 15-20 t. These values far from satisfy the needs of the local textile industry, for which alginate has to be imported from India (Huynh & Nguyen, 1998; Van Khuong, 1990).

''The farms '' The area used for ''Gracilaria '' phycoculture is around 350 ha, which is only a fraction of the potential area of more than 10 000 ha thought to be available (Van Khuong, 1990).

''Microalgae '' In Vietnam ''Arthrospira '' and some diatoms in particular are cultured. Of the diatoms ''Chaetoceros '' sp. and ''Skeletonema costatum '' are grown in culture for use in shrimp hatcheries. In 1989 ''Arthrospira '' cultivation was executed in two factories, one of which measured 1000 m2 in surface area. The two factories produced pellets or dried tablets to be used as nutritional supplements for women and children (Van Khuong, 1990).

Algae are all the autotrophic organisms other than plants, a group of 30 000 to 40 000 different and described living organisms (Norton et al., 1996). The total number of undescribed species, however, may exceed known ones by a factor of four to eight. For a long time algae have been considered as primitive plants without a strict tissue differentiation, but only some groups of green algae are related to the "real" plants. The members of the informal group called "algae" are not necessarily related. Some algae are more closely related to bacterial groups (the blue-green algae are in fact ''Cyanobacteria''), while others are more closely related to some Protozoa or to fungus-like organisms than to other algal groups. Algae occur in an incredible variety of life forms, from uni-cellular species to giant kelp which may extend to more than 60 m in length. The algal body is designated as a "thallus" or a "frond". In general, drawing conclusions about algae by analogy with plants, or even with other algal groups, is often fraught with potentially invalid assumptions.

Seaweeds are generally classified into four main groups, largely on the basis of their structure and pigmentation: red algae (division ''Rhodophyta''), brown algae (division ''Phaeophyta''), green algae (division ''Chlorophyta'') and blue-green algae (division ''Cyanophyta'', which is a group of the Prokaryotes). However, colour is only an approximate guide, e.g. red seaweeds show a variety of colours, from pink to purple and black. Botanists use structural features of the seaweeds as an accurate guide to classification.

Blue-green algae and green algae, although also present in salt water, are more commonly associated with freshwater and on land (for example, on tree trunks, in soils, etc.). The largest forms of the green algae, however, occur in the sea. Red and brown algae are usually associated with marine environments, often rocky shores, although some representatives of these groups occur in freshwater. Blue-green algae are ubiquitous members of the soil microflora. Brown seaweeds are particularly abundant in cold and temperate waters and most species of commercial interest grow best in waters below 20°C, usually at or below the intertidal zone (McHugh & Lanier, 1983). Few species are found in tropical regions, of which members of the genera ''Sargassum '' and ''Turbinaria'', however, may be locally dominant and also of commercial interest.

Green seaweeds may become dominant in pools and in the intertidal, especially in eutrophic situations. In tropical regions, the multinucleate genera (''Caulerpa '' J.V. Lamour., ''Halimeda '' J.V. Lamour. and others) especially form important constituents of both seagrass meadows and coral reefs.

Temperature, and particularly water temperature, is the main abiotic factor governing the geographical distribution of seaweeds, although interactions amongst environmental variables are the rule rather than the exception. Many marine seaweeds can not survive seawater temperatures higher than 33-35°C35 °C. Some, however, can tolerate much lower seawater temperatures than occur in their own environment (Lüning, 1990). The edaphic Cyanobacteria generally grow and fix nitrogen optimally between 30-35°C35 °C; thus temperature is not limiting to their growth in the tropics. Several blue-green algae are unique in the microbial world for their ability to simultaneously fix nitrogen in aerobic habitats and carbon by the oxygenic eukaryotic plant mechanism (Metting et al., 1988).

Most marine macroalgae grow optimally at salinities of approximately 30‰, but there are exceptions such as ''Gracilaria '' spp. and the mangrove algae of the red algal genera ''Bostrychia '' Harv. and ''Caloglossa '' J. Agardh. These species often show maximum growth at much lower salinities (Lüning, 1990). In the species of the latter two genera brackish-water ecotypes may also occur; these do not tolerate high salinities (Lüning, 1990). In these euryhaline ecotypes respiration and photosynthesis decline only slightly with decreasing salinity.

Marine agriculture has a relatively short history (approximately 250 years), especially when compared to approximately 8000-10 000 years of terrestrial agriculture. In the years 1970-1972 almost nobody would have predicted any future growth in marine agronomy and South-East Asia was barely covered in handbooks on aquaculture (Bardach et al., 1972; Doty, 1979). ''Gracilaria '' and other algae as food for milkfish were mentioned and the culture of ''Caulerpa '' in the Philippines for the fresh vegetable market was also listed. The advanced experimental culture of ''Eucheuma '' and ''Hypnea '' spp. in the Philippines was then mentioned for the first time (Bardach et al., 1972).

When harvesting wild populations of the microalga ''Arthrospira '' (''Spirulina'') several technical, ecological, and public health issues require serious evaluation before any large-scale venture can be undertaken (Jassby, 1988a).

Seaweeds grown in open lagoons and cultivation where natural populations are supported (as in ''Betaphycus gelatinus '' in Hainan, China) can be considered as a form of extensive field production, although transitions to more intensive forms of cultivation frequently occur.

In tropical regions marine phycoculture for the production of food or of phycocolloids is either by pond culture, by fixed, off-bottom monoline methods or Fragmentation by floating methods, where either rafts or longlines are used. The methods to be preferred depend on hand and growing the species fragments to be cultivated and the local circumstances, as well as on the conceptual framework for marine phycoculture mature plants (Santelices, 1999vegetative propagation). In general Gracilaria spp. and This is the case in cultivation of ''Caulerpa lentillifera are grown in ponds, while Eucheuma, Kappaphycus and several other Gracilaria spp. are cultured attached to monolines, rafts, longlines or off-bottom systems (Luxton, 1993). To account for '' and avoid the possible devastating actions of tropical storms, the areas within about 6° of the equator are considered to be the most favourable for potential seaweed cultivation sites (Doty, 1979)''Kappaphycus''.

Basically, a seaweed farm In tropical regions marine phycoculture for Eucheuma, Kappaphycus the production of food or line-grown Gracilaria comprises networks of lines suspended phycocolloids is either from mangrove stakes and then immersed a short distance below the low-tide levelby pond culture, by fixed, off-bottom monoline methods or from (bamboo) by floating methods, where either raftsor longlines are used. Fixed bottom monoline The methods to be preferred depend on the species to be cultivated and the local circumstances, as well as on the conceptual framework for marine phycoculture (constant depth farmingSantelices, 1999) is an inexpensive . In general ''Gracilaria'' spp. and easy method to establish ''Caulerpa lentillifera'' are grown in ponds, while ''Eucheuma, Kappaphycus'' and maintainseveral other ''Gracilaria'' spp. It needs bi-filament polythene twineare cultured attached to monolines, either No 5 (2.5 mm) rafts, longlines or No 6 off-bottom systems (3.0 mmLuxton, 1993) and mangrove stakes. The "seedlings" are inserted between To account for and avoid the twines to start farming. A plot can be 10 m <x> 10 m in area (two plots possible devastating actions of 5 m <x> 10 m are better)tropical storms, with a maximum of 32 lines, each the areas within about 32 cm apart. The lines 6° of the equator are stretched and tied considered to be the stakes, which are positioned approximately 5 or 10 m apart most favourable for potential seaweed cultivation sites (TronoDoty, 19901979).

Basically, a seaweed farm for ''Eucheuma, Kappaphycus'' or line-grown ''Gracilaria'' comprises networks of lines suspended either from mangrove stakes and then immersed a short distance below the low-tide level, or from (bamboo) rafts. Fixed bottom monoline phycoculture (constant depth farming) is an inexpensive and easy method to establish and maintain. It needs bi-filament polythene twine, either No 5 (2.5 mm) or No 6 (3.0 mm) and mangrove stakes. The "seedlings" are inserted between the twines to start farming. A plot can be 10 m × 10 m in area (two plots of 5 m × 10 m are better), with a maximum of 32 lines, each about 32 cm apart. The lines are stretched and tied to the stakes, which are positioned approximately 5 or 10 m apart (Trono, 1990). Material costs of the floating rafts are higher than those of off-bottom systems (Luxton, 1993). Floating raft monoline phycoculture can be used in places where the cultivated algae can be submerged in 30 cm of seawater at all times. In addition, the raft must be able to withstand the weight of the seaweeds near harvest time. Recommended material for the raft is bamboo measuring 2.5 m <x> × 5.0 m or 5.0 m <x> × 5.0 m. The lines are stretched within the rafts which need to be well anchored.

 - In the second group of intensive cultivation methods, parts of the culture work is undertaken indoors, where microscopic stages of the life cycle of the cultivated algae are handled (Pérez et al., 1992). These techniques are not yet commonly applied in South-East Asia (Trono, 1994). Indoor cultivation methods, however, can also be used in micro-propagation in ''Eucheuma denticulatum '' and ''Kappaphycus alvarezii '' (Dawes et al., 1993). - In the third group of intensive phycoculture methods all actions are on land, and the cultivated algae are grown in tanks. Up to now results with seaweeds in tanks are not very promising, but cultivation of several species of microalgae can be successful and occurs, albeit not frequently, in some countries in South-East Asia.

Carbonation (CO2CO₂-supply) of microalgal cultures is usually required, as well as continuous mixing in all shallow outdoor cultures (Vonshak, 1997). In some cases cultivation of freshwater algae in seawater-based culture media is feasible (Tseng & Xiang, 1993). Continuous flow mixing in shallow channelized ponds, driven by propeller or screw pumps, by air lift pumps or by paddle wheels are methods often chosen, particularly in tropical areas (Oswald, 1988b). However, not all microalgae can be grown adequately in such ponds. A fairly new development is the use of tubular photo-bioreactors (Borowitzka, 1994; Materassi, 1994; Torzillo, 1997). Another possibility is the vertical alveolar panel reactor (VAP), which can also be used in combination with open raceway ponds (Materassi, 1994; Pushparaj et al., 1997; Tredici & Chini Zittellii, 1997). A third alternative is heterotrophic growth of microalgae (Day & Tsavalos, 1996; Gladue & Maxey, 1994; Johns, 1994).

Phycoculture techniques have been successfully applied to several seaweed species. The successful establishment of a large phycoculture industry in the Philippines based on seaweed had its genesis in an agar supply crisis in Japan around 1960. This eventually resulted in the collection of seaweeds in hitherto untapped areas around the world and precipitated appreciation of the fact that inadequate seaweed supplies were hindering the growth of the seaweed colloid manufacturing industry. Seaweed culture offered the best solution to raw material supply problems. Development of marine phycoculture in tropical areas especially developed as a pioneering effort in the Philippines from 1965 onwards. Farming of ''Eucheuma '' species required the identification of suitable sites, identifying the people who would farm and obtaining the stability of return on the investment that would keep the farmers active and industry interested in continuing use of the farm-produced material (Doty, 1979).

The phycoculture of ''Gracilaria '' is now well-established in several South-East Asian countries (McHugh & Lanier, 1983; Trono, 1994). Nevertheless, only species from about 15 genera of seaweeds are at present grown in marine phycoculture (Ohno & Critchley, 1993; Pérez et al., 1992). The indications are, however, that most commercially important seaweeds will be produced by phycoculture before long. In fact, this is largely already the case. Virtually all commercial brown seaweeds, as well as 63% of the commercial red seaweeds and 68% of the commercial green seaweeds are now cultivated (Ohno & Critchley, 1993). For ''Eucheuma '' and ''Kappaphycus '' it is even stated that over 95% of the crop is farmed (Lobban & Harrison, 1994). Of the total 3.9 million t (wet weight) of seaweeds used in 1991 in the seaweed industry, 2.8 million t were provided by phycoculture (Pérez et al., 1992).

After a site has been identified, test planting of the desired species (or, in some species, the desired cultivar) is recommended. For ''Eucheuma, Kappaphycus '' and some ''Gracilaria '' spp. test plots consisting of a few monolines planted with 50-100 test plants each are constructed at different strategic locations in the area. The growth of the test plants must be monitored at weekly intervals and their daily growth rates determined. Areas supporting daily growth rates of 2-5% or higher are potentially good sites. A 2-3 months long monitoring period of growth rate may be enough to start a small family farm, but for commercial farms a year-round monitoring programme is necessary, considering the possibility of problems associated with the seasonality of algal growth.

About 530 000 ha in the world are already occupied by the existing 250 000 seaweed farms, many of which are small family enterprises, which provide employment for approximately 950 000 individuals. These developments augur well for the countries in South-East Asia with phycoculture potential in that the cultivation, harvesting and processing of seaweeds are highly labour-intensive activities (Doty, 1977). The farming of tropical algae is not a periodic activity, rather it is continuous and an alternative means of sustainable employment based on renewable resources. Incentive and free enterprise can be considered to be major factors in farm productivity (Doty, 1987). A large, successful farm could require several people for its proper operation and a smaller farm might be successful even though it may not require all the time of even one person. The cost of production of ''Eucheuma '' and ''Kappaphycus '' is much lower from family farms than from farms with paid employees (Doty, 1987; Trono, 1990). Although large and semi-intensive farms produce the highest yields, they are also the most expensive to maintain. The extensive farms derive higher net profits than the semi-intensive and intensive farms. Small farms generally obtain higher net profit, because of lower total costs (Llana, 1990).

Details of necessary investments and the costs of phycoculture of ''Kappaphycus alvarezii '' are readily available from the literature. These investments include the structures (poles, etc.) to fix the cultivation units, the living quarters for the farmers and the labourers, the drying house, boats, lines and nets and further miscellaneous equipment. The operating costs include mainly labour (for selecting and obtaining "seed", planting, maintenance and weeding, harvesting, drying and washing, packing, baling and shipping) as well as some costs for materials such as polythene ties and freshwater. Overhead costs such as salaries, boat and transportation rentals, communications, representation expenses, repairs, fuel and oil also have to be taken into account. Estimates of farmer returns and future farming costs can also be calculated (Doty, 1986; Llana, 1990; McHugh, 1990).

The utilization of blue-green algae as bio-fertilizers has a long history and is inherently attractive. There is widespread interest in developing technologies for mass culture and their use with crops other than rice. A methodology for accurate, rapid estimation of standing crop or productivity of filamentous blue-green algae (''Cyanobacteria'') does not exist (Metting et al., 1988). Important physical factors influencing growth and nitrogen fixation include light, pH, temperature and cycles of wetting and drying. Soil cyanobacteria grow best under neutral to alkaline conditions. Nutrients and agro-chemicals also influence their activities and growth, in particular the availability of phosphate in the soil. The addition of lime (CaCO3CaCO₃) to rice often stimulates the growth of blue-green algae. The effects of herbicides on growth and N2-fixation by free-living cyanobacteria are variable and differ widely among strains. Many components are inhibitory at high concentrations, but are stimulatory when diluted.

Although algae are never really "planted", most macroalgae need to be attached to a substrate in order to ensure survival. Cultures can be started from vegetative cuttings, spores or propagules, or by bringing young plants in from nurseries. These young plants can be seeded directly onto nets, as is usually the case with ''Porphyra '' cultures, or onto special nursery cord to be later attached to other substrates for further cultivation. The latter method is normally used in the phycoculture of large brown algae (kelps) in non-tropical waters (e.g. ''Laminaria '' spp., ''Undaria '' spp.), but is also used to grow Gracilaria sporelings. In pond cultures of ''Caulerpa '' and ''Gracilaria '' one end of a small bundle of cuttings is often buried into the mud at the bottom of the pond. When algae are to be attached to lines, this has to be done by hand. The cuttings are attached either individually or in small bundles (Trono, 1994).

 For commercial cultivation of microalgae, sufficient amounts of fresh and healthy specimens must be added to the culture medium. Some microalgae can be preserved for many years by cryo-preservation, while cyanobacteria can be immobilized in polyurethane foam or sugar-cane waste to be used as bio- fertilizer (Day et al., 1997; Kannaiyan et al., 1997).

Biotic competition with other algae, with epiphytic organisms, parasites and pathogens and with predators (herbivores) is an important factor determining the success of phycoculture. Weeding and regular pruning are necessary activities in the maintenance of every seaweed farm. If senescence of the stocks is apparent, the lines should be re-stocked with fresh "seed". Decreasing productivity of the stocks is an important problem in farming of tropical seaweeds (Dawes et al., 1993; Trono, 1994). Research on periphyton communities used as food in abalone culture in Pacific Canada has shown that nutrient enrichment usually does not change the competition of these periphyton communities. However, both productivity and protein concentrations increased (Austin et al., 1990). This may also be the case in pool polyculture of milkfish and ''Gracilaria'', which is also mainly based on the food value of the periphyton to the fish.

Monocultures of seaweeds tend to be susceptible to mycoses, bacterial activity and/or viruses. Large parts of a farm can be infected, especially where plants in phycoculture are overcrowded. Literature on diseases of tropical algae is scant, except for "ice-ice", a disease of ''Eucheuma '' and ''Kappaphycus '' (Trono, 1994). This disease is characterized as one of the "malaises" of ''Eucheuma '' cultivation (Doty, 1987). It is probably not a real disease, but a symptom of poor growing conditions (Pringle et al., 1989). "Ice-ice" can be induced by manipulation in laboratory studies, although bacteria most probably accelerate the expression of the "disease" (Correa, 1997; Largo et al., 1995a, 1995b). In species that are only propagated vegetatively, ageing of the stocks may result in reduction of growth and productivity (Dawes et al., 1993; Pérez et al., 1992; Trono, 1994).

Successful maintenance in outdoor mass culture of microalgae requires constant feedback on the state of the culture (Belay et al., 1994; Richmond, 1988). In several microalgae, especially in ''Dunaliella salina '' Teodor. and some strains of ''Arthrospira'', the ability of the algae to withstand high salt concentrations makes extensive open-air cultivation possible (being relatively free of competitors, pathogens and predators) (Borowitzka & Borowitzka, 1988; Tseng & Xiang, 1993).

These differ for the various species in cultivation. Some culture methods allow Eucheuma plants to grow to one kg or more, while other methods allow less full-grown specimens. In addition, the optimum harvest for ''Eucheuma '' and ''Kappaphycus '' varies between different locations, depending on a number of factors including the level of loss from physical damage as thalli increase in size. A short 40-45 days growing period, with the harvesting of immature thalli of less than one kg (wet weight) is still practised in some localities, in the belief that higher yields are obtained by frequent cropping. A longer harvest interval of 50-60 days, however, produces a crop which is usually more suitable for carrageenan production (Luxton, 1993). It has been suggested that higher prices should be paid for thalli with a basal diameter greater than a given size. This may result in a better quality of the seaweeds that are offered for sale (Doty, 1986).

The need for collectors and farmers to improve post-harvest treatment (cleaning, sorting, washing, drying) is crucial to provide products with a consistently high quality (Doty, 1986; Luxton, 1993; McHugh & Lanier, 1983; Trono, 1994). The algae must be well-dried shortly after collection and as rapidly as possible. Dehydration must be sufficient to guarantee preservation of the alga, otherwise anaerobic fermentation will occur, causing high temperatures and even carbonization of the seaweeds during storage in the warehouse. In general, the moisture content is best reduced to about 20% (for ''Eucheuma '' and ''Kappaphycus '' 35%) by natural or artificial drying ("bone-dry"). Commercial seaweeds are often mixed with significant quantities of impurities such as stones, shells, sand, other seaweeds, epiphytes, as well as other products added during gathering, drying and packing (such as land weeds, leaves, wood and plastic). It is important to prevent this type of contamination of the raw material. Contact with freshwater, particularly rain, should be avoided, especially for ''Eucheuma '' and ''Kappaphycus''. Sand causes severe problems during carrageenan extraction due to its abrasive properties (Blakemore, 1990).

In some cases the problem of storage is more difficult to solve. In Gracilaria enzymatic hydrolysis of agar may occur spontaneously, even at a relatively low moisture content. The rates of hydrolysis are variable depending on the species, its origin and conditions. This prevents long-term storage of stocks. Agar in ''Gelidium'', however, can be preserved indefinitely in seaweeds, provided they have been well-treated (Armisén, 1995; Armisén & Galatas, 1987). When the gathered seaweeds are treated with NaOH at an adequate concentration and for the correct duration, destruction of microbial contaminants takes place, perhaps also resulting in the in-activation of hydrolytic enzymes. Sterilization by gamma irradiation, however, often causes loss of gel strength characteristics (Armisén, 1995).

For ''Eucheuma '' and ''Kappaphycus '' it is known that the material is unstable and undergoes degradation during storage above a moisture content of 35%. Above 40% moisture content the carrageenan in the seaweed may not survive transportation to the factory, arriving with characteristics unsuitable for some applications. At 25-35% moisture content seaweeds are relatively stable for periods in excess of 12 months and the thalli are also flexible which is ideal for efficient baling. At 15-25% moisture content Eucheuma thalli are extremely stable, but are too brittle and resist compression or snap during baling. Below 15% moisture content thalli remain stable, but can cause processing problems during carrageenan extraction (Blakemore, 1990). Careful drying and good baling are essential for well-packed seaweeds and lower freight costs (McHugh, 1990).

The technical requirements for the manufacture of seaweed colloids, involving production techniques and expertise of effective marketing, put up certain barriers for producers who wish to enter the trade. However, the production technology for agar and semi-refined carrageenan is not so complex, that the development of the requisite technology is beyond the resources of most countries in South-East Asia (Bixler, 1996; McHugh, 1996; McHugh & Lanier, 1983). When used in combination with other gums different forms of phycocolloids, especially agar, may behave differently in relation to gel strength (Armisén & Galatas, 1987). Due to synergism, mixtures of ''Gelidium '' agar with "locust bean gum" (LBG) produce a more elastic gel; but this is not the case with ''Gracilaria '' agar (Armisén, 1995). Some sources of kappa carrageenan, however, show even greater synergism when mixed with LBG.

Gels of algal polysaccharides are generally made with 0.5-2% polymer per weight. Besides being characterized for gel and sol temperatures, these gels are tested for break force or gel strength and penetration. These parameters are measured on devices aptly called gel testers. These are machines which determine, by various means, the force necessary to break the surface of a gel (i.e. break force, expressed in g/cm2cm²) and the amount of deformation of the surface of the gel at break point (i.e. penetration, expressed in cm) (Lewis et al., 1988).

The discovery (around the year 1658) of the "freeze-thaw" extraction of agar is attributed to Japan (Booth, 1979). Agar extraction is a fairly simple process. Frozen agar gel liberates water as it thaws, profiting from the insolubility of agar in the cold (Armisén, 1995). Alternatively, synaeresis is often applied, especially in agars produced from Gracilaria . The term synaeresis is used in agar technology to describe the process whereby pressure is used to exclude liquid from the gel. Boiling is necessary to dissolve agar in water. The insoluble residue is usually removed by some means of filtration and the liquid, when cooled, forms a gel. Solutions with 1-1.5% agar stiffen to a firm gel when cooled to between 36 - 42°C 42 °C and the gel will not melt below 85-90°C 90 °C (Indergaard & Østgard, 1991). By alternately freezing and thawing the gel several times, the water is removed and a dried "strip" of agar is produced. Agar is sold in strip form, but also in powder form (McHugh, 1996). Details on processing techniques and analysis of physico-chemical properties of agar are widely available (Armisén, 1995; Armisén & Galatas, 1987; McHugh & Lanier, 1983; Montaño & Pagba, 1996). Agar manufacturing has the advantage of being feasible on both small or large scales, with the corresponding capital outlay.

Originally only ''Gelidium '' agar constituted what was considered genuine "agar", assigning the term "agaroids" to the products extracted from other seaweeds. This differentiation is no longer accepted. In 1984 approximately half of all agar produced came from members of the ''Gelidiaceae'', the other half were mainly from ''Gracilaria '' (Armisén & Galatas, 1987). The different seaweeds used as raw material in agar production gave rise to products with differences in their behaviour. For this reason, when agar is mentioned, it is customary to indicate its original raw material as this can influence its application. To describe the product more accurately, it is usual to mention the origin of the seaweeds, since ''Gracilaria '' agar from one area of the world has different properties from ''Gracilaria '' agar from another area.

An increase in agar gel strength was obtained through improvement of the industrial process. Treatments of the seaweeds prior to extraction are very important as they will determine to a high degree the characteristics of the agar obtained. Strong alkaline conditions increase gel strength, especially in ''Gracilaria '' agar (Armisén & Galatas, 1987). The treatment, called sulphate alkaline hydrolysis, must be adapted to the seaweed used, to obtain as much desulphation as possible whilst avoiding the yield losses that this process can cause (Armisén & Galatas, 1987; Villanueva et al., 1997). Other corrective treatments using an alkaline solution eliminate a large quantity of foreign substances. This alkaline treatment uses sodium carbonate and is milder than the alkaline treatment with sodium hydroxide which used to improve gel strength. Pre-treatment with acetic acid, however, may also result in higher agar yields and gel strength (Roleda et al., 1997). 

*Native and natural agar (from ''Gracilaria'') usually can not be classified as bacteriological grade agar, as there is a high content of methoxyl groups and consequently high gelling temperatures (Murano, 1995). Half of the world supply of agar is directly used in food. In Asia there is considerable household consumption of "natural agar", mainly in traditional cooking, which is often marketed in "strips", in bar-like "squares" or in pill form. These are mixtures in which ''Gelidium '' agar dominates, but which can not be used for industrial food agar (Armisén, 1995; Armisén & Galatas, 1987).*Industrial agar is sold world-wide in powder form, as pharmacological grade, biotechnological grade, bacteriological grade and purified agar. The source of these grades is mainly ''Gelidium '' (Armisén, 1995).

*Bacteriological agar (also known as "standard agar") needs strict physical-chemical control and requires the absence of haemolytic substances and bacterial inhibitors. Nevertheless, there are no official general specifications for universal categorization as bacteriological agar (Armisén & Galatas, 1987). It must have a good transparency in sol and gel forms and a consistent gel strength from lot-to-lot. Uses in microbiology are based on special properties: a gelling temperature of 36 ± 1.5°C5 °C, a melting temperature of 87 ± 2.5°C 5 °C and lack of hydrolysis by bacterial exo-enzymes. The above temperatures refer to culture media which contain 10-11 g agar/l. Requirements are a low content of oligomers and proteins (so that these can not form a source of nutrients for microorganisms), a low and regular content of electro-negative groups that could cause differences in diffusion of electro-positive molecules (e.g. antibiotics, nutrients, metabolites), freedom from toxic (bacterial inhibitors) and haemolytic substances that might interfere with normal haemolytic reactions in culture media as well as free from contamination by thermophilic spores (Armisén, 1991). Bacteriological agar, which is the highest agar grade, is prepared from ''Gelidium '' (and ''Pterocladia''), since ''Gracilaria '' and ''Gelidiella '' give agars with gelling temperatures above 41°C41 °C.

Interest in agarose resumed with the search for an electrically neutral polysaccharide suitable for use in electrophoresis and chromatography. Agarose is a derivate of high-quality agar. Its yield can be 70-80% of that of agar. The rather small market for this special product is expanding because of its current use in tissue culture and as a medium for electrophoresis. Demand is expected to remain high due to biotechnological and biochemical needs. A technique for agarose preparation using polyethylene glycol results in a product of good purity (Armisén & Galatas, 1987; Russell et al., 1964). Good commercial agarose is considered to have less than 0.35% sulphate; the pyruvate content is likewise very low (Armisén, 1991). The agar of Philippine ''Gracilariopsis heteroclada '' is a potential source of agarose (Hurtado-Ponce, 1994).

Some bacteria (e.g. ''Cytophaga '' and ''Pseudomonas '' spp.) can produce agar-degrading enzymes (Forro, 1987).

In the Philippines "semi-refined" carrageenan is produced from seaweeds known as Kappaphycus alvarezii. There are many different names and acronyms for this "semi-refined" carrageenan. The method used was introduced in the Philippines by Japanese chemists in 1978 (Llana, 1990). Baskets full of seaweed fronds are immersed and cooked in hot, aqueous potassium hydroxide 8.5% solutions and soaked 5-6 times in freshwater to extract most of the residual alkali. In the production process most of the non-carrageenan matter in the seaweed is dissolved and removed, leaving a solid residue of cellulose and carrageenan, which is dried and milled (Stanley, 1987). The product contains some insoluble material, and thus will not yield clear gels (Neish, 1990). Material handling in the production process is greatly reduced and energy costs are lower than in the case of the full industrial process. Thus production costs are only 25-50% for that of refined carrageenan (McHugh, 1996). Therefore, the price of semi-refined carrageenan is about 50% of that of the refined product (McHugh & Lanier, 1983). With the advantages of being easier and cheaper to manufacture, it is feasible for an entrepreneur to develop the semi-refined carrageenan-processing technology on an experimental scale for a modest capital outlay, and then gradually increase the scale of operation as is done in the Philippines (Anonymous, 1998; Neish, 1990). The process is most effective when using ''Kappaphycus alvarezii'', but with careful control it is also possible to treat ''Eucheuma denticulatum '' and its iota carrageenan. Usually, however, kappa carrageenan is the active component in semi-refined carrageenan.

The quality of carrageenan has been decreasing recently; this is thought to be the result of hybridization with native plants in the farming areas in the Philippines (Lobban & Harrison, 1994). Some bacteria, (e.g. ''Pseudomonas '' sp.) are capable of degrading carrageenans (Forro, 1987).

Usually the quality of Sargassum alginate is thought to be rather poor and not good enough to be used by the textile printing industry (McHugh & Lanier, 1983). Nevertheless, a local alginate producer in Indonesia sells about 100 t of locally produced Sargassum alginate to the textile industry (McHugh, 1996). Comparative research, moreover, suggests that alginate produced from Malaysian ''Sargassum '' is of a high quality (Fasihuddin & Siti, 1994).

Several bacteria, in particular ''Cytophaga '' spp., are capable of degrading alginates, especially under anaerobic conditions (Forro, 1987).

The potential of microalgae as a commercial source of carotenoids has been recognized for some time and extensive research and development of β carotene production using ''Dunaliella salina '' is underway in various parts of the world. After harvesting, the biomass must be processed to extract either glycerol or β carotene (Chen & Chi, 1981; Ruane, 1974). Microalgae are ideally suited as a source of stable isotopically labelled compounds (Apt & Behrens, 1999).

For species grown in phycoculture the possibility arises for cultivation of improved seaweed strains with predictable and better yields. In the ''Kappaphycus alvarezii '' group several cultivars are now known. It might be possible to cross these to provide better yields, although hybridization has not yet succeeded (Trono, 1994). In ''Eucheuma denticulatum '' all crossing experiments have also so far failed (Pérez et al., 1992). References to tropical species are generally lacking in the literature on genetic studies in marine algae (van der Meer & Patwary, 1995). However, the presence of numerous unknown ''Gracilaria '' spp. in particular offers tremendous opportunities for the development of ideal seedstock strains for the agarophytes (Shen & Wu, 1995; Trono, 1994).

Genetic engineering of microalgae has provided promising results, that are not yet fully applicable (Craig et al., 1988). Highly sophisticated molecular systems are being used to include recombinant techniques especially in ''Cyanobacteria '' and some unicellular eukaryotic algae, but these new techniques have not yet contributed directly to a commercial product.

Commercial cultivation of all promising species is not yet possible, especially cultivation of several agar-producing species in the genera ''Gelidiella, Gelidium '' and ''Pterocladia''. In addition, there are erratic results in the production of algae bearing iota carrageenan which still require more attention. Nevertheless, attempts to increase production from natural beds of ''Gelidium'', by increasing the area of rocky substrates, farming it with ropes and rafts, and even cultivating free-floating plants in onshore tanks have met with some success (Lobban & Harrison, 1994; Melo et al., 1990).

The market for phycocolloids largely depends on relatively low prices. Agar, carrageenan and alginate may compete directly with one another in some end-use markets. Specialty gums, such as carrageenans, are sold on the basis of their functionality in specific applications. For this reason carrageenan manufacturers have to devote a substantial portion of their budget to maintaining active applications and technical marketing groups to serve the ever-changing needs of their customers. In that way the carrageenan industry tries to sustain the growth that it has enjoyed for the last 30 years. A survey of market factors (food product innovations, especially different forms of liquid diets and fat substitutes) and the successful application of carrageenan in further-processed poultry and meat has been documented (Bixler, 1996). Phycocolloids compete with gums from flowering plants, such as guar gum and locust bean gum and cheaper cellulose derivates. A variety of agar substitutes have been developed for gel media in microbiology, including alginate and polysaccharides of non-nonalgal algal origin, such as plantgar and gellan. For a number of important food uses, however, no synthetic or other natural gums have been found that can replace phycocolloids on a cost-effective basis. This fact seems to assure the continued viability of the industry. New food applications will require gums which demonstrate pH and temperature stability, salt tolerance and cold stability; these properties are found in seaweed extracts (Guist, 1990).

The potential for culturing carrageenan-bearing seaweeds in countries in South-East Asia is very large. However, expansion in the number of growers has resulted already in an oversupply over-supply of certain forms of carrageenan and may lead only to the spreading of sales among a greater number of producers (at lower prices) as well as to disputes amongst the different carrageenan producers (McHugh & Lanier, 1983).