Gracilaria (PROSEA)

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Plant Resources of South-East Asia
Introduction
List of species


Gracilaria arcuata - 1, habit; 2, longitudinal section of a cystocarp. G. gigas - 3, habit. G. manilaensis - 4, cross-section of a thallus with deeply pot-shaped spermatangial conceptacula (Verrucosa-type); 5, detail of a surface view of a mature male thallus. G. textorii - 6, cross-section of shallow cup-shaped spermatangial conceptacula (Textorii-type); 7, detail of a surface view of a mature male thallus; 8, cross-section of a vegetative part of a thallus; 9, longitudinal section of a cystocarp; 10, detail of a cross-section of a tetrasporangial thallus. G. edulis - 11, cross-section of joint, multiple, deeply pot-shaped spermatangial conceptacles (Polycavernosa-type).

Gracilaria Grev.

Protologue: Alg. brit.: 54 (1830).
Family: Gracilariaceae
Chromosome number: x= 24; G. bursa-pastoris, G. coronopifolia, G. foliifera(all non-Asiatic), G. manilaensis (Philippine): 2n= most probably 48; G. "verrucosa" (European): 2n= 64 (probably mis-identified Gracilariopsis); G. arcuata, G. firma, G. salicornia (from the Philippines), G. "verrucosa" (European and Japanese): x= 24.
Nuclear genome sizes of Gracilaria spp. (G. arcuata, G. edulis, G. firma and G. salicornia) from the Philippines are in agreement with the assessment of 2 n = 48, except for G. eucheumatoides, of which the nuclear genome size comes close to that of representatives of the genus Gracilariopsis.

Major species and synonyms

  • Gracilaria bursa-pastoris (S.G. Gmelin) P.C. Silva - see under G. chouae.
  • Gracilaria changii (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia - see separate article.
  • Gracilaria chouae C.F. Zhang & B.M. Xia, in I.A. Abbott, Taxon. econ. seaweeds 3: 196-203, figs 9-14, 16-18, 22, 23, 25 (1992), synonyms: G. bursa-pastoris auct. non (S.G. Gmelin) P.C. Silva (1952, South-East and East Asian material), G. parvispora I.A. Abbott (1985, South-East and East Asian material).
  • Gracilaria coronopifolia J. Agardh, Spec. gen. ord. alg. vol. 2, part 2(2): 592 (1852); probably the material recorded under this name in South-East Asia is another, yet undescribed Gracilaria sp.
  • Gracilaria fisheri (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia - see separate article.
  • Gracilaria textorii (Suringar) De Toni, Phyceae jap. nov.: 27 (1895), synonym: Sphaerococcus textorii Suringar (1867).
  • Gracilaria vermiculophylla (Ohmi) Papenf., Phycos 5: 101 (1967), synonym: G. asiatica C.F. Zhang & B.M. Xia (1985), probably including G. asiatica var. zhengii C.F. Zhang & B.M. Xia (1992).

Vernacular names

For all Gracilaria and Gracilariopsis :

  • Philippines: gulaman, caocaoyan, gargararao.

Origin and geographic distribution

Gracilaria can be found on almost all coasts of the world. It comprises probably more than 150 species, of which about 45 have been recorded from South-East Asia. For separate countries the approximate number of recorded species of Gracilaria is as follows: Vietnam 13, Thailand 14, Malaysia and Singapore 8, Indonesia 18, the Philippines 27 and less for Burma (Myanmar) and Papua New Guinea. In the older literature names often used are G. verrucosa or G. confervoides (L.) Grev. These names have also been used to include several Gracilaria spp. of more limited distribution. Of those species for which there is no separate treatment, G. arcuata seems to be an eastern species, not occurring in Burma (Myanmar), Thailand or Malaysia.

It is difficult to summarize the distribution of G. chouae, G. coronopifolia and G. foliifera: due to taxonomic and nomenclatural uncertainties fully reliable data are not yet available. A species known as G. coronopifolia, however, is probably the most common one of the genus in the Philippines.

G. gigas has been recorded from northern Vietnam, the Philippines (Sulu Islands) and eastern Indonesia, as well as from China and Japan.

G. manilaensis is only known to occur in the Philippines.

G. textorii has been recorded from many Asian countries; in South-East Asia it is listed for most countries, but not for Papua New Guinea.

G. vermiculophylla mainly occurs in China, Japan and Korea, but has also been recorded from Vietnam.

Uses

Most Gracilaria are used both for food and as raw material for the agar industry. Several species, especially G. coronopifolia, G. edulis, G. eucheumatoides, G. firma, G. salicornia, G. tenuistipitata and G. vermiculophylla (as G. asiatica) are often eaten raw or blanched as a vegetable (salad) or for the preparation of home-made jelly or sweet soup. Many of these algae are also used as feed for cultivated fish and invertebrates, as medicine (laxative, pulmonary complaints), as insect repellent, as fertilizer, in biogas production and for tertiary treatment of sewage. A seaweed-based pesticide and fertilizer is prepared from Gracilaria sp. It is very effective in preventing insect infestation, which is said to be due to the odour and taste of the crop plants sprayed with the pesticide. It is also mentioned that spraying hardens the stems and leaves of the crop plants, making them unpalatable for the insects. This might be due to the polysaccharide content of the seaweeds, which coats the plants. Additional insecticidal properties may be based on the production of hydrogen sulphide and other sulphides by anaerobic fermentation of the sulphate content of seaweed extracts. Crops sprayed with this seaweed as a fertilizer have shown increases in yield of up to 10-15%.

The world's first source of agar, from the middle of the seventeenth century, was Gelidium sp. from Japan. However, demand for agar exceeded the supply of these algae. Since then Gracilaria has played an important role in the production of this phycocolloid. The development of production processes through alkaline hydrolysis of sulphates has allowed a good quality food agar to be obtained from Gracilaria. The term "agaroids" can be applied to Gracilaria agars produced without alkaline hydrolysis. These agaroids have greater sulphate content and much less gel strength.

Production and international trade

Although several Gracilaria are collected and/or cultivated in the different countries of South-East Asia, in many reports only the genus name (Gracilaria) is provided. Data are then only available for the genus as a whole, and often include representatives of Gracilariopsis.

In 1991 3450 t of "Gracilaria" (dry weight) was harvested in Indonesia, which at the time was 13.5% of the world's Gracilaria production used for the agar industry. In 1993 5500 t of Gracilaria was farmed in Indonesia, especially in South Sulawesi (2000 farmers).

In the mid-1990s the Gracilaria harvest in Vietnam, mainly from cultivated stock, yielded 1500-2000 t (dry weight) per year. A large part is exported mainly to Russia, Japan and China and a smaller portion is used domestically. The total cultivated area of Gracilaria in Vietnam is about 1000 ha. In Malaysia only G. changii is cultivated on an experimental scale. In the Philippines production is mainly from natural populations, although Gracilariopsis heteroclada C.F. Zhang & B.M. Xia (often as Gracilaria heteroclada C.F. Zhang & B.M. Xia or as Gracilariopsis bailinae C.F. Zhang & B.M. Xia) is cultivated locally.

Gracilaria imports to Japan during the period 1984-1993 show a steady decline from over 10 000 t of dry seaweed in 1985 to about 3500 t in 1993, due to new agar production facilities in Gracilaria-producing countries. Exports recorded from South-East Asian countries to Japan (all dry weight) include: Indonesia 70 t (1984), 1120 t (1989) and 420-815 t/y (1990-1993), Malaysia 20 t (1989), Singapore 1 t (1993), the Philippines 1470 t (1984), 950 t (1989) and 285-850 t/y (1990-1993), Thailand 3 t(1984), and Vietnam 15 t (1984), 150 t (1987) and 60 t (1991). Cost and Freight (CF) prices for these seaweeds varied considerably between averages of 1.23-2.21 US$/t, although exports from countries in South-East Asia usually earned less, due to the greater tendency to become hydrolyzed during transport and storage.

Worldwide up to 5000 t of agar are processed annually from 25 000-30 000 t Gracilarioid algae which contain 15-20% agar on a dry weight basis. Of this, about 50% is harvested mainly from wild-growing algae in the cool-temperate waters of Chile and Argentina, and the remainder comes from cultures, mainly from fish pond culture in China, Taiwan and South-East Asia. For some years, more food grade agar has been obtained worldwide from Gracilaria than from any other agarophyte. In Indonesia total agar production increased from 150 t in 1984 to 450 t in 1993 and 980 t in 1994, produced in 11 agar-processing plants. In the Philippines there is a market for agar strips ("gulaman" bars), that are made by the extraction of Gracilaria seaweed. The actual demand is at least 30 t annually. Thailand produces about 3540 t per year of agar from locally collected Gracilaria. Local agar production in Vietnam, mainly from cultivated G. tenuistipitata and G. vermiculophylla, is estimated to be between 80 and 100 t/y and is mostly used for domestic purposes.

Properties

Fatty acids in Gracilaria may have longer chains of carbon atoms then those occurring in most other seaweeds, although palmitic acid (16:0) is still the predominant saturated fatty acid also in Gracilaria seaweeds. Gracilaria agar has the advantage of greater synaeresis than Gelidium agar. Agar produced using properly applied synaeresis has a greater purity than that produced using the freezing-drying technique. This is due to the fact that the synaerized gel is superior to that obtained by draining the defrosted gel. Impurities remain in the dry agar in quantities directly proportional to the quantity of water remaining in the gel. These quantities of water are lower in well-synaerized gels.

Gelling temperatures of Philippine Gracilaria are ranging from 38-44°C, and melting temperatures from 86.6-94.6°C, while gel strength varies between 188 and 876 g/cm2. Publications containing comparative data on worldwide agar yield and gel strength of Gracilaria often lack data from South-East Asia.

Most agaroids extracted directly from Gracilaria show a higher gelling temperature than Gelidium agar because of methoxylation, but also have a higher sulphate content. They also have a much lower gel strength. The higher sulphate content of Gracilarioids, however, can be greatly reduced by alkaline hydrolysis, which converts the 1-galactose-6-sulphate to 3,6-anhydro-L-galactose. In this way, the transformation of agaroid into genuine agar can be done by treating the seaweed before dissolving the polysaccharide contents.

From the chemist's point of view, seven kinds of agar from Gracilarioid algae can be recognized, and these can be separated into three major categories:

  • high gel-strength agars with much non-ionic agarose,
  • lesser gel-strength agars with ionic agarose,
  • low gel-strength agars that have a high sulphate content.

Agar of G. arcuata is of low quality. It produces a viscous solution that forms a soft gel (200 g/cm2) with a high gelling point of 60 °C. This G. arcuata agar consists of alternating 3-linked 6-0-methyl-β-D-galactopyranosyl and 4-linked 3,6-anhydro-2-0-methyl-α-L-galactopyranosyl units.

The agar melting points, gel strengths and ionic nature of the agarose are associated with the amounts of pyruvate, sulphate and methoxyl groups.

For Indonesian G. coronopifolia, gel strength and viscosity of the agar is lower than those for G. edulis and G. fisheri.

Aqueous extracts of G. coronopifolia revealed high gibberellin-like activity in laboratory bioassays, and also considerable auxin-like and cytokinin-like activity. Similar activity was measured for G. arcuata, while these values were much less in extracts of G. eucheumatoides and G. salicornia.

Extracts of some Gracilaria show antibiotic activity against a number of pathogenic bacteria and fungi.

Description

  • Plants usually bushy, arising from a small discoid base; frond cylindrical, compressed or flattened, fleshy to cartilaginous; branching dichotomous, irregular or proliferous; branches basically constricted or not.
  • Medulla parenchymatous, consisting of large cells; cortex narrow, small-celled and assimilative.
  • Life cycle triphasic, diplo-haplontic, isomorphic and dioecious.
  • Tetrasporangia just below the surface of the frond.
  • Spermatangia cut off from surface cells arranged in sori or in shallow or deeper conceptacles.
  • Cystocarps hemispherical, with large cellular basal placenta tissue (gonimoblast) and prominent superficial pericarp composed of several layers of radiating cells; discharging through a pore.


G. arcuata.

  • Plant 6-10 cm tall, dark red; frond cylindrical, 3-4 mm wide, fleshy, robust, main axes gradually curved; branching irregularly pinnate, dichotomous or secund; branches and branchlets of almost the same diameter, attenuated towards the apex, branch bases constricted.
  • Medullary cells 340-820 μm in diameter, cell walls 3-8 μm thick, cell transition to small cortex cells (6.7-13.5 μm × 5.5-8.8 μm) abrupt.
  • Tetrasporangia not known.
  • Spermatangia in deeply pot-shaped conceptacles, covering the entire inner surface of the conceptacle.
  • Cystocarps globose, rostrate; pericarp 250-280 μm thick.

G. chouae.

  • Plant 15-20(-40) cm tall, light red, erect, solitary or caespitose, bushy, arising from a small discoid base; frond cylindrical or slightly compressed, with fleshy texture; branching dichotomous, alternate or unilateral; branches brittle when fresh, 2-3 mm in diameter, branch bases constricted.
  • Medullary cells 232-600 μm in diameter, cell walls 6.6-8.3 μm thick, cell transition to small cortex cells abrupt; cells of inner cortex 23-33 μm in diameter, outer cortex cells 10-17 μm × 7-10 μm.
  • Tetrasporangia cruciate, up to 45 μm × 24 μm.
  • Spermatangia covering the floor of depressed shallow conceptacles, 30-53 μm deep and 30-43 μm in diameter.
  • Cystocarps conical, rounded, prominently protruding, 700-900 μm × 700-900(-1200) μm, slightly rostrate or non-rostrate; pericarp 100-191 μm thick, cells arranged in two distinct layers.

G. coronopifolia.

  • Plant 4-20 cm tall, purplish-red to green-brown, cartilaginous to succulent, with a small discoid holdfast; frond umbrella-shaped or forming an entangled mass; axes cylindrical, not articulated; branching dichotomous and anastomosing; branches without constricted bases, 0.5-3 mm in diameter, tertiary branches often much thicker than main axes, apices often bifurcate.
  • Medullary cells 183-470(-1000) μm in diameter, cell walls 6-9 μm thick, cell transition to small cortex cells (6.7-10.8 μm × 6.8-8 μm) gradual or abrupt.
  • Tetrasporophytes often more robust than gametophytes.
  • Tetrasporangia cruciate, spherical to oval, 20-23 μm × 30-40 μm.
  • Spermatangia covering entire inner surface of deeply pot-shaped narrow-mouthed conceptacles, 33-60 μm deep.
  • Cystocarps conspicuous, rostrate at ostiole, constricted at the base, 1000-1600 μm in diameter; pericarp 100-150 μm thick, with undifferentiated thick-walled cells.

G. foliifera.

  • Plant more than 5 cm tall, rigid; frond flattened, regularly dichotomously branched with entire margins; branches 2-15 mm wide.
  • Tetrasporangia not known.
  • Spermatangia in deeply pot-shaped conceptacles, covering entire inner surface of the conceptacle.
  • Cystocarps with traversing filaments.

G. gigas.

  • Plant more than 30 cm tall, robust, coarse, cartilaginous, with disc-shaped holdfast; frond cylindrical; branching at short intervals, profusely or more scarcely irregular; branches 4-7 mm in diameter, thick, succulent, branch bases of only the long branches constricted.
  • Medullary cells 560-1138 μm × 437-910 μm, cell walls 8-13 μm thick, cell transition to small cortex cells (5.5-13.5(-18) μm × 5.5-9.5 μm) abrupt; gland cells ovoid, 15-25 μm × 20-32 μm.
  • Tetrasporangia cruciate, ovoid to oblong, 20-40 μm × 23-70 μm.
  • Spermatangia covering the floor of depressed shallow conceptacles, 21-35(-40) μm deep, 26-40 μm wide.
  • Cystocarps protruding when ripe, hemispherical, 1000-1500 μm in diameter, rostrate, slightly constricted; pericarp 100-145 μm thick, composed of somewhat compressed cells.
  • G. manilaensis .
  • Plant up to 60 cm or taller, purplish-red to greenish, caespitose; frond cylindrical, main axes up to 1.5 mm thick, fleshy to somewhat cartilaginous, not articulated; branching profuse, alternate or secund; branches similar to main axes, branch bases sharply constricted.
  • Medullary cells polygonal, up to 570 μm in diameter, cell transition to small cortex cells (7.2-12.8 μm × 8.8-13.6 μm) abrupt.
  • Tetrasporophytes often more robust than gametophytes.
  • Tetrasporangia regularly cruciate, 35-40 μm × 24-27 μm.
  • Spermatangia covering entire inner surface of deeply pot-shaped conceptacles, up to 71 μm deep, up to 50 μm wide.
  • Cystocarps globose, up to 1000 μm in diameter.

G. textorii.

  • Plant 5-20 cm tall, dull or brownish-red to somewhat yellowish-red, coriaceous to membranous, caespitose; frond flattened, with cylindrical stipe, irregularly dichotomous or flabellate-cuneate below with round axillae, segments 1-2 mm wide, 500-800(-1000) μm thick, occasionally undulate, becoming slender in upper parts, margins entire or proliferous, apices blunt, bifurcate, or rather ligulate, fleshy to somewhat cartilaginous, not articulated; branching profuse, alternate or secund; branches similar to main axes, branch bases sharply constricted.
  • Medullary cells 200-310 μm × 150-270 μm, cell transition to small outermost cortex cells (9.5-13.5(-16) μm × 6.5-11 μm) abrupt; hairs present.
  • Tetrasporangia borne on both surfaces of almost entire frond, cruciate, 40-50 μm × 23-30 μm.
  • Spermatangia covering the floor of shallow, cup-like conceptacles, 20-30 μm deep.
  • Cystocarps globose, constricted at base, slightly rostrate, 1800 μm × 2000 μm.

G. vermiculophylla.

  • Plant 20-60 cm tall, purplish-brown to dark brown, occasionally greenish to yellowish; frond cylindrical, main axes 1-3 mm thick, cartilaginous, occasionally vermiform, not articulated; branching profuse, alternate or secund, with 3-4 orders of branches, last branching order tending toward slender unilateral laterals, branch bases not constricted.
  • Medullary cells up to 400-490 μm × 270-365 μm, cell transition to small cortex cells (9-16 μm × 4.5-6 μm) abrupt.
  • Tetrasporangia cruciate or tetrahedral, 49-66 μm × 29-40 μm.
  • Spermatangia covering entire inner surface of deeply pot-shaped conceptacles, up to 100-150 μm deep, up to 60-100 μm wide.
  • Cystocarps triangular when young, becoming broader with age, slightly rostrate, constricted at base, 1500 μm × 1800 μm in greatest dimensions.

Growth and development

Occasionally both tetrasporangia and gametangia are found on the same thallus in Gracilaria. Explanations include in situ germination of tetraspores, coalescence of spores or developing discs (formation of chimaeras), mitotic recombination, a mutation or initial failure of cell walls to form during development of tetraspores. Gametophytes and their parts may be smaller than tetrasporophytes. In general, diploids are often favoured in survival and growth rates.

Seasonal variation of biomass of wild populations of G. manilaensis has been studied in a coastal area in Iloilo, western Visayas (the Philippines) and compared with other algae. This alga was only frequent during three months during the dry season (November-May), occurring in quantities of 8.9-35.7 g/m2. Growth rates of several Gracilaria have been measured in both an outdoor cultivation tank with flowing seawater and in an indoor closed recirculation system (aquatron). In material from Manila Bay, the Philippines (as G. verrucosa) maximum daily growth rates (24.4%) occurred in July at seawater temperatures of 25-26°C. At higher or lower temperatures growth was lower but still considerable. The mean daily growth rate for this alga in these outdoor tanks was 22.7 ± 0.1%, which was much higher than that for other tropical Gracilaria. Growth rates in the aquatron were much lower. In other experiments maximum daily growth rates for G. manilaensis (4.5 ± 0.4%) occurred at 25 °C, while the alga grew less well at all other temperatures between 23-30 °C. During cultivation experiments in the intertidal zone and in semi-enclosed ponds in the Philippines, however, daily growth rates of 1.6-5.5% were recorded for G. manilaensis (as G. verrucosa).

Other botanical information

Because of the economic interest in its products, viz. phycocolloids, the study of Gracilarioid algae has spread rapidly throughout the world, resulting in numerous proposals for taxonomic and nomenclatural change. There has not yet been a monographic revision of Gracilaria. Its species are notably difficult in their taxonomy owing to poorly understood species limits, considerable variation in morphological features selected for classification, large number of taxa mostly studied only in a narrow geographic range and misapplication of species names due to lack of reference to type specimens. Gracilaria was established by Greville at which time it contained four species, while no type was designated. Schmitz lectotypified the genus with G. confervoides (L.) Grev., based on Fucus confervoides L., a later homonym of Fucus verrucosus Huds. Papenfuss advocated that the earliest correct name was Fucus verrucosus Huds. and made the combination Gracilaria verrucosa (Huds.) Papenf. Different lines of investigation have been followed, resulting in proposals on delimitation of taxa and in the search for new specific characters and characters that may be used to break up this large and highly variable genus into subgenera and add segregate genera. Names of genera that have been separated from Gracilaria include Gracilariopsis E.Y. Dawson, Hydropuntia Mont. and Polycavernosa C.F. Chang & B.M. Xia. In South-East Asia, however, most described differences between these genera are considered to be too technical for general acceptance, resulting in proposals to retain all species in the single genus Gracilaria. Nevertheless, the separation of the genera Gracilaria and Gracilariopsis is nowadays often followed, as is done in the present volume. The names Polycavernosa and Hydropuntia, however, which are full synonyms, are considered as included in the genus Gracilaria. On the basis of the diagnostic value of the characters of spermatangial structures, several types are established, resulting in distinguishing four subgenera:

  • The Chorda-type, with continuously superficial spermatangia: subgenus Gracilariella H. Yamam. According to some authors, this subgenus includes the genus Gracilariopsis, although in the present volume this is considered as a separate genus.
  • The Polycavernosa-type, with the occurrence of spermatangia in multiple cavities, borne near the periphery of the thallus: subgenus Hydropuntia (Mont.) C.K. Tseng & B.M. Xia.
  • The Textorii-type, with the spermatangia forming shallow cup-shaped conceptacles: subgenus Textoriella H. Yamam.
  • The "Verrucosa"-type, with the spermatangia forming deep pot-shaped conceptacles: the type subgenus Gracilaria.

Recent taxonomic research on type material has made it clear that the original material of Fucus verrucosus Huds. belongs to the separate genus Gracilariopsis.

The species G. manilaensis is similar to G. blodgettii, the occurrence and taxonomic position of which is still uncertain in South-East Asia. Frond length and width, as well as the number of branches of G. blodgettii are different, however G. changii is also rather similar and probably closely related to "G. blodgettii". The same holds for G. vermiculophylla, but this species has been shown to be incompatible in reciprocal crosses.

Material identified as G. bursa-pastoris from Vietnam is in most cases re-identified as Gracilariopsis heteroclada C.F. Zhang & B.M. Xia and material identified as G. coronopifolia from Vietnam is often G. edulis.

Ecology

Most Gracilaria grow abundantly in the intertidal zone, on rocky, sandy and muddy bottoms. These algae are often attached to small stones or shells, but also on larger rocks. Some Gracilaria, however, including G. firma, G. tenuistipitata and G. vermiculophylla can also grow abundantly when unattached in brackish pools and lagoons. These unattached algae are often especially suitable for cultivation. G. manilaensis occurs in sandy-muddy to coralline bottoms in shallow coves, well protected by surrounding islets and is permanently submerged. It is often associated with Caulerpa (green algae), such as Caulerpa racemosa (Forssk.) J. Agardh var. peltata (J.V. Lamour.) Eubank and C. taxifolia (Vahl) C. Agardh. It disappears at the beginning of the wet season, when heavy rainfall and storms start to occur.

Propagation and planting

In Gracilaria phycoculture the usual method of propagation is by vegetative fragmentation, although propagation by spores has been described especially for G. coronopifolia and G. chouae (as G. parvispora). Thalli are chopped or split and bunches of branches or individual branches are used for planting (bottom-stocking) or broadcasting or for cultivation on ropes, either in fixed off-bottom frames, on floating long lines or on rafts. Gracilarioids usually regenerate easily when cut, a fortunate characteristic for the purpose of mass cultivation.

Selection of "seedlings" is one of the most important factors for successful seaweed farming. Fresh, healthy and strong branches must be chosen and cut with a sharp stainless knife. Then the branches have to be washed with clean water before they are used. Stock seedlings can be maintained in a net pen or a floating cage. Initial planting for bottom monoline culture can be done on a beach or a platform near the selected site where the lines are to be placed. About 16 "seedlings" can be inserted in one 5 metre line. This operation should be done with the utmost speed, in order to prevent the algae from drying out. For pond culture cuttings are broadcast directly onto the muddy or sandy bottom of the impoundments.

Success in the farming of Gracilaria is highly dependent on the selection of an appropriate site. In open sea or bays the site should be protected from strong currents and wind. Areas with heavy freshwater runoff are to be avoided, but brackish water with good water flow can be used for pond culture. Ponds used for shrimp culture are usually also suitable for Gracilaria farming. Polyculture with shrimps, milkfish and/or crabs has been frequently mentioned to be successful. The ground should be firm and stable enough to permit easy installation of stakes. The most favourable factor in selecting a site is the availability of areas with natural Gracilaria stock. However, the absence of Gracilaria growth is not necessarily a negative sign. Before setting up a farm in a non-tested location, a test plant of 2.5 m × 2.5 m or a raft should be set up to determine the feasibility of the area chosen. When line cultures are used in areas with considerable tidal currents, the lines should be set to run parallel to these currents. These lines and their anchorage must be checked regularly and drift seaweeds that become entangled should be removed carefully.

Phycoculture

Gracilaria cultivation can be carried out in tanks or raceways, excavated ponds with pumped water, the sea or semi-enclosed ponds and in suspended, spray, or drip culture.

Commercial phycoculture in South-East Asia is mainly done in the sea or semi-enclosed ponds.

Ponds for Gracilaria cultivation are usually rather small because thalli tend to be blown into concentrated areas in larger ponds. Wind-breaks perpendicular to the prevailing wind direction and short bamboo sticks (30-40 cm), anchored in the soft bottom substrate of the pond can help to prevent such concentrations. Water changes made every 2-3 days can help regulate salinity, mineral nutrient supplies and water temperature. Urea or ammonium sulphate fertilizer may be added at a rate of 3 kg/ha. Pig or chicken manure can be added in much higher quantities: 120-180 kg/ha. The pH of the water should be in the range of 7.0-8.0, preferably near 8.0. In tropical mangrove areas, where ponds are often built, the organic content of the mud used or exposed in the construction is initially high, which usually makes the pH too low. In time, the pH rises and the water quality may stabilize; only then will Gracilaria thrive. The depth of the pond water is usually kept at 30-40 cm, but at higher air temperatures, the pond depth may be increased to 50-60 cm.

In Vietnam especially G. tenuistipitata and G. vermiculophylla (as G. asiatica) are grown, mainly in brackish water lagoons and man-made ponds. They can grow in a wide range of salinities (5-22‰), but the best production is obtained within the salinity range of 15-22‰ and a temperature range of 25-28 °C. In conditions of low salinity (below 10‰) and high temperatures (32-34 °C) fertilizers have been shown to improve crop growth significantly. The growth of Gracilaria can, however, be drastically reduced or even ceased due to very low salinities during the rainy season.

Diseases and pests

Contamination by agarolytic bacteria in warm water Gracilaria occurs easily, the most important being Bacillus cereus. This is, however, not the only cause of agar hydrolysis; the seaweeds' own agarolytic enzymes probably also cause enzymatic degradation. "Ice-ice", referring to crop decay in Eucheuma and Kappaphycus spp., also occurs in Gracilarioid algae under conditions of low light, low water motion and low inorganic micronutrients.

Grazing by fish, sea urchins and molluscs can cause problems, as can sediment accumulation and epiphytes. In outdoor tank growth experiments Gracilaria is often heavily grazed by isopods.

Harvesting

Because of the seasonal occurrence of most of the Gracilarioid agarophytes in the coastal areas of South-East Asia, continuous harvesting of natural populations of Gracilaria will result in a loss of productivity. Proper management of the natural stock by keeping to appropriate harvesting time, frequency and rules on quantities to be removed will improve the productivity of each species. Harvesting of natural stock should not be more than 50-75% of the available biomass, in order to allow enough "seedlings" to regenerate for the next growing period.

Both in rope-farmed stock and in pond culture of unattached plants it is necessary to allow sufficient material for regeneration. In rope-farmed cultures this can be done by cutting thalli a few cm from the rope, in ponds by selective gathering or by special cultivation of "seed" material.

When harvesting in ponds, 1/3-1/2 of the total biomass can be removed every 30-35 days in the best season for perennial species, and every 45 days in other months. For rope-farmed material, a growing period of 45-50 days is required for the first harvest. The individual harvested plants in rope cultivation should weigh approximately 500 g and about 100 g should be left on the line for continued growth. Successive harvesting may be done after about 35-40 days.

Yield

An average yield of approximately 410 kg (wet weight) of seaweed can be expected after 45-50 days, from each 10 m × 10 m plot of line-grown Gracilaria, or 41 t/ha (wet weight) in pond culture. For Gracilaria wet to dry weight ratios vary between 8:1 and 12:1, resulting in a calculated production of about 4 t/ha (dry weight) every 45-50 days. In natural populations maximum production is probably 5t/ha/y (dry weight). Standing crops of most tropical Gracilarioid agarophytes yield up to 2 kg/m2 of seaweed. This is considerably lower than for Gracilaria in temperate areas, which can reach up 7 kg/m2. In general, a lower quality of agar (low gel strength, low 3,6-anhydro-L-galactose content, much sulphate and 4-0-methyl-L-galactose) is obtained from tropical pond-grown Gracilaria than from wild-growing algae.

The quality and yield of agar from natural populations of G. manilaensis show monthly differences. In May, when average monthly biomass is highest, the agar yield (about 20% of dry weight) is lower than in the other months (about 25% of dry weight), but gel strength is distinctly higher (about 350 g/cm2 in May, 80-150 g/cm2 in April and June). In May the agar gelling temperature is also higher for G. manilaensis than in the other months (about 42 °C and 38-40 °C, respectively), while the melting temperature of its agar is lower in May than during the other months (78.5 °C and 80-83 °C, respectively). Data on sulphate content of G. manilaensis from the Philippines are only available for May and June (about 16 μg/mg and 24,5 μg/mg, respectively).

The quality of the agar produced from G. manilaensis is similar to that of G. changii, but G. manilaensis gives a higher agar yield. In the Philippines, however, the quality of the agar produced from Gracilariopsis heteroclada is better. Moreover, the latter species occurs year-round whereas the Gracilaria spp. are seasonal in occurrence.

Handling after harvest

Harvested Gracilaria should be cleaned on site and transported to a drying area. Before drying, the contaminants on the harvested thalli must be removed. Drying platforms with floorings made of straw or bamboo mat are best for obtaining good quality products. Rain water must be avoided. These seaweeds should never be dried directly on sand or soil. Once dried, they should be packed in bags, stored in a dry place and sold and transported as soon as possible. Bone-dry Gracilaria contains 18% of water. When used as a raw material for agar production all species of Gracilaria, unlike Gelidium spp., have to be processed without much delay and cannot be stored for use during years of lower availability. The agar contained in Gracilaria has a strong tendency to become hydrolyzed during storage, even under favourable conditions. This state of hydrolysis can totally ruin it as an industrial raw material. The agar content of Gracilaria from warm waters is likely to decrease within a few months, even when the algae are dried and stored under adequate conditions.

These agarophytes can be preserved for longer after collection by treating them with a sufficient concentration of NaOH at temperatures between 50 and 90°C, but marked losses in yield may occur. Sterilization by γ-irradiation may result in loss of agar characteristics, particularly gel strength. The duration of the extraction period after 2% NaOH treatment of dried Gracilaria samples has considerable influence on agar yield and rheological properties of the extracted agar. Each species has an optimum length of the extraction period, during which agar with good rheological properties can be extracted. These rheological properties of agar gels are difficult to evaluate, however, especially because there is a lack of uniformity of measurement.

Genetic resources

Cultures of Gracilarioids are not always stable. Different morphological strains isolated from a single Gracilaria plant may show differences in their agar characteristics. So far very little genetic selection in Gracilaria has been practised.

Prospects

Gracilaria has now practically replaced Gelidium as the most important source of agar in the world. However, the short seasonal occurrence of several species (G. changii and G. manilaensis in the Philippines, G. fisheri in Thailand) makes these species unreliable sources of raw material for the agar industry, which requires a sufficient supply year-round. Indonesia has a growing agar market and agar-extraction industry, and demands already exceed the present supply. Research and development are needed to find the best Gracilaria spp. to grow. Optimum conditions for pond culture also require attention. Gracilaria cultivation in the Philippines and Malaysia may stimulate the development of agar strip production in the region. Gracilaria grown in culture in Thailand may also be used for that purpose, while part of the production of the seaweed can be exported to Indonesia.

The necessity of performing alkaline hydrolysis to obtain good quality agar for food use can cause considerable pollution in the outflows of the treatment installation. To lower the amount of waste, much research effort is being devoted to induce the transformation of 1-galactose-6-sulphate into 3,6-anhydro-L-galactose during the life of the seaweed.

Agarose is a derivate of high quality agar. Its yield can be 70-80% of that of the 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 increased biotechnological and biochemical needs.

Gracilaria production for hydrocolloids means producing thousands of dry tonnes per year economically, in order for it to be cost competitive in the raw materials world market. Gracilaria farming is particularly attractive in low-labour-cost areas. Nevertheless, it is reported that most Indonesian Gracilaria are suitable material for agar extraction and that enough raw material is available to develop more agar extraction plants in Indonesia. Many Gracilaria are believed to form a potential source of methane.

Literature

  • Abbott, I.A., 1995. A decade of species of Gracilaria (sensu latu). In: Abbott, I.A. (Editor): Taxonomy of economic seaweeds 5. pp. 185-195.
  • Armisén, R., 1995. World-wide use and importance of Gracilaria. Journal of Applied Phycology 7: 231-243.
  • Chirapart, A. & Ohno, M., 1993. Growth in tank culture of species of Gracilaria from the Southeast Asian waters. Botanica Marina 36: 9-13.
  • Critchley, A.T., 1993. Gracilaria (Rhodophyta, Gracilariales): an economically important agarophyte. In: Ohno, M. & Critchley, A.T. (Editors): Seaweed cultivation and marine ranching. First edition. Japan International Cooperation Agency, Yokusuka, Japan. pp. 89-112.
  • Kain (Jones), J.M. & Destombe, C., 1995. A review of the life history, reproduction and phenology of Gracilaria. Journal of Applied Phycology 7: 269-281.
  • Kapraun, D.F., Lopez-Bautista, J., Trono, G.C. & Bird, K.T., 1996. Quantification and characterization of nuclear genomes in commercial red seaweeds (Gracilariales) from the Philippines. Journal of Applied Phycology 8: 125-130.
  • Pondevida, H.B. & Hurtado-Ponce, A.Q., 1996. Assessment of some agarophytes from the coastal areas of Iloilo, Philippines. I & II. Botanica Marina 39: 117-122 & 123-127.
  • Tseng, C.K. & Xia, B.-M., 1999. On the Gracilaria in the Western Pacific and Southeastern Asia region. Botanica Marina 42: 209-217.
  • Uy, W.H., Balanay, M., Dagapioso, D. & Ologuin, M., 1992. Studies on the culture of Gracilaria coronopifolia J. Agardh from carpospores. In: Calumpong, H.P. & Meñez, E.P. (Editors): Proceedings of the second RP-USA Phycology symposium/workshop. Cebu and Dumaguete, The Philippines. pp. 169-180.
  • Yamamoto, H., Terada, R. & Muraoka, D., 1999. On so-called Gracilaria coronopifolia from the Philippines and Japan. In: Abbott, I.A. (Editor): Taxonomy of economic seaweeds 7. pp. 89-97.

Sources of illustration

Chang, C.F. & Xia, B.M., 1963. Polycavernosa, a new genus of the Gracilariaceae. Studia Marina Sinica 3: Plate 1, p. 127 (spermatangia of G. edulis - named Polycavernosa fastigiata); Chang, C.F. & Xia, B.M., 1964. A comparative study of Gracilaria foliifera (Forssk.) Børgs. and Gracilaria textorii (Suring.) De Toni. Acta Botanica Sinica 12: Plate 1 (G. textorii: spermatangia, sections of cystocarp, thallus and tetrasporangia); Xia, B. & Zhang, J., 1999. Flora algarum marinarum sinicarum, vol. 2, Rhodophyta, 5. Academiae Sinicae Edita, Beijing, China. Fig. 15, p. 26 (G. arcuata, habit, cystocarp), Fig. 13, p. 53 (habit G. gigas); Yamamoto, H. & Trono, G.C., 1994. Two new species of Gracilaria from the Philippines. In: Abbott, I.A. (Editor): Taxonomy of economic seaweeds with reference to some Pacific and Caribbean species. Vol. 4. California Sea Grant College Program, La Jolla, United States. Figs. 7 & 8, p. 98 (spermatangia of G. manilaensis). Redrawn and adapted by P. Verheij-Hayes.

Authors

  • W.F. Prud'homme van Reine
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