Gracilaria tenuistipitata (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

var. liui - 1, habit; 2, cross-section of a thallus; 3, longitudinal section of a cystocarp; 4, longitudinal section of the pericarp of a cystocarp; 5, cross-section of a tetrasporophyte with tetrasporangia; 6, cross-section of a male gametophyte with spermatangial conceptacles of the Textorii-type.

Gracilaria tenuistipitata C.F. Chang & B.M. Xia

Protologue: Studia Mar. Sin. 11: 102-105, 161, figs 6, 7 (1976).

Family: Gracilariaceae

Chromosome number: 2n= unknown

Vernacular names

• Philippines: huganot
• Thailand: sarai woon, sarai phom nang.

Origin and geographic distribution

G. tenuistipitata is found in China and in South-East Asia in Indonesia, the Philippines, Thailand and Vietnam.

Uses

In Thailand and Vietnam fresh G. tenuistipitata is commonly used as a vegetable. In both fresh and dried forms it is used in various prepared dishes. It is used for extraction of good quality agar as the gel strength is as good as that of the agar of G. fisheri (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia. In Vietnam, the dried form is used for agar processing for local consumption. It is used as feed in shellfish culture, especially for abalone (Haliotis diversicolor) and as a test organism for toxicity assessment in marine and brackish environments. Other uses are as fertilizer and in traditional medicine.

Production and international trade

In the Philippines G. tenuistipitata is mainly produced from natural stock. The quality of its agar is comparable to that of several commercially cultivated Gracilaria spp. Production of G. tenuistipitata in Thailand often occurs where G. fisheri is produced but it is not in itself a significant crop plant. In Vietnam, 3 Gracilaria species are cultivated: G. verrucosa (Huds.) Papenf. (synonym: G. vermiculophylla (Ohmi) Papenf.) (70%), G. blodgettii Harv. or G. firma C.F. Chang & B.M. Xia (18%) and G. tenuistipitata var. liui C.F. Zhang & B.M. Xia (19%). In 1987, Vietnam produced about 1500 t of dried Gracilaria; in 1993 the quantity reached 2800-3000 t (dry weight). This amount was used for agar processing for local consumption and some was exported to China, Russia and Japan. Other recently published data, however, assessed the cultivation area to be about 1000 ha, and annual production 1500-2000 t (dry weight). The agar extraction industry in South-East Asia using Gracilaria has grown. In 1987, Vietnam produced only 20 t of agar, but by 1993 this figure had risen to about 250 t. In addition to industrial-scale factories, many small-scale and family-scale ones have appeared with highly economic efficiency. However, this industry meets local demands only, being used for confectionery and other foodbased industries. Agar export from Vietnam is experiencing difficulties because of its low quality, especially low gel strength. In Taiwan G. tenuistipitata var. liui is the main taxon in cultivation. This variety grows much faster than var. tenuistipitata.

Properties

Pretreatment with 5% alkali (NaOH) at 90 °C for 3 hours in the extraction of agar gave average mean agar yield of 16.2% in G. tenuistipitata from the Philippines, 12.7% from Thailand and 19.7% from Vietnam. Gel strength determination using 1.5% agar was 304-473 g/cm² for natural agar from the Philippines and 606-726 g/cm² for agar after alkali treatment, and 338 g/cm² and 768 g/cm², respectively for those from Vietnam and Thailand. The agar from G. tenuistipitata possesses regular agarobiose-repeating units with partial methylation at the 6-position of the D-galactosyl residues.

Description

• Thalli filiform, 20-40 cm tall, rarely up to 1 m, arising from a small disklike holdfast, fleshy red, cartilaginous, adhering well to paper on drying; branching simple or moderately alternate near the base, with only few orders of branches; branches becoming like main axis, terete throughout, very slender and gradually tapering towards the base, above expanding to 0.5-1.5 mm in diameter, 1-2 times branched.
• Medulla broad, with large thick-walled cells, 225-390 μm in diameter, cell walls 13-16 μm thick in the centre, towards the surface considerably smaller; cortex of 1-2 layers of rounded cells 10-20 μm in diameter, with cuticle 10-30 μm thick.
• Tetrasporangia scattered among surface layer of the frond, round in surface view, 30-33 μm in diameter, ovoid or oblong in transverse-section, 30-46 μm × 18-30 μm, cruciately divided, surrounded by modified cortical cells.
• Spermatangia scattered over surface of branch in small shallow, well-defined depressions 10-23 μm in diameter, separated by modified, elongated cortical cells.
• Cystocarps prominently protruding, 830-950 μm tall, globose, markedly rostrate, constricted at the base.
• Gonimoblast composed of large parenchymatous cells, connected with pericarp by very rare nutritive filaments; pericarp 72-102 μm thick, consisting of 8-11 layers of cells, with outer cells of 1-2 layers elliptical to roundish, with obscured cell wall, with the innermost few layers of cells roundish to horizontally oval with distinct cell wall.
• Carpospores round or ovoid, 33-49 μm in diameter.

Growth and development

In pond culture the mean annual growth rate of G. tenuistipitata in Taiwan is 2.4% per day, with a maximum of 3.3% per day under favourable conditions. The average daily growth rate of pond-cultivated G. tenuistipitata in the Philippines is 6.2%, which is lower than the daily growth rates of other Philippine Gracilaria species that were grown in experimental rope cultures.

Other botanical information

G. tenuistipitata var. liui differs from var. tenuistipitata, having slender thalli bearing numerous, delicate, short to long flagelliform lateral branchlets, frequently only 0.3 mm in diameter, branching mostly from percurrent axes.

Ecology

G. tenuistipitata var. tenuistipitata grows on gravel and shells in the sublittoral region of lower salinity, while var. liui frequently occurs naturally in fish ponds and shallow intertidal areas on muddy substrate, sometimes exposed during low tide. It can survive temperatures as high as 34 °C (best growth is between 25 and 28 °C), while var. liui can grow under salinities of 4-47‰ and thus can often be found in areas with an intrusion of freshwater. It grows best in salinities of 15-22‰. When cultivated the thalli are usually detached and without holdfasts.

Propagation and planting

Propagation of G. tenuistipitata in cultures is usually by vegetative fragmentation. Thalli are split or individual branches broadcast in shallow ponds or grown in rectangular net cages.

Phycoculture

Pond culture of G. tenuistipitata has been successfully practised in China, Vietnam, Thailand, Taiwan and the Philippines. This alga is considered to have potential for culture or farming in brackish water ponds due to its tolerance for a wide range of salinity and turbid waters. Of the two varieties, var. liui is far better than var. tenuistipitata, because it grows faster and is adaptable to brackish water. The ponds used for culture should be about one hectare and have a water management system. The water depth is kept at 30-70 cm and the salinity range from 15-25‰. The water should be changed once every two weeks. In the Philippines G. tenuistipitata and Gracilariopsis heteroclada C.F. Zhang & B.M. Xia (usually as Gracilaria heteroclada C.F. Zhang & B.M. Xia) are the most suitable algae for pond culture. The minimum water depth in the pond should be kept at 20-30 cm. If monoculture is practised, fertilization of the pond water is required. Three kg of urea is needed per hectare of pond every week, and the water should be changed once every two weeks. About 4500 kg/ha of seaweed seedlings are needed to produce 12 t. G. tenuistipitata was identified as a potential alga for culture or farming based on its ecological characteristics, wide availability in many areas of the country and its high yield of good-quality agar.

In Thailand, pond culture is common in Pattani Province. G. tenuistipitata collected from the wild is separated into small tufts or fragments and broadcast uniformly on the bottom of the pond with a muddy-sandy substrate, at a density of 500 g/m² of seedlings. The water is maintained at a suitable depth of about 50-70 cm. Daily water exchange by tidal flow ensures adequate nutrient supply for the seaweed and maintains the optimum water temperature in the pond.

In Vietnam and Taiwan, monoculture and polyculture in ponds with shrimp (often grassshrimp, Panaeus monodon), fish (often milkfish, Chanos chanos) and crabs (often Scylla serrata) have been carried out. The optimum range of salinity is 12-24‰ and the optimum temperature range is 20-30 °C. Seed stock is 500-600 g/m². The water level is maintained at a depth of 30-45 cm, increasing to 40-50 cm during sunny months, and a maximum level of 60 cm deep. Water exchange occurs naturally at high and low tides. Addition of nitrogen is effective until a total N concentration of 4.3 μM is reached, and fertilizers significantly improve crop growth under combined conditions of low salinity (< 10‰) and high temperatures (above 32 °C).

Harvesting

Under optimum conditions, G. tenuistipitata may be harvested 2-3 months after seeding in Thailand. In Vietnam the alga is harvested by hand or by rake when the density rises above 1 kg/m².

Yield

In Thailand yield of G. tenuistipitata is 1.5-2.5 kg/m² (wet weight) or more when harvested after 2-3 months.

Handling after harvest

When used for agar production G. tenuistipitata has to be sun-dried, free of extraneous matter, washed in freshwater and redried before being packed for storage or transport. When sold as feed for abalone culture the fresh product can be sold directly to abalone farms, without any form of processing, thus reducing labour costs.

Chemical analyses reveal higher 3,6-anhydrogalactose and lower sulphate contents in alkali-modified agar than in natural agar. There is a considerable decrease in agar yield upon alkali treatment, but the agar quality is certainly better, with a two-fold increase in gel strength.

Prospects

G. tenuistipitata has a good-quality agar useful for food and other specialized agar products. Its production for the agar industry for local consumption and the international market seems to have a bright future. Labour shortage, however, may result in direct selling of the fresh seaweed for feed in cultures of marine animals.

Literature

• Ajisaka, T. & Chiang, Y.-M., 1993. Recent status of Gracilaria cultivation in Taiwan. Hydrobiologia 260/261: 335-338.
• Chang, C.F. & Xia, B.M., 1976. Studies on Chinese species of Gracilaria. Studia Marina Sinica 11: 91-163.
• Haglund, K., Björklund, M., Gunnare, S., Sandberg, A., Olander, U. & Pédersen, M., 1996. New method for toxicity assessment in marine and brackish environments using the macroalga Gracilaria tenuistipitata (Gracilariales, Rhodophyta). Hydrobiologia 326/327: 317-325.
• Huynh, Q.N. & Nguyen, H.D., 1998. The seaweed resources of Vietnam. In: Critchley, A.T. & Ohno, M. (Editors): Seaweed resources of the world. Japan International Cooperation Agency, Yokosuka, Japan. pp. 62-69.
• Khuong, D.V., 1995. Studies on taxonomy, ecology and processing technology of economic Gracilaria species. A country status report for final workshop of the Regional Study on the Taxonomy, Ecology and Processing of Commercially Important Red Seaweeds, Bangkok, Thailand. 14 pp.
• Montaño, N.E., Villanueva, R.D. & Romero, J.B., 1999. Chemical characteristics and gelling properties of agar from two Philippine Gracilaria spp. (Gracilariales, Rhodophyta). Journal of Applied Phycology 11: 27-34.

Sources of illustration

var. liui: Zhang, J. & Xia, B., 1988. On two new Gracilaria (Gigartinales, Rhodophyta) from South China. In: Abbott, I.A. (Editor): Taxonomy of economic seaweeds. Taxonomy of economic seaweeds with reference to some Pacific and Caribbean species. Vol. 2. California Sea Grant College Program, La Jolla, United States. Fig. 1, p. 132 (habit), figs. 3-6, p. 133 (all other drawings). Redrawn and adapted by P. Verheij-Hayes.

Authors

• K. Lewmanomont & S.-M. Phang