Gracilariopsis heteroclada (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

1, habit; 2, cross-section of a thallus; 3, detail of a cross-section of a thallus; 4, non-median longitudinal section of a cystocarp; 5, longitudinal section of the pericarp of a cystocarp; 6, cross-section of a tetrasporophyte with tetrasporangia; 7, cross-section of a male gametophyte with spermatangial sori of the Chorda-type.

Gracilariopsis heteroclada C.F. Zhang & B.M. Xia

Protologue: Pacific Sci. 45: 22 (1991).

Family: Gracilariaceae

Chromosome number: 2n= most probably 64; assessment of nuclear genome sizes of Gracilarioid algae from the Philippines suggests that the size of the nuclear genome is larger for Gracilariopsis heteroclada than for most Gracilaria spp. except Gracilaria eucheumatoides Harv.

Synonyms

• Gracilaria heteroclada C.F. Zhang & B.M. Xia (1988), non G. heteroclada (Mont.) Feldmann & Feldm.-Maz. (1943),
• Gracilariopsis bailinae C.F. Zhang & B.M. Xia (1991),
• Gracilaria bailinae (C.F. Zhang & B.M. Xia) C.F. Zhang & B.M. Xia (1994, nom. inval.).

Origin and geographic distribution

G. heteroclada seems to be restricted to southern Asia, with records from China and in South-East Asia from the Philippines and Vietnam.

Uses

In the Philippines agar of G. heteroclada (usually as Gracilaria heteroclada) is produced commercially. It is strong enough to be used as industrial and bacteriological agar and thus as a potential source of agarose. In Vietnam G. heteroclada is eaten raw and used in cooking, for the preparation of sour vegetables, jellies, and sweet soups. In the Philippines the alga is also used for human food, animal feed, fertilizer and in traditional medicine.

Production and international trade

In the Philippines and in central and southern Vietnam G. heteroclada (as Gracilaria heteroclada) is grown in cultures, which are used for commercial agar production. The total area for G. heteroclada in Vietnam is about 100 ha, with an annual production of 150-200 t (dry weight).

Properties

Gelling temperature of the agar from G. heteroclada and its melting temperature have been determined as 38-46°C and 83-97°C respectively, while gel strength of this agar was assessed to be 810-905 g/cm². Thus gel strength of G. heteroclada is comparable to that of industrial agar, but lower than the specified gel strength of agarose (> 900 g/cm²). Seasonality in gel strength has been observed in the Philippines, with low values during the late part of the dry season (April-May, with high air temperatures) and high ones during the early part of the dry season (October-March). The source of the material in the Philippines (Manila Bay, Iloilo in western Visayas) and the quality of the pretreatment significantly influence the rheological properties of the produced agar. Except for gel strength, which on average is higher than in most Philippine Gracilaria spp., the other values fit within the variation of data found for the Philippine Gracilarioid algae. In Vietnam, G. heteroclada (as G. bailinae) is considered to be one of the best raw materials for agar processing, due to its high yield of high quality agar.

Description

• Plant usually erect, bushy, 10-50(-70) cm tall, solitary to caespitose, arising from a small discoid base.
• Fronds with a long, filiform, fleshy, brittle, cylindrical axis, 1-3 mm in diameter; branching with 2-4 orders of branches, irregular to almost alternate; branchlets (= branches of the third order) short, succulent, brittle, breaking easily, spinose and non-constricted at their bases.
• Medullary cells 200-530 μm in diameter; cell walls 8-10 μm thick, with abrupt cell transition to small cortex cells, 7-10 μm × 4-7 μm.
• Tetrasporangia scattered in the cortex, ovoid or oblong, 33-36 μm × 16-26 μm in surface view, 20-30 μm × 16-26 μm in longitudinal section.
• Spermatangia superficial, continuously over the frond surface.
• Cystocarps prominently protruding or subconical, 500-780 μm × 830-1000 μm, rostrate or not, non-constricted at base; pericarp 76-100 μm thick.

Growth and development

Seasonal variation in biomass of wild populations of G. heteroclada has been studied in coastal areas in Iloilo (the Philippines) and compared with those of two Gracilaria spp. (G. changii (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia and G. manilaensis H. Yamam. & Trono). In contrast to the latter two Gracilaria, which were only present during the dry season, the populations of G. heteroclada were present year-round, although its abundance and properties were not uniform during the year. G. heteroclada seaweed was especially abundant in Estancia (northern Iloilo) in the wet season, but more abundant in the dry season in Zarraga (mid Iloilo). The standing crop in Estancia was 13.4-119.3 g/m², with highest values from May to September, whereas in Zarraga it was 6.9-43 g/m², with highest values from November to March. In all cases the algae behaved as a perennial. Plants temporarily exposed to air usually decreased in productivity. In the rainy season G. heteroclada disappeared from the littoral zone. Regeneration of G. heteroclada after the rainy season is by vegetative fragmentation. Tetrasporangia and cystocarps occur seasonally; in Iloilo, tetrasporangia are dominant in April and May (late dry season, with high water temperatures) and cystocarps in January (after the wet season).

When cultured in outdoor tanks, the highest growth rates were reached (0.3-1.1%/d), especially at salinity levels of 15-25‰ and with addition of 0.5 mM urea. Growth in indoor tanks is always somewhat lower (maximum 0.2-0.7%/d). In biculture with milkfish, G. heteroclada can reach mean daily growth rates of 4.7 ± 1.6% (maximum 5.9%) in indoor laboratory culture and of 3.7 ± 0.4% (maximum 4.7%) in outdoor ponds.

Ecology

G. heteroclada grows in muddy to sandy-muddy environments with changing and occasionally rather low salinities, where it forms almost mono-specific beds, which are sometimes exposed to air during the lowest low tides. It also occurs, however, attached to hard substrates in the intertidal region, often in a sexually reproducing form. However, in the lagoon habitats the alga is mostly found in an unattached, vegetatively reproductive state.

Usually, its biomass does not show any significant relationship with salinity (15-35‰), temperature (27-32 °C), pH (7.3-8.7), turbidity (39-92%) and nitrogen (0-0.26 ppm NO₂-N and 0-0.44 ppm NH₃-N), but higher phosphate levels are positively correlated with higher biomass. G. heteroclada grows well in Vietnam in conditions where salinity ranges from 15-30‰ and water temperatures of 24-33 °C prevail. At low salinities (< 10‰) and high water temperatures (> 34 °C) growth is inhibited.

Propagation and planting

In phycoculture of G. heteroclada in Vietnam "seedlings" or vegetative propagules (cuttings) are normally broadcast onto the muddy or sandy bottoms of the culture ponds. In the Philippines bunches of thalli are stacked into the sandy-muddy substrate of ponds.

Phycoculture

In some shrimp cultivation ponds in central Vietnam G. heteroclada and black tiger shrimp (Palaemon monodon) are cultivated in rotation in the same pond (6 months each). Both crops benefit significantly from this approach. In bi-culture with milkfish (Chanos chanos) in the Philippines performance of both organisms is significantly better than in monoculture.

Diseases and pests

G. heteroclada is excessively grazed by herbivorous fish including siganids, mullets and puffers. Epiphytes such as Cladophora spp. were observed to cling to Gracilariopsis spp. The presence of these epiphytes can result in significantly lower yields of G. heteroclada.

Harvesting

Harvesting techniques used for Gracilaria are also used for G. heteroclada, i.e. mainly hand-collecting.

Yield

The biomass of G. heteroclada is not more than 2 kg/m² as expected for tropical regions. This is considerably lower than for Gracilarioid algae in temperate areas. The yield and quality of agar from natural populations of G. heteroclada varies considerably during the year. For areas in the Philippines the neutral (i.e. non-alkaline) agar yield is 6-23% of dry weight. In July, when agar yield is often optimal, gel strength is mostly lower than in other months. Gelling temperature and melting temperature change irregularly during the year, and the sulphate percentage especially varies throughout the year for Zarraga (Iloilo, the Philippines) from 4.8-29.0 μg/mg.

Handling after harvest

Plants of G. heteroclada grown in tanks kept at 26.5-31 °C and salinities of 8-32‰, when treated with different concentrations of aqueous NaOH produce different agar yields. The extracted agars vary considerably in gel strength and gelling and melting temperatures. Of the statistically significant differences in gel strength, dynamic gelling and melting temperatures observed for the various treatments, both the strongest and the weakest gel were extracted after pre-treatment with 3% NaOH. The difference was in the salinity: plants grown in water with a salinity of 24‰ produced maximum gel strength, and those grown at a salinity of 32‰ produced agar with a very low gel strength. Generally, the best quality agar can be extracted after cold aqueous NaOH 4% pretreatment for 7 days.

Prospects

The quality of agar of G. heteroclada is better than that of several Gracilaria spp. in South-East Asia. This together with its year-round occurrence, make G. heteroclada a promising seaweed for commercial purposes.

Literature

• Alcantara, L.B., Calumpong, H.P., Martinez-Goss, M.R., Meñez, E.G. & Israel, A., 1999. Comparison of the performance of the agarophyte, Gracilariopsis bailinae, and the milkfish, Chanos chanos, in mono- and biculture. Hydrobiologia 398/399: 443-453.
• Hurtado-Ponce, A.Q., 1994. Agar production from Gracilariopsis heteroclada (Gracilariales, Rhodophyta) grown at different salinity levels. Botanica Marina 37: 97-100.
• Hurtado-Ponce, A.Q. & Laio, L.-M., 1998. The genus Gracilariopsis (Rhodophyta, Gracilariales) in the Philippines: morphological and taxonomic confirmations. The Philippine Scientist 35: 141-151.
• Hurtado-Ponce, A.Q. & Pondevida, H.B., 1997. The interactive effect of some environmental factors on the growth, agar yield and quality of Gracilariopsis bailinae (Zhang et Xia) cultured in tanks. Botanica Marina 40: 217-223.
• Luhan, Ma. R.J., 1996. Biomass and reproductive states of Gracilaria heteroclada Zhang et Xia collected from Jaro, Central Philippines. Botanica Marina 39: 207-211.
• 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.
• Rabanal, S.F., Azanza, R. & Hurtado-Ponce, A.Q., 1997. Laboratory manipulation of Gracilariopsis bailinae Zhang & Xia (Gracilariales, Rhodophyta). Botanica Marina 40: 547-556.

Sources of illustration

Rabanal, S.F., Azanza, R. & Hurtado-Ponce, A.Q., 1997. Laboratory manipulation of Gracilariopsis bailinae Zhang et Xia (Gracilariales, Rhodophyta). Botanica Marina 40: fig. 13, p. 553 (habit); Zhang, J. & Xia, B., 1988. On two new Gracilaria (Gigartinales, Rhodophyta) from South China. In: Abbott, I.A. (Editor): Taxonomy of economic seaweeds with reference to some Pacific and Caribbean species. Vol. 2. California Sea Grant College Program, La Jolla, United States. Figs. 11, 12, 13, 15, 17, p. 135 (as Gracilaria heteroclada) (all other drawings). Redrawn and adapted by P. Verheij-Hayes.

Authors

• W.F. Prud'homme van Reine