Development of Culture of Microalgae as Live Food Source for Larvae Culture in Brackishwater Hatcheries

Abstract

Three standard microalgae species viz., Nannochloropsis oculata, Tetraselmis chui and Chaetoceros muelleri, commercially used in aquaculture in many countries, were cultured in small scale batch culture with initial inoculation density of 44 x 104 cells/ml for 28 days to determine the effect of salinitics (05 ppt to 30 ppt) on the growth under laboratory condition. The BFRI (Bangladesh Fisheries Research Institute) microalgae culture nutrient medium was used as the culture medium. N. oculata showed maximum cell growth (345 x 104 cells/ml) at 20 ppt salinity. T. chui showed better growth (458 x 104 cells/ml) at 30 ppt but failed to grow at salinities from 5 to 10 ppt. C. muelleri did not tolerate a salinity of 5 ppt but grew well at the salinity range of 20-30 ppt and the highest cell density (395 x 104 cells/ml) of this alga was found at 25 ppt. N. oculata was the most curyhaline species of microalgae tested here and can tolerate a wide range (5-30 ppt) of salinities. This species is one of the most important species in aquaculture, so the findings from this study have positive implications for brackishwater hatcheries in Bangladesh. The growth of three microalgae species mentioned above was also investigated using three different inorganic nutrient media: (i) Modified Guillard's f/2 medium; (ii) Rix Mix medium and (iii) BFRI medium. Each microalgae species was cultured at 25 ppt salinity for 24 days in small scale batch system with initial inoculation density of 17x 104 cells /ml in the three media with three replications in each. N. oculata cultured in modified Guillard's f/2 medium showed superior growth with a mean peak density of 221 x 104 cells/ml than in Rix Mix medium (141 x 104 cell/ml) and BFRI mcdium (47 x 104 cells/ml) on the 16th day of culture at stationary phase. Considering the increase in cell density at the 20th day of culture in Rix Mix medium, C. muelleri was significantly (P<0.05) highest than in other two media. N. oculata cultured in BFRI medium resulted in the poorest growth with a mean peak increase in density of 84 x 104 cell/ml at the 12th day of culture. However, with an increase in cell density, growth of T. chui (182 x104cells/ml) was significantly (P<0.05) higher in BFRI medium than in modified Guillard's f/2 medium. N. oculata and C. muelleri can be grown very well in both the modified Guillard's f/2 medium and Rix Mix medium. Better growth of T. chui can be obtained while culturing either in BFRI and Rix Mix medium. These three nutrient media used in the present study may be useful for microalgae species culture for all the target zooplankton, fish and crustacean larvae in marine and brackishwater hatcheries. Unicellular algae, Chlorella sp., were collected from the local brackishwater environment and then isolated by the combination of dilution, single cell collection by capillary glass pipette and agar media used techniques. Pure stock culture protocols of the isolated Chlorella sp. was developed to maintain stock culture in the microalgal culture laboratory by using modified Guillard's f/2 medium. The maximum cell density of Chlorella sp. reached at 16 X 10" cells /ml under pure culture condition within two weeks of culture period. Small scale culture of Chlorella sp. at 5-30 ppt salinities showed that it could grow equally well in all the salinities tested with significantly highest cell density (2530 X 104 cells/ml) at 20 ppt salinity, which might have the potentiality for development of medium scale culture and mass culture system of Chlorella sp. for different finfish/crustacean larvae rearing in hatchery.