The amino-acid and sugar composition of 16 species of microalgae used in mariculture

The biochemical compositions of 16 species of microalgae commonly used in mariculture were determined. The species examined were six diatoms [Chaetoceros calcitrans (Paulsen) Takano, Chaetoceros gracilis Schutt, Nitzchia closterium (Ehrenb.) W. Smith, Phaeodactylum tricornutum Bohlin, Skeletonema costatum (Greville) Cleve, Thalassiosira pseudonana (Hustedt, clone 3H) Hasle and Heimdal], four prymnesiophytes [Isochrysis galbana Parke, Isochrysis aff. galbana Parke (T-iso), Pavlova lutheri (Droop) Green, Pavlova salina (N. Carter) Green], two prasinophytes [Tetraselmis chui Butcher, Tetraselmis suecica (Kylin) Butcher], two chlorophytes [Dunaliella tertiolecta Butcher, Nannochloris atomus Butcher], one eustigmatophyte [Nannochloropsis oculata (Droop) Green] and one cryptophyte [Chroomonas salina (Wislouch) Butcher]. The microalgae were grown under defined conditions and harvested during the log phase of growth. Levels of protein, lipid and carbohydrate were quite variable between species. In particular, N. atomus contained almost twice the proportion of carbohydrate compared to all other species. The amino-acid compositions of the microalgae were quite similar, although T. chui and T. suecica were richer in arginine, while N. atomus had high levels of proline. The amino-acid compositions were compared with those of oyster Crassostrea gigas Thunberg larvae to assess the protein quality of the microalgae. The levels of the essential amino-acids in the microalgae were either similar to or greater than the levels of the same amino acids in oyster larvae, thus indicating a high protein quality for all microalgal species. Therefore, in relation to differences in the nutritional value of the microalgae, the amino-acid composition may not be critical. The sugar composition of the polysaccharides from microalgae did however show major differences between species and between classes. Glucose was the principal sugar, while rhamnose, fucose, ribose, arabinose, xylose, mannose, and galactose were detected in varying proportions. The variations in the sugar composition could contribute to differences in the nutritional value of some species, since animals digest polysaccharides of different composition at different rates.