In the present work we focused on three species of microalgaeisolated from Tunisian coast in which bivalves reproduced naturally: Tetraselmis sp. (Tetra), Picochlorum sp. (Pico) and Chaetoceros sp. (Chaeto). Class, family and genus were identified and their potential utilization in aquaculture examined. Growth and biochemical composition have been accordingly studied. Thus, the three isolated microalgae exhibited adequate growth as well as protein and lipid contents for potential hatchery uses. Relative protein content, ranged from 50 to 53%, 39 to 45% and 42 to 45% respectively for Chateoceros sp., Tetraselmis sp. and Picochlorum sp. in exponential and stationary phases. Lipids content reached 15 to 18%, 12% and 15 to 20% respectively for Chateoceros sp., Tetraselmis sp. and Picochlorum sp. Relative fatty acid composition of Chateoceros sp. was characterized by the presence of EPA (20:5n-3) with 10 to 19% and low amount of DHA (22:6n-3) with 1 to 2%. Low EPA content was also recorded in Tetraselmis sp (5%); whereas fatty acid profile of Picochlorum sp was dominated by linolenic and linoleic acids. Sterol analysis showed that Picochlorum sp was composed of94% of stigmasterol; whereas the major components was campeserol (71-73%) and 24-methylene (25-27%) in Tetraselmis sp. More sterol diversity was found in Chaetoceros sp. in its exponential growth phase with 24-methylene (51%), isofucosterol (15%) and cholesterol (15%). Molecular identification was carried out on Picochlorum sp; the 18S rRNA gene phylogeny revealed that such strain was a sequence for Picochlorum.
