An important role of the sea ice microbial community (SIMCO), with respect to higher trophic levels, is the biosynthesis of essential dietary polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid [20:5ω3 (EPA)] and docosahexaenoic acid [22:6ω3 (DHA)]. This study investigates the changes in fatty acid composition of several sea ice diatom communities during the 1989/90 summer bloom in McMurdo Sound, together with the incidence of PUFA production by psychrophilic sea ice bactena isolated from Prydz Bay, Eastern Antarctica. Both the diatom communities and bacterial strains isolated produced high levels of essential PUFA. Bacterial production of PUFA was widespread (27% of isolates) and in certain strains the percentage level of EPA approached the level found in the diatom communities. The role of microalgae as the sole source of PUFA in natural and mariculture food chains is under review.