The lipid and fatty acid composition of Porphyridium cruentum was determined as a function of light intensity, temperature, pH, and salinity. In cultures cultivated at the optimal temperature under non-limiting light conditions, eicosapentaenoic acid was the main polyunsaturated fatty acid. When growth rate was reduced by decreased light intensity, increased cell concentration, suboptimal temperature, suboptimal pH, or increased salinity, the content of eicosapentaenoic acid decreased and that of arachidonic acid increased, the latter becoming the major polyunsaturated fatty acid.

The red microalga Porphyridium cruentum is a potential source for pharmaceutically valuable fatty acids eicosapentaenoic acid and arachidonic acid. The conditions promoting a high content of either fatty acid were investigated. The fatty acid composition was effected by cell concentration. In outdoor cultures maintained at a relatively low cell concentration, eicosapentaenoic was the main polyunsaturated fatty acid. At a higher cell concentration, arachidonic acid was dominant. The maximal daily temperature exerted a similar (although less intensive) effect on fatty acid composition. In winter, eicosapentaenoic acid constituted up to 50% of the fatty acids and up to 2.5% of the biomass dry weight but its output rate was only 0.12 g/m2·d. In summer, the content of eicosapentaenoic acid was only 1.3% but the output rate increased to 0.28 g/m2·d resulting from the elevated biomass yield which increased to 24 g/m2·d (AFDW). The highest arachidonic acid content was obtained in summer at high temperatures and cell concentrations reaching 30% of the fatty acids and up to 1.4% of the dry weight.