Variations in lipid and fatty acid content in relation to acetyl CoA carboxylase in the marine prymnesiophyte isochrysis galbana

Lipid and fatty acid content was determined in Isochrysis galbana cultures grown under various environmental conditions in steady state continuous cultures. Lipid and fatty acid accumulation was observed under severe nitrogen limiting conditions. The rate of in situ lipid synthesis, determined from ¹⁴C bicarbonate incorporation into lipid fraction, decreased under nitrogen limited growth (μ<0.72day^-1), concomitant with a reduction in the in vitro activity of the enzyme acetyl CoA carboxylase. Cellular lipid and fatty acid content remained fairly constant over a wide range of irradiance levels. The rate of lipid synthesis, however, increased as irra-diance level was elevated to a maximal value at a light intensity of 150μmol quanta m^-2s^-1 and slightly decreased at a higher photon flux. The in vitro activity of ACCase roughly followed the same pattern of lipid synthesis in response to light intensity. The relative abundance of acetyl CoA carboxylase significantly decreased in nitrogen limited cultures grown at low dilution rates (μ<0.72day^-1). A fairly good linear correlation was measured between the cellular content of ACCase and the enzyme activity in cultures grown under nitrogen limiting conditions. Furthermore, in nitrogen limited cultures, the cellular fatty acid content was linearly related to the cell capacity to produce malonyl CoA, the end product of ACCase and the building block in fatty acid synthesis.