RADIOLABELING STUDIES OF LIPIDS AND FATTY-ACIDS IN NANNOCHLOROPSIS (EUSTIGMATOPHYCEAE), AN OLEAGINOUS MARINE ALGA

The synthesis of fatty acids and lipids in Nannochloropsis sp. was inv estigated by labeling cells in vivo with [C-14]-bicarbonate or [C-14]- acetate. [C-14]-bicarbonate was incorporated to the greatest extent in to 16:0, 16:1, and 14:0 fatty acids, which are the predominant fatty a cids of triacylglycerols. However, more than half of the [C-14]-acetat e was incorporated into longer and more desaturated fatty acids, which are constituents of membrane lipids. [C-14]-acetate was incorporated most strongly into phosphatidylcholine, which rapidly lost label durin g a 5-h chase period. The label associated with phosphatidylethanolami ne also decreased during the chase period, whereas label in other memb rane lipids and triacylglycerol increased. The dynamics of labeling, a long with information regarding the acyl compositions of various lipid s, suggests that 1) the primary products of chloroplast fatty acid syn thesis are 14:0, 16:0, and 16:1; 2) C-20 fatty acids are formed by an elongation reaction that can utilize externally supplied acetate; 3) p hosphatidylcholine is a site for desaturation of C-18 fatty acids; and 4) phosphatidylethanolamine may be a site for desaturation of C-20 fa tty acids.