RADIOLABELING STUDIES OF LIPIDS AND FATTY ACIDS IN NANNOCHLOROPSIS (EUSTIGMATOPHYCEAE), AN OLEAGINOUS MARINE ALGA1

The synthesis of fatty acids and lipids in Nannochloropsis sp. was investigated by labeling cells in vivo with [C-14]-bicarbonate or [C-14]-acetate. [C-14]-bicarbonate was incorporated to the greatest extent into 16:0, 16:1, and 14:0 fatty acids, which are the predominant fatty acids of triacylglycerols. However, more than half of the [C-14]-acetate 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 during a 5-h chase period. The label associated with phosphatidylethanolamine also decreased during the chase period, whereas label in other membrane lipids and triacylglycerol increased. The dynamics of labeling, along with information regarding the acyl compositions of various lipids, suggests that 1) the primary products of chloroplast fatty acid synthesis 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) phosphatidylcholine is a site for desaturation of C-18 fatty acids; and 4) phosphatidylethanolamine may be a site for desaturation of C-20 fatty acids.