COMPARATIVE EFFECTS OF EXOGENOUS FATTY-ACID SUPPLEMENTATIONS ON THE LIPIDS FROM THE CYANOBACTERIUM SPIRULINA-PLATENSIS

The accumulation of gamma-linolenic acid (gamma-C18:3) in membrane lip ids of Spirulina platensis PCC 8005 was studied in cultures supplement ed with large amounts of linoleic acid (C18:2), the immediate precurso r of gamma-C18:3. Analyses of lipid and fatty acid compositions showed that the percentages of C18:2 and gamma-C18:3 were largely affected i n all lipid classes, specially in monogalactosyldiacylglycerol (MGDG), the major galactolipid and digalactosyldiacylglycerol (DGDG). The dis tribution of fatty acids at positions 1 and 2 of the glycerol moiety o f MGDG was modified. In sharp contrast with the classical structure of cyanobacterial lipids, C18:2 and gamma-C18:3 were found esterified on both carbons 1 and 2 of the glycerol. Consequently, the molar ratio o f C18/C16 to C18/C18 molecular species of MGDG which was 99/1 in contr ol cultures, was decreased to 68.2/31.8 in linoleate-supplemented cult ures. This result brings evidence that exogenous linolenic acid supple mentation can induce this prokaryotic photosynthetic microorganism to synthesize high levels of the so-called ''eukaryotic'' molecular speci es of glycerolipids (C18/C18) instead of the usual ''prokaryotic'' one s (C18/C16). Furthermore, the presence of gamma-C18:3/gamma-C18.3 mole cular species of MGDG indicates that the Delta 6 fatty acid desaturati on was not restricted to carbon 1 as expected from the structure of S. platensis lipids in control cultures, but can readily occur on fatty acids esterified on both carbons 1 and 2 of the glycerol backbone. Com parison of lipids from cultures supplemented with different fatty acid s clearly shows that linoleate-supplemented S. platensis is the best ' 'algal'' source for gamma-linolenic acid.