Biosynthesis of eukaryotic lipid molecular species by the cyanobacterium Spirulina platensis.

This report brings evidence that a prokaryotic photosynthetic organism can synthesize eukaryotic molecular species of glycerolipids. When Spirulina platensis PCC 8005 was supplemented with oleic acid, the sum of the percentages of 18 carbon (C18) fatty acids in monogalactosyldiacylglycerol (MGDG), the major lipid class, became largely higher than 50 mol%. This was absolutely unexpected from the well-known structure of cyanobacterial lipids. In these organisms, C18 fatty acids usually account for less than 50 mol% because they are esterified on carbon 1 of the glycerol, exclusively. This classical feature was 99% confirmed in control as well as in palmitate-supplemented cultures. The major molecular species of MGDG, which resulted from the different distributions of fatty acids on carbons 1 and 2 of glycerol, were C18/C16 type, belonging to the so-called "prokaryotic" type of lipids. By contrast, the molecular species of MGDG from oleate-supplemented cultures consisted of only 74 mol% of C18/C16 and the complementary 26 mol% were C18/C18, the so-called "eukaryotic" type of lipids. Furthermore, such C18/C18 lipids were also evidenced as traces (< 1%) in control cultures. These results underline the fact that the fatty acid specificity of 1-monoacylglycerol-3-phosphate-acyltransferase (in Spirulina) is not as absolute as the widely accepted concept of prokaryotic lipid would suggest. Oleate, supplemented at high concentration, can be compelled to act as a substrate for the acyltransferase and this results in the appearance of C18/C18 "eukaryotic" lipids in a prokaryotic organism.