Glycerolipid synthesis in Chlorella kessleri 11 h : II. Effect of the CO 2 concentration during growth

Abstract

In the accompanying paper, we demonstrated that Chlorella kessleri uses prokaryotic and eukaryotic pathways to synthesize sn-1-C 18– sn-2-C 16 (C 18/C 16, prokaryotic lipids) and sn-1-C 18– sn-2-C 18 (C 18/C 18, eukaryotic lipids) species, respectively, in chloroplast lipids such as monogalactosyl diacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG). In this study, to examine the effect of CO 2 on lipid metabolism, we compared the fatty acid distributions at the sn-1 and sn-2 positions of each major lipid, i.e. MGDG, DGDG, phosphatidylcholine (PC), and phosphatidylethanolamine (PE), and the patterns of incorporation of [ 14C]acetate into fatty acids and lipids in vivo between cells of C. kessleri grown under ordinary air (low-CO 2 cells) and ones grown under CO 2-enriched air (high-CO 2 cells). Low-CO 2 cells, as compared with high-CO 2 cells, showed elevated contents of 18:3(9,12,15), especially at both the sn-1 and sn-2 positions of MGDG and DGDG, and also at the sn-2 position of PC and PE. When the cells were labeled with [ 14C]acetate, slower rates of 18:3 synthesis in the respective major lipids with lower incorporation of 14C into total membrane lipids were observed in low-CO 2 cells than in high-CO 2 cells. These results thus indicate that the higher unsaturation levels in low-CO 2 cells are at least partially due to repressed fatty acid synthesis, which promotes the desaturation of pre-existing fatty acids, rather than to up-regulation of desaturation activity. It was also noted that, in both MGDG and DGDG, the contents of eukaryotic lipids were higher at the expense of prokaryotic lipids in low-CO 2 cells than in high-CO 2 cells, suggesting relatively greater metabolic flow in the eukaryotic pathway compared to the prokaryotic pathway for galactolipid synthesis in low-CO 2 cells. We propose that, together with the repression of fatty acid synthesis, the increased synthesis of C 18/C 18 species of galactolipids, which are suitable substrates for chloroplast desaturation, through the eukaryotic pathway, contributes to the higher contents of 18:3 in low-CO 2 cells than in high-CO 2 cells.