The effect of the reversed tricarboxylic acid cycle on the 13C contents of bacterial lipids

Free and esterified lipids of a green sulfur bacterium, Chlorobium limicola, and a purple sulfur bacterium, Thiocapsa roseopersicina, were investigated to examine the effect of the reversed tricarboxylic acid cycle on the 13C contents of their lipids. The lipids of C. limicola are 2 to 16‰ enriched in 13C relative to the total cell material, with the isoprenoid lipids 13 to 14‰ depleted in 13C relative to the straight-chain lipids. Although T. roseopersicina mainly uses the Calvin cycle for CO2 fixation and only partly uses the reversed tricarboxylic acid cycle, the same trends are observed with straight-chain lipids 2 to 4‰ enriched in 13C relative to the cell material and the isoprenoid lipids 7 to 9‰ depleted in 13C relative to the straight-chain lipids. These trends are consistent with the (partial) use of the reversed tricarboxylic acid cycle by C. limicola and T. roseopersicina, resulting in a reversed sequence of fractionation effects on the 13C content of the lipids compared to organisms using ribulose bisphosphate carboxylase to fix CO2.