Fig. 4

Variations of the TCA cycle in photosynthetic microorganisms. a Complete and fully functional TCA cycle. b TCA cycle observed in microalgae, such as Synechococcus sp., which lacks the enzymes α-ketoglutarate dehydrogenase and succinyl-CoA synthetase (enzymes highlighted in red). A bypass via succinate-semialdehyde dehydrogenase, as observed in Synechocystis sp., is shown in blue. c Split TCA cycle as reported for C. reinhardtii [30]. The two branches producing 2-oxoglutarate and malate for downstream biosynthesis. Oxaloacetate is provided via anaplerotic activity of phosphoenolpyruvate carboxylase in this split TCA cycle [46]