Diversity in photosynthetic electron transport under [CO]-limitation: the cyanobacterium Synechococcus sp. PCC 7002 and green alga Chlamydomonas reinhardtii drive an O-dependent alternative electron flow and non-photochemical quenching of chlorophyll fluorescence during CO-limited photosynthesis.

Some cyanobacteria, but not all, experience an induction of alternative electron flow (AEF) during CO-limited photosynthesis. For example, Synechocystis sp. PCC 6803 (S. 6803) exhibits AEF, but Synechococcus elongatus sp. PCC 7942 does not. This difference is due to the presence of flavodiiron 2 and 4 proteins (FLV2/4) in S. 6803, which catalyze electron donation to O. In this study, we observed a low-[CO] induced AEF in the marine cyanobacterium Synechococcus sp. PCC 7002 that lacks FLV2/4. The AEF shows high affinity for O, compared with AEF mediated by FLV2/4 in S. 6803, and can proceed under extreme low [O] (about a few µM O). Further, the transition from CO-saturated to CO-limited photosynthesis leads a preferential excitation of PSI to PSII and increased non-photochemical quenching of chlorophyll fluorescence. We found that the model green alga Chlamydomonas reinhardtii also has an O-dependent AEF showing the same affinity for O as that in S. 7002. These data represent the diverse molecular mechanisms to drive AEF in cyanobacteria and green algae. In this paper, we further discuss the diversity, the evolution, and the physiological function of strategy to CO-limitation in cyanobacterial and green algal photosynthesis.