Cyclic electron flow in cyanobacteria and eukaryotic algae

In oxygenic photosynthesis light energy is largely captured in linear electron flow (LEF) between the photosystems and drives ATP formation via a thylakoid proton-driven ATP synthase. In addition, for over 50 years there has been good evidence that an additional cyclic electron flow (CEF) around photosystem I (PSI) is harnessed to provide extra ATP in addition to that produced by LEF. The evidence comes from all oxygenic organisms, cyanobacteria, eukaryotic algae and embryophytic plants. However, the CEF mechanism has been difficult to investigate because of the cyclic nature of the EF and confusion with other pathways not using oxygen as a terminal electron acceptor, and the MAPS, flavodiiron and chlororespiration pathways to oxygen. This article discusses the current evidence for CEF in all oxygenic organisms and suggests future experiments by which the situation can be clarified.