The plastoquinone pool, poised for cyclic electron flow?

The relative contribution of cyclic compared to linear electron flow (LEF) depends on the redox poise of electron carriers, and is also dependent upon the relative turnovers of the two photosystems (only photosystem I is involved in cyclic). Although these views have been built on solid experimental facts gathered over the years∼1963–1987, they have sometimes been overlooked in the recent literature. In the microalga Chlamydomonas reinhardtii, a PSI-cyt b6f supercomplex is formed when cells are incubated under anaerobic conditions and PSI antenna size is large (state 2), conditions that are thought to be favorable to cyclic electron flow (CEF). Although amore reductive poise of the chloroplast and an increase in PSI antenna size would expectedly speed up cyclic turnovers in limiting light, the role for supercomplex formation in favoring cyclic flow has not yet been fully demonstrated. Linear and cyclic are indeed concurrent and competing flows, however, evidence is still lacking that the sequestration of PSI inside a cyclic supercomplex decreases linear flow.