RT Journal Article SR Electronic T1 Cyclic electron flow in Chlamydomonas reinhardtii JF bioRxiv FD Cold Spring Harbor Laboratory SP 153288 DO 10.1101/153288 A1 W. J. Nawrocki A1 B. Bailleul A1 P. Cardol A1 F. Rappaport A1 F.-A. Wollman A1 P. Joliot YR 2017 UL http://biorxiv.org/content/early/2017/06/21/153288.abstract AB Cyclic electron flow (CEF), one of the major alternative electron transport pathways to the primary linear electron flow (LEF) in chloroplasts has been discovered in the middle of the last century. It is defined as a return of the reductants from the acceptor side of the Photosystem I (PSI) to the pool of its donors via the cytochrome b6f, and has proven essential for photosynthesis. However, despite many efforts aimed at its characterisation, the pathway and regulation of CEF remain equivocal, and its physiological significance remains to be properly defined. Here we use novel spectroscopic approaches to measure CEF in transitory conditions in the green alga Chlamydomonas reinhardtii. We show that CEF operates at the same maximal rate regardless of the oxygen concentration, and that the latter influences LEF, rather than CEF in vivo, which questions the recent hypotheses about the CEF supercomplex formation. We further reveal that the pathways proposed for CEF in the literature are inconsistent with the kinetic information provided by our measurements. We finally provide cues on the regulation of CEF by light.