Flavodiiron-Mediated O2 Photoreduction Links H2 Production with CO2 Fixation during the Anaerobic Induction of Photosynthesis

Some microalgae, such as Chlamydomonas reinhardtii , harbor a highly flexible photosynthetic apparatus capable of using different electron acceptors, including carbon dioxide (CO 2 ), protons, or oxygen (O 2 ), allowing survival in diverse habitats. During anaerobic induction of photosynthesis, molecular O 2 is produced at photosystem II, while at the photosystem I acceptor side, the reduction of protons into hydrogen (H 2 ) by the plastidial [FeFe]-hydrogenases primes CO 2 fixation. Although the interaction between H 2 production and CO 2 fixation has been studied extensively, their interplay with O 2 produced by photosynthesis has not been considered. By simultaneously measuring gas exchange and chlorophyll fluorescence, we identified an O 2 photoreduction mechanism that functions during anaerobic dark-to-light transitions and demonstrate that flavodiiron proteins (Flvs) are the major players involved in light-dependent O 2 uptake. We further show that Flv-mediated O 2 uptake is critical for the rapid induction of CO 2 fixation but is not involved in the creation of the micro-oxic niches proposed previously to protect the [FeFe]-hydrogenase from O 2 . By studying a mutant lacking both hydrogenases (HYDA1 and HYDA2) and both Flvs (FLVA and FLVB), we show that the induction of photosynthesis is strongly delayed in the absence of both sets of proteins. Based on these data, we propose that Flvs are involved in an important intracellular O 2 recycling process, which acts as a relay between H 2 production and CO 2 fixation.