Water oxidation by photosystem II is the primary source of electrons for sustained H 2 photoproduction in nutrient-replete green algae

Significance Photosynthetic H 2 production in the green alga Chlamydomonas reinhardtii is catalyzed by O 2 -sensitive [FeFe]-hydrogenases, which accept electrons from photosynthetically reduced ferredoxin and reduce protons to H 2 . Since the process occurs downstream of photosystem I, the contribution of photosystem II (PSII) in H 2 photoproduction has long been a subject of debate. Indeed, water oxidation by PSII results in O 2 accumulation in chloroplasts, which inhibits H 2 evolution. Therefore, clear evidence for direct water biophotolysis resulting in simultaneous H 2 and O 2 releases in algae has never been presented. This paper demonstrates that sustained H 2 photoproduction in C. reinhardtii is directly linked to PSII-dependent water oxidation and brings insights into regulation of PSII activity and H 2 production by CO 2 /HCO 3 – under microoxic conditions.