FIG. 1.

Photobiological hydrogen production in Chlamydomonas reinhardtii. Under aerobic conditions, electrons derived from the water-splitting reaction of PSII are passed along the photosynthetic ETC via PQ, cytochrome b_6f (Cyt b₆f), PSI, and ferrodoxin (Fd) before being used in the production of NADPH and starch, which can be used for mitochondrial respiration. H⁺ released into the thylakoid lumen by PSII and the PQ/PQH2 cycle (H⁺ flow; dashed lines), generates an H⁺ gradient, which drives ATP production via ATP synthase. Under anaerobic conditions, mitochondrial respiration is not functional, H⁺/e⁻ is stored in starch, and NADPH is used by hydrogenase for bio-H₂ production.