Hydrogen production in the C. reinhardtii chloroplast. Normally, the protein ferredoxin (FD) transfers electrons to an enzyme that reduces NADP+ to NADPH, which is required for chloroplast metabolic processes. Reduced ferredoxin (FD(red)) can instead transfer electrons to a chloroplast hydrogenase, which produces molecular hydrogen (H2) from protons (H+). Hydrogen production thus acts as an alternate electron sink. Reduced ferredoxin can also be produced via glycolysis from the breakdown of starch, which enables hydrogen production in the absence of photosynthesis. FNR, ferredoxin NADP+ oxidoreductase; PFOR, pyruvate ferredoxin oxidoreductase. Figure courtesy of and adapted from M. Posewitz.