Light-Driven H 2 Production in Chlamydomonas reinhardtii : Lessons from Engineering of Photosynthesis.

In the green alga Chlamydomonas reinhardtii , hydrogen production is catalyzed via the [FeFe]-hydrogenases HydA1 and HydA2. The electrons required for the catalysis are transferred from ferredoxin (FDX) towards the hydrogenases. In the light, ferredoxin receives its electrons from photosystem I (PSI) so that H 2 production becomes a fully light-driven process. HydA1 and HydA2 are highly O 2 sensitive; consequently, the formation of H 2 occurs mainly under anoxic conditions. Yet, photo-H 2 production is tightly coupled to the efficiency of photosynthetic electron transport and linked to the photosynthetic control via the Cyt b 6 f complex, the control of electron transfer at the level of photosystem II (PSII) and the structural remodeling of photosystem I (PSI). These processes also determine the efficiency of linear (LEF) and cyclic electron flow (CEF). The latter is competitive with H 2 photoproduction. Additionally, the CBB cycle competes with H 2 photoproduction. Consequently, an in-depth understanding of light-driven H 2 production via photosynthetic electron transfer and its competition with CO 2 fixation is essential for improving photo-H 2 production. At the same time, the smart design of photo-H 2 production schemes and photo-H 2 bioreactors are challenges for efficient up-scaling of light-driven photo-H 2 production.