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

In the green alga , 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 production becomes a fully light-driven process. HydA1 and HydA2 are highly O sensitive; consequently, the formation of H occurs mainly under anoxic conditions. Yet, photo-H production is tightly coupled to the efficiency of photosynthetic electron transport and linked to the photosynthetic control via the Cyt 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 photoproduction. Additionally, the CBB cycle competes with H photoproduction. Consequently, an in-depth understanding of light-driven H production via photosynthetic electron transfer and its competition with CO fixation is essential for improving photo-H production. At the same time, the smart design of photo-H production schemes and photo-H bioreactors are challenges for efficient up-scaling of light-driven photo-H production.