Light-Driven H₂ 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 H₂ production becomes a fully light-driven process. HydA1 and HydA2 are highly O₂ sensitive; consequently, the formation of H₂ occurs mainly under anoxic conditions. Yet, photo-H₂ production is tightly coupled to the efficiency of photosynthetic electron transport and linked to the photosynthetic control via the Cyt b₆f 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 H₂ photoproduction. Additionally, the CBB cycle competes with H₂ photoproduction. Consequently, an in-depth understanding of light-driven H₂ production via photosynthetic electron transfer and its competition with CO₂ fixation is essential for improving photo-H₂ production. At the same time, the smart design of photo-H₂ production schemes and photo-H₂ bioreactors are challenges for efficient up-scaling of light-driven photo-H₂ production.