Advances and challenges in photosynthetic hydrogen production.

The vision to replace coal with hydrogen goes back to Jules Verne in 1874. However, sustainable hydrogen production remains challenging. The most elegant approach is to utilize photosynthesis for water splitting and to subsequently save solar energy as hydrogen. Cyanobacteria and green algae are unicellular photosynthetic organisms that contain hydrogenases and thereby possess the enzymatic equipment for photosynthetic hydrogen production. These features of cyanobacteria and algae have inspired artificial and semi-artificial in vitro techniques, that connect photoexcited materials or enzymes with hydrogenases or mimics of these for hydrogen production. These in vitro methods have on their part been models for the fusion of cyanobacterial and algal hydrogenases to photosynthetic photosystem I (PSI) in vivo , which recently succeeded as proofs of principle. Biotechnological solar hydrogen production has the aim to utilize electrons from the light reactions of photosynthesis. Recently, cyanobacterial and algal H 2 ases, the enzymes that catalyze production of H 2 , were successfully fused to PSI. They intercept electrons from the photosynthetic electron transport chain. Both engineered strains produce photosynthetic hydrogen for prolonged periods. They maintained their ability to grow photoautotrophically. The processes do not exclusively rely upon water oxidation as desired, which is mostly due to the fact that the H 2 ases are sensitive to oxygen. [ABSTRACT FROM AUTHOR]