Cyanobacterial in vivo solar hydrogen production using a photosystem I–hydrogenase (PsaD-HoxYH) fusion complex

Photosynthetically produced hydrogen is an attractive, sustainable fuel. Semiartificial in vitro techniques have been successfully implemented in which hydrogenases were attached to isolated photosystems for hydrogen production. However, in vitro systems are in general short lived as metabolic processes that support self-repair and maintenance are missing. So far, photosystem–hydrogenase fusions have been tested in vitro only. Here, we report photosynthetic hydrogen production using a photosystem I–hydrogenase fusion in vivo. The NiFe-hydrogenase HoxYH of the cyanobacterium Synechocstis sp. PCC 6803 was fused to its photosystem I subunit PsaD in close proximity to the 4Fe4S cluster FB, which ordinarily donates electrons to ferredoxin. The resultant psaD-hoxYH mutant grows photoautotrophically, achieves a high concentration of photosynthetically produced hydrogen of 500 μM under anaerobic conditions in the light and does not take up the generated hydrogen. Our data indicate that photosynthetic hydrogen production in psaD-hoxYH is most likely based on both oxygenic and anoxygenic photosynthesis. Using photosynthetic microorganisms may be a route to sustainable hydrogen production from solar energy, but hydrogen generation is typically short lived. Here the authors address this challenge by engineering cyanobacteria to construct photosystem I–hydrogenase fusions that produce hydrogen in vivo.