The Potential of Using Cyanobacteria as Producers of Molecular Hydrogen

Abstract

For photobiological hydrogen (H2) production, cyanobacteria are among the ideal candidates because they have minimal nutritional requirements: they can thrive on air, water, and mineral salts with light as the only energy source. Cyanobacteria may possess several enzymes directly involved in H2 metabolism: nitrogenase(s) catalyzing the production of H2 concomitantly with the reduction of nitrogen to ammonia; an uptake hydrogenase, catalyzing the consumption of H2 produced by the nitrogenase; and a bidirectional hydrogenase, which has the capacity to both take up and produce H2. Two strategies are possible for cyanobacterial biohydrogen: (1) H2 produced by nitrogenase and (2) H2 produced by the bidirectional hydrogenase. Although hydrogenase genes have been identified and characterized in several cyanobacteria, very little is known about the cyanobacterial hyp genes and/or the corresponding hyp proteins. Efficient photoconversion of water to hydrogen by cyanobacteria is influenced by many factors. For example, it is known that immobilized cells produce more hydrogen than free-living cultures and that non-Mo-containing nitrogenases allocate more electrons to the production of hydrogen. The gas phase, the age and density of the culture, as well as the composition, pH, and temperature of the growth medium are also crucial for the final result.