Cyanobacterial H2 metabolism: Knowledge and potential strategies for a photobiotechnological production of H2

Abstract

Molecular hydrogen is an environmentally clean source of energy that may be a valuable alternative to the limited fossil fuel resources of today. For photobiological H2 production, cyanobacteria are among the ideal candidates since they have the simplest nutritional requirements: they can grow in air (N2 and CO2), water (electrons and reducing agents), and simple mineral salts with light as the only source of energy. In N2-fixing cyanobacteria, H2 is mainly produced by nitrogenases, but its partial consumption is quickly catalyzed by a unidirectional uptake hydrogenase. In addition, a bidirectional (reversible) enzyme may also oxidize some of the molecular hydrogen. Filamentous cyanobacteria have been used in bioreactors for the photobiological conversion of H2O to H2. However, the conversion efficiencies achieved are low because the net H2 production is the result of H2 evolution via a nitrogenase and H2 consumption mainly via an uptake hydrogenase. Consequently, the improvements of the conversion efficiency are achieved through the optimization of the conditions for H2 evolution by the nitrogenase and through the production of mutants deficient in H2 uptake activity. Symbiotic cells are of fundamental interest since they in situ 'function as a bioreactor', possess a high metabolism and there is transfer of metabolite(s) from symbiont to host, but almost no growth. This communication presents the general knowledge about hydrogen metabolism,' hydrogenases in filamentous cyanobacteria, outline strategies for improving the capacity of H2 production by filamentous strains, and stresses the importance of international cooperations and networks.