hydrogenases

Hydrogenases

Interest in the generation of renewable fuels has gained momentum in the last decades in the face of global warming associated with the continued use of fossil fuels, and because of the finite nature of their reserves.

Biohydrogen production from photosynthetic organisms constitutes a conceptually promising avenue in renewable bioenergy, because it would couple directly solar radiant energy, essentially inexhaustible, to the generation of clean, carbon-neutral biofuels, particularly if water-splitting (oxygenic) phototrophs were used.

Cyanobacteria, the only group of oxygenic phototrophs among the bacteria, have been regarded as good models for research and eventual application in this area for several reasons: they are capable of growth with minimal nutritional requirements, they are demonstrable producers of hydrogen gas under certain physiological conditions, and some can be genetically modified with ease. Among cyanobacteria, three different enzymes participate in hydrogen metabolism nitrogenase, and two types of Ni-Fe hydrogenases (uptake and bidirectional).

hydrogenase

 

In principle, production of biohydrogen based on nitrogenase systems requires significant modifications of the enzyme or cumbersome growth conditions in order to promote proton reduction and prevent N2 reduction. The hydrogen produced by nitrogenase is often recycled back into metabolic reducing equivalents by means of the uptake hydrogenase. Under physiological conditions, and as the name suggests, the latter enzyme can only consume, rather than produce H2, and so does not constitute a viable platform for biohydrogen production; in fact, it needs to be inactivated to improve yields of nitrogenase-based hydrogen production. Certain cyanobacteria, however, host a bidirectional hydrogenase that can catalyze both the production and the uptake of hydrogen under physiological conditions. One of the major disadvantages for sustained H2 production via the bidirectional hydrogenase is the easy reversal of the reaction direction.

Therefore, our lab focuses on surveying newly isolated cultures from diverse environments for the presence of the bidirectional hydrogenase gene and a concurrent quantitative comparison of their hydrogenase activities under non nitrogen-fixing conditions. We target cyanobacteria from terrestrial environments, since no bidirectional hydrogenase genes originating in these environments were known from public databases, suggesting that they may have been differentially under-sampled. Marine microbial intertidal mats were also of special interest since a high flux of hydrogen had been reported from these cyanobacterial mats. To this we add a survey of freshwater plankton, a habitat well known to harbor cyanobacteria with bidirectional hydrogenases and hydrogen producing capabilities.

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