Hydrogenesis in hyperthermophilic microorganisms: Implications for biofuels

Hydrothermal microbiotopes are characterized by the consumption and production of molecular hydrogen. Heterotrophic hyperthermophilic microorganisms (growth Topt⩾80 °C) actively participate in the production of H2 in these environments through the fermentation of peptides and carbohydrates. Hyperthermophiles have been shown to approach the theoretical (Thauer) limit of 4 mol of H2 produced per mole of glucose equivalent consumed, albeit at lower volumetric productivities than observed for mesophilic bacteria, especially enterics and clostridia. Potential advantages for biohydrogen production at elevated temperatures include fewer metabolic byproducts formed, absence of catabolic repression for growth on heterogeneous biomass substrates, and reduced loss of H2 through conversion to H2S and CH4 by mesophilic consortia containing sulfate reducers and methanogens. To fully exploit the use of these novel microorganisms and their constituent hydrogenases for biohydrogen production, development of versatile genetic systems and improvements in current understanding of electron flux from fermentable substrates to H2 in hyperthermophiles are needed.