Formate-driven H₂ production by whole cells of Thermoanaerobacter kivui

In times of global warming there is an urgent need to replace fossil fuel-based energy vectors by less carbon dioxide (CO₂)-emitting alternatives. One attractive option is the use of molecular hydrogen (H₂) since its combustion emits water (H₂O) and not CO₂. Therefore, H₂ is regarded as a non-polluting fuel. The ways to produce H₂ can be diverse, but steam reformation of conventional fossil fuel sources is still the main producer of H₂ gas up to date. Biohydrogen production via microbes could be an alternative, environmentally friendly and renewable way of future H₂ production, especially when the flexible and inexpensive C1 compound formate is used as substrate.

In this study, the versatile compound formate was used as substrate to drive H₂ production by whole cells of the thermophilic acetogenic bacterium Thermoanaerobacter kivui which harbors a highly active hydrogen-dependent CO₂ reductase (HDCR) to oxidize formate to H₂ and CO₂ and vice versa. Under optimized reaction conditions, T. kivui cells demonstrated the highest H₂ production rates (qH₂ = 685 mmol g^-1 h^-1) which were so far reported in the literature for wild-type organisms. Additionally, high yields (Y_(H2/formate)) of 0.86 mol mol^-1 and a hydrogen evolution rate (HER) of 999 mmol L^-1 h^-1 were observed. Finally, stirred-tank bioreactor experiments demonstrated the upscaling feasibility of the applied whole cell system and indicated the importance of pH control for the reaction of formate-driven H₂ production.

The thermophilic acetogenic bacterium T. kivui is an efficient biocatalyst for the oxidation of formate to H₂ (and CO₂). The existing genetic tool box of acetogenic bacteria bears further potential to optimize biohydrogen production in future and to contribute to a future sustainable formate/H₂ bio-economy.