Efficient whole cell biocatalyst for formate-based hydrogen production.

Background: Molecular hydrogen (H) is an attractive future energy carrier to replace fossil fuels. Biologically and sustainably produced H could contribute significantly to the future energy mix. However, biological H production methods are faced with multiple barriers including substrate cost, low production rates, and low yields. The C1 compound formate is a promising substrate for biological H production, as it can be produced itself from various sources including electrochemical reduction of CO or from synthesis gas. Many microbes that can produce H from formate have been isolated; however, in most cases H production rates cannot compete with other H production methods.

Results: We established a formate-based H production method utilizing the acetogenic bacterium . This organism can use formate as sole energy and carbon source and possesses a novel enzyme complex, the hydrogen-dependent CO reductase that catalyzes oxidation of formate to H and CO. Cell suspensions reached specific formate-dependent H production rates of 71 mmol g h (30.5 mmol g h) and maximum volumetric H evolution rates of 79 mmol L h. Using growing cells in a two-step closed batch fermentation, specific H production rates reached 66 mmol g h with a volumetric H evolution rate of 7.9 mmol L h. Acetate was the major side product that decreased the H yield. We demonstrate that inhibition of the energy metabolism by addition of a sodium ionophore is suitable to completely abolish acetate formation. Under these conditions, yields up to 1 mol H per mol formate were achieved. The same ionophore can be used in cultures utilizing formate as specific switch from a growing phase to a H production phase.

Conclusions: reached one of the highest formate-dependent specific H productivity rates at ambient temperatures reported so far for an organism without genetic modification and converted the substrate exclusively to H. This makes this organism a very promising candidate for sustainable H production and, because of the reversibility of the enzyme, also a candidate for reversible H storage.