Abstract
BACKGROUND: Molecular hydrogen (H(2)) is an attractive future energy carrier to replace fossil fuels. Biologically and sustainably produced H(2) could contribute significantly to the future energy mix. However, biological H(2) 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(2) production, as it can be produced itself from various sources including electrochemical reduction of CO(2) or from synthesis gas. Many microbes that can produce H(2) from formate have been isolated; however, in most cases H(2) production rates cannot compete with other H(2) production methods. RESULTS: We established a formate-based H(2) production method utilizing the acetogenic bacterium Acetobacterium woodii. This organism can use formate as sole energy and carbon source and possesses a novel enzyme complex, the hydrogen-dependent CO(2) reductase that catalyzes oxidation of formate to H(2) and CO(2). Cell suspensions reached specific formate-dependent H(2) production rates of 71 mmol g(protein)(-1) h(-1) (30.5 mmol g(CDW)(-1) h(-1)) and maximum volumetric H(2) evolution rates of 79 mmol L(-1) h(-1). Using growing cells in a two-step closed batch fermentation, specific H(2) production rates reached 66 mmol g(CDW)(-1) h(-1) with a volumetric H(2) evolution rate of 7.9 mmol L(-1) h(-1). Acetate was the major side product that decreased the H(2) 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(2) 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(2) production phase. CONCLUSIONS: Acetobacterium woodii reached one of the highest formate-dependent specific H(2) productivity rates at ambient temperatures reported so far for an organism without genetic modification and converted the substrate exclusively to H(2). This makes this organism a very promising candidate for sustainable H(2) production and, because of the reversibility of the A. woodii enzyme, also a candidate for reversible H(2) storage.