Carboxydothermus hydrogenoformans is a thermophilic strictly anaerobic bacterium that catalyses the water gas shift reaction, the conversion of carbon monoxide with water to molecular hydrogen and carbon dioxide. The thermodynamically favorable growth temperature, compared to existing industrial catalytic processes, makes this organism an interesting alternative for production of cheap hydrogen gas suitable to fuel CO-sensitive fuel cells in a future hydrogen economy, provided sufficiently low levels of CO are reached. Here we study CO conversion and final CO levels in cultures of C. hydrogenoformans grown in batch cultures that were started with a 100% CO gas phase with and without removal of formed CO2. Final CO levels were 117？ppm without CO2 removal and below 2？ppm with CO2 removal. The Gibbs free energy change calculated with measured end concentrations and the detection of acetate suggest that C. hydrogenoformans shifted from a hydrogenogenic to an acetogenic metabolism. 1. Introduction Carboxydothermus hydrogenoformans is a strictly anaerobic carboxydotrophic hydrogenogenic thermophilic bacterium [1] that conserves energy for growth by performing the water gas shift reaction, the conversion of carbon monoxide with water to hydrogen and carbon dioxide (reaction (1)). The standard Gibbs free energy change of the reaction per mol of CO is relatively small, ？20？kJ？mol？1. Production of acetate from CO (reaction (2)) or from H2 and CO2 (reaction (3)) is not reported for C. hydrogenoformans, despite the presence of the required genes in the genome of C. hydrogenoformans [2] C O + H 2 O ？ C O 2 + H 2 ( 1 ) 4 C O + 2 H 2 O ？ 2 C O 2 + C H 3 C O O H ( 2 ) 4 H 2 + 2 C O 2 ？ C H 3 C O O H ( 3 ) The water gas shift reaction is applied in industry for production of relative cheap hydrogen gas from synthesis gas. Synthesis gas is a mixture of mainly H2, CO, and CO2 which is produced by partial oxidation or autothermal reforming of hydrocarbon-rich sources such as fossil fuels, domestic and agricultural wastes, and other biomass sources [3–5]. Water-gas shift catalysis is applied in successive high and low temperature steps at 400 and 200°C to convert the bulk of CO to final concentrations of not below 1000？ppm. Low-temperature fuel cells are sensitive to CO and require CO levels of below 10？ppm or of a few hundred ppm for novel types that apply improved membranes [6]. Hydrogen gas derived from synthesis gas, while relatively cheap, is therefore not suitable for these types of fuel cells. Highly desired is a water-gas shift process capable of removing CO from
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