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Carbon monoxide-fermenting microorganisms can be used for the production of a wide range of commodity chemicals and fuels from syngas (generated by gasification of, e.g., wastes or biomass) or industrial off-gases (e.g., from steel industry). Microorganisms are normally more resistant to contaminants in the gas (e.g., hydrogen sulfide) than chemical catalysts, less expensive and self-regenerating. However, some carboxydotrophs are sensitive to high concentrations of CO, resulting in low growth rates and productivities. We hypothesize that cultivation of synthetic cocultures can be used to improve overall rates of CO bioconversion. As a case study, a thermophilic microbial coculture, consisting of Carboxydothermus hydrogenoformans and Methanothermobacter thermoautotrophicus was constructed to study the effect of cocultivation on conversion of CO-rich gases to methane. In contrast to the methanogenic monoculture, the coculture was able to efficiently utilize CO or mixtures of H2/CO/CO2 to produce methane at high efficiency and high rates. In CSTR-bioreactors operated in continuous mode, the coculture converted artificial syngas (66.6% H2:33.3% CO) to an outflow gas with a methane content of 72%, approaching the 75% theoretical maximum. CO conversion efficiencies of 93% and volumetric production rates of 4 m3methane/m3liquid/day were achieved. This case shows that microbial cocultivation can result in a significant improvement of gas-fermentation of CO-rich gases.