Proton‐motive‐force‐driven formation of CO from CO2 and H2 in methanogenic bacteria

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Abstract
Cell suspensions of methanogenic bacteria (Methanosarcina barkeri, Methanospirillum hungatei, Methanobrevibacter arboriphilus, and Methanobacterium thermoautotrophicum) were found to form CO from CO2 and H2 according to the reaction: CO2+ H2→ CO + H2O; ΔG0=+20 kJ/mol. Up to 15000 ppm CO in the gas phase were reached which is significantly higher than the equilibrium concentration calculated from ΔG0 (95 ppm under the experimental conditions). This indicated that CO2 reduction with H2 to CO is energy‐driven and indeed the cells only generated CO when forming CH4. The coupling of the two reactions was studied in more detail with acetate‐grown cells of M. barkeri using methanol and H2 as methanogenic substrates. The effects of the protonophore tetrachlorosalicylanilide (TCS) and of the proton‐translocating ATPase inhibitor N,N′‐dicyclohexylcarbodiimide (cHxN)2C were determined. TCS completely inhibited CO formation from CO2 and H2 without affecting methanogenesis from CH3OH and H2. In the presence of the protonophore the proton motive force Δp and the intracellular ATP concentration were very low. (cHxN)2C, which partially inhibited methanogenesis from CH3OH and H2, had no effect on CO2 reduction to CO. In the presence of (cHxN)2C Δp was high and the intracellular ATP content was low. These findings suggest that the endergonic formation of CO from CO2 and H2 is coupled to the exergonic formation of CH4 from CH3OH and H2 via the proton motive force and not via ATP. CO formation was not stimulated by the addition of sodium ions.