Oxidation of ethanol by methanogenic bacteria

Abstract

Methanogenium organophilum, a non-autotrophic methanogen able to use primary and secondary alcohols as hydrogen donors, was grown on ethanol. Per mol of methane formed, 2 mol of ethanol were oxidized to acetate. In crude extract, an NADP⁺-dependent alcohol dehydrogenase (ADH) with a pH optimum of about 10.0 catalyzed a rapid (5 μmol/min·mg protein; 22°C) oxidation of ethanol to acetaldehyde; after prolonged incubation also acetate was detectable. With NAD⁺ only 2% of the activity was observed. F₄₂₀ was not reduced. The crude extract also contained F₄₂₀: NADP⁺ oxidoreductase (0.45 μmol/min·mg protein) that was not active at the pH optimum of ADH. With added acetaldehyde no net reduction of various electron acceptors was measured. However, the acetaldehyde was dismutated to ethanol and acetate by the crude extract. The dismutation was stimulated by NADP⁺. These findings suggested that not only the dehydrogenation of alcohol but also of aldehyde to acid was coupled to NADP⁺ reduction. If the reaction was started with acetaldehyde, formed NADPH probably reduced excess aldehyde immediately to ethanol and in this way gave rise to the observed dismutation. Acetate thiokinase activity (0.11 μmol/min·mg) but no acetate kinase or phosphotransacetylase activity was observed. It is concluded that during growth on ethanol further oxidation of acetaldehyde does not occur via acetylCoA and acetyl phosphate and hence is not associated with substrate level phosphorylation. The possibility exists that oxidation of both ethanol and acetaldehyde is catalyzed by ADH. Isolation of a Methanobacterium-like strain with ethanol showed that the ability to use primary alcohols also occurs in genera other than Methanogenium.