Organic acids and ethanol inhibit the oxidation of methane by mire methanotrophs

Aerobic methane (CH4) oxidation reduces the emission of CH4 from mires and is regulated by various environmental factors. Organic acids and alcohols are intermediates of the anaerobic degradation of organic matter or are released by plant roots. Methanotrophs isolated from mires utilize these compounds preferentially to CH4. Thus, the effect of organic acids and ethanol on CH4 oxidation by methanotrophs of a mire was evaluated. Slurries of mire soil oxidized supplemental CH4 down to subatmospheric concentrations. The dominant pmoA and mmoX genotypes were affiliated with sequences from Methylocystis species capable of utilization of acetate and atmospheric CH4. Soil slurries supplemented with acetate, propionate or ethanol had reduced CH4 oxidation rates compared with unsupplemented or glucose-supplemented controls. Expression of Methylocystis-affiliated pmoA decreased when CH4 consumption decreased in response to acetate and was enhanced after acetate was consumed, at which time the consumption of CH4 reached control levels. The inhibition of methanotroph activity might have been due to either toxicity of organic compounds or their preferred utilization. CH4 oxidation was reduced at 5 and 0.5 mM of supplemental organic compounds. Acetate concentrations may exceed 3 mM in the investigated mire. Thus, the oxidation of CH4 might decrease in microzones where organic acids occur.