Role of interspecies H2 transfer to sulfate and ferric iron-reducing bacteria in acetate consumption in anoxic paddy soil

Abstract

Addition of sulfate resulted in complete inhibition of methanogenesis in anoxic paddy soil. About 20% of the CH4 was produced from H2/14CO2, the rest from acetate. Inhibition of H2-dependent methanogenesis was explained by successful competition by sulfate reducers for H2, as the H2 partial pressures decreased upon addition of sulfate. However, acetate concentrations did not decrease. Sulfate reduction was stimulated by H2, but not by acetate. Counts of acetate-utilizing sulfate reducers were relatively low both in fresh and pasteurized soil indicating that the bacteria were only present as spores. Inhibition of methanogenesis by chloroform resulted in accumulation of both H2 and acetate. When sulfate was added in addition, H2 accumulation stopped, but acetate still accumulated indicating that the activity of the methanogens was necessary for acetate conversion and that acetate could not be utilized by the sulfate reducers directly. Conversion of [2-14C]acetate resulted in formation of relatively more 14CO2, when sulfate was added, indicating that the methyl group of acetate was now being oxidized instead of reduced. Addition of chloroform strongly inhibited the conversion of [2-14C]acetate, even in the presence of sulfate. A conceivable explanation is sulfate-dependent interspecies H2 transfer between acetate-utilizing methanogens and H2-utilizing sulfate reducers, changing the electron flow from CH4 production to sulfate reduction. Addition of ferrihydrite resulted only in incomplete inhibition of methanogenesis which could be explained by successful competition of ferric iron reducers for H2. The ferric iron reducers were able to use acetate directly, but did not outcompete the methanogens.