Mesophilic syntrophic acetate oxidation during methane formation in biogas reactors

Abstract

The reaction pathway for the formation of methane from acetate was investigated in sludge from 13 different biogas reactors. By following the conversion of [2-14C]acetate and [14C]bicarbonate it was shown that methane formation by syntrophic acetate oxidation was the dominating mechanism for acetotrophic methanogenesis in sludge containing high levels of salts, mainly ammonium, and volatile fatty acids. In one biogas reactor the degree of syntrophic acetate oxidation increased as the concentrations of salts and volatile fatty acids increased. Statistical evaluation with principal component analysis showed a close correlation between the degree of syntrophic acetate oxidation and the concentrations of ammonium and potassium. The acetate degradation rate was lower (10–800 times) in biogas reactors in which methane was formed through syntrophic acetate oxidation than in biogas reactors where acetate was directly cleaved to methane and carbon dioxide. Microscopic observations revealed a predominance of one type of methanogen in the reactors with syntrophic acetate oxidation. Isolation and characterization, including substrate spectrum, mol% G+C, polyamine and 16S rRNA analysis, showed that the strains were hydrogenotrophic methanogens belonging to the genus Methanoculleus.