A stable isotope titration method to determine the contribution of acetate disproportionation and carbon dioxide reduction to methanogenesis

A novel stable isotope titration approach was developed to determine the contributions to total methane production made by CO2 reduction and the disproportionation of acetate in anoxic environments. 13CH4, 12CH4, 13CO2 and 12CO2 production rates were measured in the head space of replicate anaerobic microcosms titrated with increasing amounts of 13C-labelled substrates. The contribution of CO2 reduction was calculated from the linear relationship between ratios of labelled and total CH4 production and ratios of labelled and total CO2 after the addition of 13C-bicarbonate. In the case of acetoclastic methanogenesis rates of 13CH4 and 12CH4 production were fitted to a model based on an assumption that the relationship between the concentration of 13C-labelled acetate and the rates of labelled and unlabelled methane production followed Michaelis–Menten kinetics. A comparison of the raw data with the model supported the assumption and provided both an estimate of the contribution of acetate to methane production and an estimate of the size of the indigenous acetate pool without the need to measure acetate directly. The method was applied to a freshwater sediment in the English Lake District where it was found that 66.3% (se 4.9) of methane production was due to acetate disproportionation and 28.9% (se 1.9) of methane production resulted from CO2 reduction. This is in agreement with theoretical predictions and other empirical measurements of methanogenesis.