Sediment chemistry controls methane emissions from lake littoral zones

Methane emissions from the nearshore zones of lakes are relatively large but can vary by several orders of magnitude. Here we compared predictions for how sediment chemistry and microbial communities influenced methane emissions from 19 littoral sites in the UK varying in organic matter sources and microbial composition. Our approach was to compare multiple predictions to explain methane fluxes from sediment chemistry and microbial composition using path analysis. We found that the prediction that organic matter composition, namely the concentration of polyphenolics, controls methane emissions by changing electrochemical conditions to favour certain methanogen taxa was hundreds of times better supported than predictions involving abundances of all methanogens and methanotrophic bacteria, methanogen diversity, or other physicochemical conditions. Diffusive CH₄ fluxes were estimated to increase by 3.1– to 16.6-times (95% confidence interval) with increasing polyphenolic concentrations, almost entirely because they lower reduction-oxidation potentials that shift methanogen composition towards widespread taxa positively associated with methanogenesis. Rather than strongly inhibiting methane-producing microorganisms, our results suggest polyphenolics change reduction-oxidation potentials to favour acetoclastic and methylotrophic methanogens. These results help explain conflicting evidence about the responses of methane to sediment chemistry and can improve future predictions of aquatic carbon cycling.