Hydrogen-independent CO reduction dominates methanogenesis in five temperate lakes that differ in trophic states.

Emissions of microbially produced methane (CH) from lake sediments are a major source of this potent greenhouse gas to the atmosphere. The rates of CH production and emission are believed to be influenced by electron acceptor distributions and organic carbon contents, which in turn are affected by anthropogenic inputs of nutrients leading to eutrophication. Here, we investigate how eutrophication influences the abundance and community structure of CH producing and methanogenesis pathways across time-resolved sedimentary records of five Swiss lakes with well-characterized trophic histories. Despite higher CH concentrations which suggest higher methanogenic activity in sediments of eutrophic lakes, abundances of methanogens were highest in oligotrophic lake sediments. Moreover, while the methanogenic community composition differed significantly at the lowest taxonomic levels (OTU), depending on whether sediment layers had been deposited under oligotrophic or eutrophic conditions, it showed no clear trend in relation to distributions of electron acceptors. Remarkably, even though methanogenesis from CO-reduction was the dominant pathway in all sediments based on carbon isotope fractionation values, taxonomic identities, and genomes of resident methanogens, CO-reduction with hydrogen (H) was thermodynamically unfavorable based on measured reactant and product concentrations. Instead, strong correlations between genomic abundances of CO-reducing methanogens and anaerobic bacteria with potential for extracellular electron transfer suggest that methanogenic CO-reduction in lake sediments is largely powered by direct electron transfer from syntrophic bacteria without involvement of H as an electron shuttle.