Methanogenesis by CO ₂ reduction dominates lake sediments with different organic matter compositions

Abstract. Microbial methane production is a key reaction involved in the terminal step of anaerobic degradation of organic matter. The energy substrates of methane-producing microorganisms are largely generated during the breakdown of larger organic molecules by fermentative microorganisms, wherein the products of fermentation may vary with the chemical compositions of these larger molecules. Due to differences in energy substrates among methane-producing microorganisms, it is thus possible that organic matter compositional variations select for different communities of methane producers. Here, we investigate the sources and compositions of OC in sediments of Lake Geneva and how both are potentially linked to methane production. Differences in dominant long-chain fatty acid abundances and carbon isotopic compositions suggest the predominance of diagenetically altered phytoplankton-derived OC at a profundal site (PS) and temporally highly variable sources of both aquatic and terrestrial OC in a deltaic location. Despite these differences, radiotracer-based methanogenesis rate measurements and stable isotopic signatures of methane indicate significant methane production that is dominated by CO2 reduction (>95 % of total methanogenesis) in both locations. Matching this interpretation, members of well-known CO2-reducing Methanoregula sp. dominate both sites. Similarly, no clear effect of OC source on methane production rates was evident. Our data demonstrate that OC of diverse sources and diagenetic states supports microbial methane production, but the data do not indicate a clear impact of the OC source on the dominant methanogenic pathway or the community structure of methanogenic microorganisms in lacustrine sediments.