Methanogenesis by CO₂ reduction dominates lake sediments with different organic matter compositions

Abstract. Microbial methane production is a respiration reaction involved in the terminal step of anaerobic degradation of organic matter. Due to the dependency of methanogenic substrate production on fermentation reactions that produce different end productions, different sources and compositions of organic carbon (OC) may impact the methanogenic potential in lake sediments. 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 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. No clear effect of OC source on methane production rates was evident. Our data demonstrate that OC of diverse sources and diagenetic states support microbial methane production, but 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.