Abstract

Microbial decomposition of thawed permafrost carbon in thermokarst lakes leads to the release of ancient carbon as the greenhouse gas methane (CH4), a potential greenhouse gas, yet potential mitigating processes are not understood. Here, we report on δ13C-CH4 enrichment in the pore water of a thermokarst lake sediment core that points towards in situ occurrence of anaerobic oxidation of methane (AOM). Analysis of the microbial communities showed naturally enriched CH4-oxidizing archaeal communities that occur in sediment horizons at temperatures near 0 °C. These archaea also showed high rates of AOM in laboratory incubations. Calculation of the stable isotopes suggests that 41 to 83% of in situ dissolved CH4 is consumed anaerobically. Quantification of functional genes (mcrA) for anaerobic methanotrophic communities revealed up to 6.7 ± 0.7 x 105 copy numbers g-1 wet weight and showed similar abundances to bacterial 16S rRNA gene sequences in the sediment layers with the highest AOM rates. We conclude that these AOM communities are fueled by CH4 produced from permafrost organic matter degradation in the underlying sediments that represent the radially expanding permafrost thaw front beneath the lake. If these communities are widespread in thermokarst environments, they could have a major mitigating effect on the global CH4 emissions.