Anaerobic oxidation of methane does not attenuate methane emissions from thermokarst lakes

The ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene-aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice-rich permafrost thaw. In cores of surface (similar to 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4 degrees C) and high (16 degrees C) temperatures. Methanogenesis rates were measured by the accumulation of methane over similar to 90 d, whereas AOM rates were measured by adding labeled-(CH4)-C-13 and measuring the produced dissolved inorganic C-13. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility ("back-flux") during CH4 production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1-m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.