| Summary: | Thermokarst (thaw) lake sediments are natural chronosequences presenting an outlook of future changes for one of the largest terrestrial carbon stocks in an ever warming Earth. Permafrost thawing and microbial breakdown of organic matter is expected to release a large fraction as methane, a potent greenhouse gas. This feedback can accelerate climate change, but potential mitigating processes in anaerobic environments remain uncertain. Here we present stable isotope methane measurements and tracer incubations that identify sediment layers with active anaerobic oxidation of methane. By using high-resolution sequencing of phylogenetic marker genes we show that these layers exhibit microbial assemblages responsible for scavenging methane in deeply thawed permafrost sediments beneath a high methane emitting thermokarst lake in central Alaska. Quantification of responsible methane oxidizers using specific mcrA primer revealed numbers equaling the remaining microbial community in the same sediment layers. Calculating of isotopic fractionation revealed the potential microbial oxidation of 40 to 80% of in situ methane at the deep permafrost thaw front. Altogether our results shows that in deeply thawed subaquatic permafrost with near 0°C temperature anaerobic oxidation of methane takes place, which is important to understand the global methane budget.
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