| Summary: | Permafrost soils currently store about twice as much carbon as deposited in the atmosphere, and alone Arctic permafrost is estimated to capture 1850 Gt carbon. As these usually remain frozen in summer, often even for millennia, decomposition rates are low. With global warming though, both the seasonal thaw depth and abrupt thaw event frequencies increase, leading to bioavailability of now thawed soil carbon for remineralisation. Insights about microbial carbon sequestration and ecological cascades of functional groups in thawing permafrost is crucial for our understanding of Arctic greenhouse gas fluxes. In Zackenberg, Northeast Greenland, a thermal erosion valley was samples two years after its abrupt collapse. We used total RNA sequencing to reveal the putatively active community (rRNA) as well as its expressed functional genes (mRNA) present in the up to 26’500-year-old permafrost material. The impact of changing physicochemical soil parameters with depth, such as pH, age, soil moisture and organic matter content was compared to determine possible metabolic and community-level responses. A pronounced RNA/DNA ratio in freshly thawed permafrost layers indicated recently increased gene activity. While strongest driver of community variation was the age of the soil layers, their recent dynamic thaw patterns explained 60-90% of the variation in the dataset. Higher abundance of protozoa in especially one-year old as opposed to recently thawed material highlights the ecological succession of not only important prokaryotic taxa, but also their predators in short time scales.
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