Microbial community dynamics following riverbank erosion across permafrost floodplains in the Yukon River basin

Much of the organic-rich permafrost deposits in the Arctic lie in riverine floodplains where thaw due to polar amplification of climate change has accelerated bank erosion, leaving permafrost carbon deposits vulnerable to degradation by microorganisms. As sediments harbored in the riverbank erode, they are subjected to sediment transport processes, interact with the water column, and are eventually re-deposited in barforms on an opposing riverbank and incorporated to build new land with ensuing forest succession.  Using a space-for-time substitution across these deposits and a suite of culture-independent amplicon and shotgun metagenomic sequencing of samples with ages from modern to several thousand years old collected from the Yukon River and its major tributary the Koyukuk River, we set out to understand microbial community succession associated with this process, and connect this with rates of carbon cycling in the subsurface.  Because dioxygen is a special molecule concerning the fate of organic carbon in soils and sediments, we also developed a useful ’sequencing-as-sensing’ approach that leverages recent developments in protein language models to assess the time-integrated fraction of the microbial community capable of aerobic biology and oxidative attack of extracellular organic matter.  Results revealed that permafrost deposits operate as a ‘seed bank’ that generates a pattern of succession toward an aerobic community capable of rapid carbon degradation during erosion and transport—a pattern that may help explain why carbon burial in river floodplains is so efficacious.