Abrupt permafrost thaw triggers microbial bloom and increased activity of microbial predators

Abstract Permafrost soils store a substantial part of the global soil carbon and nitrogen. However global warming causes abrupt erosion and gradual thaw, which make these stocks vulnerable to microbial decomposition into greenhouse gases. Here, we investigated the microbial response to abrupt in situ permafrost thaw. We sequenced the total RNA of a 1 m deep soil core consisting of up to 26’500-year-old permafrost material from an active abrupt erosion site. We analysed the microbial community in the active layer soil, the recently thawed, and the intact permafrost and found maximum RNA:DNA ratios indicating a microbial bloom in recently thawed permafrost. Several fast-growing prokaryotic taxa dominated thawed permafrost, including Sphingobacteriales, Burkholderiales, and Nitrosomonadales. Overall, the thaw state and soil moisture consistently explained changes in community composition, with especially the permafrost community being significantly distinct from thawed soils. Predation correlated with changes in prokaryotic composition. Bacterial grazers were dominated by Myxococcales and abundant in the active layer. In contrast, protozoa, especially Cercozoa and Ciliophora, doubled in relative abundance in thawed layers. Our findings highlight the ecological importance of a rapid development of microbial blooms as well as the successive predation as biological control mechanism in abruptly thawing permafrost. One sentence summary Using total RNA from an up to 26’500-year-old abruptly eroding permafrost site in Greenland, we described a microbial bloom and its controls, including bacterial and microeukaryotic predators. Abstract Figure