Permafrost 16S Raw sequence reads

Permafrost, perennially frozen earth material, is an environment where microbes must survive oligotrophic conditions, sub-zero temperatures, low water availability, and high salinity over millennia. Viable microbial life has been found in permafrost tens of thousands of years old. However, we know little about the metabolic and physiological strategies utilized by microbial communities in response to the challenges presented by life in frozen ground over geologic time. In this study we asked if age and the associated stressors have accumulating effects causing adaptive changes in community composition and function. We conducted deep metagenomic and 16S rRNA gene sequencing across a Pleistocene permafrost chronosequence ranging in age from 16,000 - 33,000 years before present (kyr). We found age-related differences in community composition and diversity that may reflect adaptation to life in permafrost over four millennia. Reconstruction of paleovegetation from metagenomic sequence suggests vegetation differences in the paleo record were not the cause for shifts in community composition and function. Rather, compositional and functional shifts were observed that were consistent with long-term survival strategies in extreme cryogenic environments. These include increased reliance on scavenging detrital biomass, horizontal gene transfer, chemotaxis, dormancy, environmental sensing, and stress response. Our results identify functional traits that may enable long-term survival in ancient cryoenvironments with no influx of energy or new materials. This has ramifications for the search for life on other planets—by understanding how microbial communities adapt to the stresses associated with frozen conditions over geologic time we may gain insight into survival strategies life (if it exists) might use on other cryogenic bodies.