Contrasting variable and stable subsurface microbial populations: an ecological time series analysis from the Deep Mine Microbial Observatory, South Dakota, USA

The deep subsurface contains a vast reservoir of microbial life. While recent studies have revealed critical details about this biosphere including the sheer diversity of microbial taxa and their metabolic potential, long-term monitoring of deep subsurface microbial populations is rare, thus limiting our understanding of subsurface microbial population dynamics. Here we present a four-year time series analysis of subsurface microbial life from the Deep Mine Microbial Observatory (DeMMO), Lead, SD, USA. We find distinct and diverse populations inhabiting each of 6 sites over this ~1.5 km deep slice of terrestrial crust, corresponding to distinct geochemical habitats. Alpha diversity decreases with depth and beta diversity measures clearly differentiate samples by site over time, even during substantial perturbations. Population dynamics are driven by a subset of variable (and often relatively abundant) OTUs, but the vast majority of detected OTUs are stable through time, constituting a core microbial community. The phylogenetic affiliations of both stable and variable taxa, including putative sulfate reducers, methanogens, spore formers, and many uncultivated lineages, are similar to those found previously in subsurface environments. This work reveals the dynamic nature of the terrestrial subsurface, contributing to a more holistic understanding than can be achieved when viewing shorter timeframes.