The Rock-Hosted Biosphere

Our understanding of Earth's rock-hosted subsurface biosphere has advanced over the past two decades through the collection and analysis of fluids and rocks from aquifers within the continental and oceanic crust. Improvements in cell extraction, cell sorting, DNA sequencing, and techniques for detecting cell distributions and activity have revealed how the combination of lithology, permeability, and fluid mixing processes controls the diversity and heterogeneous distribution of microbial communities in fractured rock systems. However, the functions of most organisms, and the rates of their activity and growth, remain largely unknown. To mechanistically understand what physiochemical and hydrological factors control the rock-hosted biosphere, future studies are needed to characterize the physiology of microorganisms adapted to mineral-associated growth under energy- and nutrient-limited conditions. Experiments should be designed to detect synergistic interactions between microorganisms, and between microorganisms and minerals, at highly variable turnover rates. ▪ The heterogeneous distribution of the rock-hosted biosphere is controlled by variations in porosity, permeability, and chemical disequilibrium. ▪ Several imaging and chemical techniques can sensitively detect microbial activity within the rock-hosted biosphere. ▪ The physiology and turnover rates of the subsurface rock-hosted biosphere remain poorly known.