Evidence of metabolism and metabolic potential in the terrestrial deep subsurface biosphere

Deep biosphere ecosystems have earned focused attention in the last decade. Interest stems from recently improved estimates of biomass in the subsurface, which vary and derive primarily from studies of marine systems. It is becoming clear, however, that the terrestrial deep biosphere is equally worthy of notice, likely hosts considerable biomass planet-wide, and has directly influenced surface processes and global biogeochemical cycles throughout Earth’s history. Conditions in the terrestrial deep biosphere typically combine one or more environmental ‘extremes.’ These challenges, such as extremes of temperature, pH, and anaerobicity, may engender the subsurface with additional environmental pressures like low nutrient concentration and flux, and limited energy availability.

This Research Topic provides a platform for collecting contributions from diverse disciplines pertinent to terrestrial deep biosphere ecosystems. We consider the deep terrestrial biosphere to include non-marine ecosystems that are partially or fully closed to photosynthetically derived energy and carbon. As such, studies involving environments such as hot and cold springs, aquifers, deep fracture fluids, cave and mine systems, and sub-ice habitats can all provide insight. We seek to expand the previous research topic on the terrestrial deep biosphere (Terrestrial Portals into the Deep Biosphere) by focusing here on work that illuminates sources and uses of energy and carbon, and biogeochemical cycling in these systems. For example, energy flux and elemental cycling in the deep biosphere are still virtual unknowns. Rates of carbon fixation vs. availability and use of organic carbon in metabolic processes are undefined. We seek submissions that explore these topics with a creative use of a wide variety of tools such as isotope geochemistry, biomarker analysis, and surveys of taxonomic, genomic, and functional diversity that are directly tied to geochemical metadata.