Geochemically generated, energy-rich substrates and indigenous microorganisms in deep, ancient groundwater

Recent studies have shown that the biosphere extends to depths that exceed 3 km, raising questions regarding the age of the microbes in these deep ecosystems and their sources of energy for metabolism. Abiogenic energy sources that are derived from in situ, purely geochemical sources and thus independent from photosynthesis have been suggested. We sampled saline fracture water emanating from a 3.1-km deep borehole in a Au mine in the Witwatersrand Basin of South Africa and characterized the chemical constituents (including stable isotopes), groundwater age, and indigenous microorganisms. Salinity data and ratios of dissolved noble gases indicate that extremely ancient (2.0 Ga) saline fracture water has mixed with meteoric water to yield an average subsurface residence time of 20-160 Ma, the oldest age of any waters collected to date in the Witwatersrand Basin. H2 isotope data suggest the water originated from a depth of 4 to 5 km. Sulfur isotope fractionation indicates biological sulfate reduction. Calculations of free energies and steady state energy fluxes based on water chemistry data also support sulfate reduction as the dominant terminal electron accepting process. Lipid and flow cytometry data indicate a sparse microbial community (103 cells ml-1), despite the presence of relatively high concentrations of energy-rich compounds (H2, CH4, CO, ethane, propane, butane, and acetate). The H2 can be explained by radiolysis of water. Stable isotopic signatures of the CH4 and short chain hydrocarbons indicate abiogenic synthesis. The persistence of energy-rich compounds suggests that other factors are limiting to microbial metabolism and growth, e.g., availability of an inorganic nutrient, such as Fe or phosphate. Copyright © Taylor & Francis Inc.