| The dynamics of the subsurface biosphere of South Africa were characterized from a metabolic, nutritional, and energetic perspective. I surveyed the indigenous subsurface community by enriching for methanogenic, Fe3+-reducing, and SO42--reducing microorganisms using media-enriched agar/sand cartridges attached to a device inserted into a flowing borehole. The device concentrated organisms which allowed the successful amplification of the 16s rDNA signatures of bacterial and archaeal microorganisms phylogenetically similar to known methanogenic, sulfate reducing, and Fe reducing microbes. Geochemical measurements further supported the occurrence of proposed metabolic pathways and suggested that the environment was one in which H2-based acetogenesis supported acetoclastic methanogenesis and sulfate reduction. Upon identifying the microbial community constituents, the indigenous nature of their nutrient source came into question. Since Au mining utilizes explosives, the issue of N contamination was addressed by compositionally and isotopically characterizing the organic and inorganic N species within fracture water, pore water, fluid inclusion, and total rock samples from the Witwatersrand strata. A concentration gradient emerged such that fluid inclusions yielded the highest dissolved N species with pore and fracture water demonstrating lower concentrations. Explosive-derived mining water exhibited high NO 3- and NH4+ concentrations and yielded delta15N and delta18O NO 3- values falling outside the delta18O range observed within the fracture (20.4 to 34.0‰), pore (23.7 to 31.1‰), and fluid inclusion water (27.2-41.3‰). Such unexpectedly high delta18O values for non-industrially produced NO 3- could only be explained by the radiolytic oxidation of fluid inclusion NH3 combined with denitrification. With an indigenous nutrient and electron acceptor source identified, we investigated the thermodynamic and hydrologic viability of various metabolic pathways given a subsurface aqueous environment which derives its nutrient and energy substrates from the host rock and is subsequently diluted by meteoric recharge. Our mixing model incorporated hydrologic, radiolytic, and metabolic processes and showed good agreement with observed values. SO42- reduction was determined to be the dominant metabolic process at all salinities. Ambient carboxylic acid concentrations are likely the result of production via the step-wise oxidation of hydrocarbon gases and consumption by acetoclastic and formate utilizing metabolic reactions. Minimum ATP requirements are the limiting factor for subsurface microbes. |
