HYDROGEOCHEMICAL SUPPORT FOR MICROBIAL HABITABILITY IN AN ANCIENT, HYPERSALINE, THERMAL, AND RADIOGENIC SUBSURFACE BRINE IN SOUTH AFRICA

Abstract

Fractured rock aquifers within the continental crust can harbor low nutrient environmentsat great depth (>2 km) that are largely isolated from surface photosphere input. Depending on the extent of isolation, deep fluids may retain large quantities of abiotic reduced gas species (e.g. H2 and CH4), high temperatures, and in regions of low permeability, brine-level salinities. Microbial biomass is often low in deep systems, with estimates of 10-10^3 cells/mL in previously characterized Precambrian Shield fluids. Abiotic processes may heavily influence or dominate biotic signatures in these systems, but due to their low biomass and relative inaccessibility, habitability and microbiology of deep fracture fluids remain poorly characterized. To constrain abiotic-biotic interactions and how they promote habitable conditions, the hydrogeologic history, carbon and redox availability, and microbial diversity for deep fluid systems must be considered. This dissertation examines the above parameters through characterization of an ancient (1.2-Ga), high temperature (45 – 55 ℃), and radiogenic hypersaline brine system at 2.9 – 3.2 km depth in Moab Khotsong gold and uranium mine, located in the Witwatersrand Basin of South Africa. Major findings included the significant contribution of water-rock interactions (alpha particle-driven water radiolysis, silicate hydration, and isotopic exchange with fracture lining minerals) to salinity formation, stable isotopic composition, and redox species availability. Characterization of brine carbon revealed a large dissolved organic matter pool composed of old biotic, recalcitrant organics that experienced extensive alteration by radiolytic oxidation and abiotic hydrocarbon contribution. Energetically feasible microbial metabolic strategies, modeled under redox and carbon conditions of the brine, included anerobic/aerobic heterotrophy, nitrate reduction, and thiosulfate oxidation despite maintenance costs imposed by high salinity, temperature, and radiolysis. These metabolic and stress defense strategies complemented current microbial community composition, revealed through taxonomic identification and metabolic annotation of single cell genomes. This accumulated effort revealed the Moab Khotsong brine as a novel, ancient fluid system where dominance of water-rock interaction, primarily in the form of radiolysis, provides energetic support for low biomass microbial life despite high abiotic stress exposure over geologic time in the subsurface.