Planktonic Microbial Communities Associated with Fracture-Derived Groundwater in a Deep Gold Mine of South Africa

The vertical distribution and function of terrestrial planktonic microbial communities at depths greater than 600 m remain poorly established. Culture-independent methods using 16S rRNA genes and geochemical approaches were employed to investigate the heterogeneity and potential function of microbial communities residing within fractures at 0.7 to 1.4 kilometers below land surface of Beatrix Au Mine, South Africa. The salinity (26 to 47 mM Cl−), temperature (33 to 40°C) and age (1 to 5 Ma) of these fracture water increased with depth. The δD and δ18O values of fracture water ranged from −44 to −39‰ and from −7 to −4‰ VSMOW, respectively, and exhibited a mixing trend with fracture water collected from the same mine in a previous study where isotopic signatures were indicative of hydrothermal origin. Fracture water from Beatrix Mine was distinct from the groundwater in the overlying Karoo sedimentary strata in terms of its Cl−, He and CH4 concentrations, and its δD and δ18O signatures and from Vaal River (s...