The deep biosphere of the Witwatersrand Basin, Republic of South Africa

The Witwatersrand Basin in the Republic of South Africa is a geological region that is host to the largest gold deposits on Earth. These mines, scattered thoughout the area, are the deepest in the world. They allow unprecedented access into the underground environment, facilitating the study of the terrestrial deep subsurface biosphere. Exploratory boreholes within these mines create opportunities for sampling in situ microbial populations and offer windows into the subsurface. Besides bacteria, analysis with Time of Flight---Secondary Ion Mass spectrometry has revealed a mobile hydrocarbon phase which resembles the carbon seams found within the basin. The presence of biomarkers within these hydrocarbons (i.e., hopanes) supports the theory that the hydrocarbons formed from the burial and subsequent diagenesis of the ancient biosphere (∼2.9--2.7 Ga). A freshly intersected water-bearing fracture zone was sampled, providing an opportunity to examine the natural, sessile, deep subsurface biosphere. The fracture, intersected by an advancing tunnel, was colonized by highly dispersed individual bacteria or by microcolonies containing up to 5 cells, with an overall cell density of 5 x 104 bacteria cm-2. This 'biofilm' population, although low, was 2 orders of magnitude greater than the bacteria present within the aqueous phase and provides the first direct observation of the sessile population from the terrestrial deep subsurface. When subsurface waters enter the mining environment they invariably become host to large, often metre scale, biofilms. Examination of these mine-slimes by electron microscopy revealed a new bacterial morphotype: a novel, branching, filamentous, star-shape bacterium. Geochemical modeling of these mine-slimes shows that their internal structure is governed by the diffusion of dissolved substrates. This model suggests that oxygen from the mining environment is quickly consumed in the upper layers of the biofilm opening up the lower regions to anaerobic metabolism. Mine-slimes are generally considered to be microbiologically compromised, subsurface samples due to the likelihood of contamination from the mining environment. However, careful examination of these biofilms has demonstrated that they can possess diverse bacterial population, including organisms consistent with the deep subsurface, suggesting that mine slimes represent an underutilized, 'natural' bacterial enrichment.;Keywords: Deep Subsurface Biosphere; Biofilms; Witwatersrand Basin; Mine-slimes...