The deep gold mines of South Africa: Windows into the subsurface biosphere

Recent investigations have identified microorganisms in various crustal environments to 2800 meters below the surface (mbls.). Relatively few deep samples of the continental crust (>800 mbls.) have been collected for microbiological analyses, however, because coring is technically difficult and expensive. The gold mines into the 2.9 Ga Witwatersrand Supergroup in South Africa may provide an alternative means of studying microbial communities at depths up to 3500 meters. Uranium-rich, Au-bearing, carbonaceous rock and water from a gallery borehole at a mined depth of 3200 mbls. and an ambient temperature of 50°C were collected for microbial analyses. Measures were taken to avoid contamination during mining and sampling. Samples were shipped to the U.S.A. in sterile, anaerobic canisters on ice, processed under sterile anaerobic conditions and distributed to participating labs. Microscopic observations revealed the presence of intact cells including filamentous microorganisms. Phospholipid fatty acid and DNA analyses indicated that the samples contain cyanobacteria, sulfate-reducing bacteria (SRB) and iron-reducing bacteria (IRB). The water sample yielded a strain of Thermus (IRB-SA) that is the first reported Thermus to reduce Fe(III) and the first facultative, thermophilic Fe(III) reducer.