Mineralogical, chemical and biological characterization of an anaerobic biofilm collected from a borehole in a deep gold mine in South Africa

A biofilm sample was collected from an anaerobic water and gas-flowing borehole, 1.474 km below land surface in the Evander Au mine, Republic of South Africa. The biofilm was 27 wt% ZnS, which was ∼2 × 10⁷times more concentrated than the dissolved Zn measured in the borehole water. X-Ray diffraction indicated that the Zn was present in the form of fine grained, 4.7 ± 0.9 nm particles with smaller amounts of pyrite (FeS₂). Scanning electron microscopy, coupled with energy-dispersive X-ray spectroscopy confirmed the identity of these minerals in the biofilm. Using transmission electron microscopy, the fine-grained ZnS minerals were found to coat the 1 μm-diameter rod-shaped bacteria that made up the primary substructure of the biofilm. The FeS₂ was present as framboids (spherical aggregates of 0.5-1 μm FeS₂ crystals) up to 10 μm in diameter and as large, 2-3 μm euhedral crystals that were not nucleated on the bacterial surfaces, but were found within the biofilm. Analyses of 16S rDNA utilizing clone libraries and a phylochip indicates that the ZnS rich biofilm is dominated by methanogens with a significant sulfate-reducing bacterial population and minor sulfide and CH₄-oxidizing chemolithotrophs. This biofilm community is sustained by sulfate, bicarbonate and H₂-bearing paleometeoric water.