Deep subsurface biodiversity: South African deep gold mines

Microbial populations in the deep subsurface extend to depths below where nutritional and environmental parameters limit the ability of microbes to survive. Highly diverse microorganisms occur in expanses where carbon sources are not evident. These studies of the deep subsurface exposed valuable information of the ecology of microbes, the chemical and geological factors associated with these thriving and diverse microbial populations and insight was gained into their metabolism using genomic and proteomic tools.

Low cell mass samples were concentrated by either massive or tangential flow filtration systems. Biodiversity studies were done using next generation sequencing as well as clone libraries, operational taxonomic units (OTU) were calculated using distance based and richness parameters. Microbial isolations were attempted using selective media followed by pyrosequencing and annotation analyses. Proteome analyses were completed on various proteins from subsurface microbes. Each geological setting provides new insight into its particular associated geochemistry and how it influences the biodiversity. With increasing depth and temperature lower diversity is recovered, however several sites reveal surprisingly high interspecies diversity.

We extend the current knowledge on subsurface cycling by comparing anaerobic and chemolithotrophic metabolic reactions to indicate that SO₄^2- reduction is the dominant metabolic process at high and low salinities and that matrix diffusion can sustain planktonic fracture microbial concentrations of ~10⁸ cells mL^-1. Methanogenesis becomes more dominant with declining salinities. Neither acetogenesis nor syntrophic degradation of abiogenic hydrocarbons appears to be the source of the observed carboxylic acids, but regardless respiration leads to direct in situ carbonation within the fractures.