MICROBIAL CARBON CYCLING IN THE DEEP BIOSPHERE OF THE KAAPVAAL CRATON, SOUTH AFRICA

Several important questions regarding the deep continental biosphere remain largely unanswered, including: 1) What factors control the lowest depth limit of life? 2) What are the sources of carbon? 3) What processes regulate the energy? and 4) Which organisms are actively recycling C and which are inactive, e.g., spores? We have been sampling ultradeep fracture water (1-4 km) in the Kaapvaal Craton of South Africa via boreholes in gold, diamond, and platinum mines. We have found that 1) deep, ancient fracture waters contain diverse, indigenous microbial communities with cell abundances of ~10^3-4 cells/mL, 2) cell turnover times are ~1-2 years at 3 km and 55°C, 3) deep saline fracture waters, ~2.0 Ga, are rich in radiolytically generated H₂ and abiogenic hydrocarbons, 4) H₂ flux is accentuated by tectonic activity, 5) CH₄ and CO₂ are the primary C sources for biomass, and 6) more complex organic C appears to be generated in situ by microbes growing on these energy sources and carbon substrates. Culture-independent analyses (clone libraries and pyro-tag sequencing of 16S rDNA, metagenomic sequencing, metatranscriptomic sequencing, and single cell genome sequencing) have revealed indigenous, novel microbial communities with active metabolic and evolutionary processes. Firmicutes dominate, but may be present mainly as spores. Sulfate reduction and methanogenesis are the dominant terminal electron accepting processes, in some cases with ANME sequences indicating anaerobic CH₄ oxidation. Most novel microbial species are only found within a single fracture zone. Other microbes, including the sulfate-reducing firmicute Candidatus “Desulforudis audaxviator”, however, have been detected globally in other deep fracture waters, suggesting that very similar H₂-driven subsurface microbial ecosystems are actively cycling C and other elements. The next big questions to address in terrestrial deep biosphere research are the rates of microbial activity and evolution.