DCO Project Summary

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Project Title
South African Terrestrial Deep Subsurface
Start DateEnd Date
2012-01-01 2013-12-31
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Recent studies of the continental subsurface microbial ecosystem present in the Witwatersrand Basin, South Africa have shown that with increasing depth and fracture water age and salinity, biogenic methane diminishes and abiogenic hydrocarbons and H2 increase and the concentration of planktonic cells slowly declines. Similarly, studies of the 16S rRNA gene in the planktonic community revealed a decrease in the relative abundance of methanogens and an increase in the relative abundance of low G+C Firmicutes with increasing depth.   Metagenome analyses of one member of this Firmicutes community, D. Audaxviator, indicated that it was capable of both heterotrophic and chemoautotrophic activity. An in situ incubation experiment suggests that some Firmicutes are acetogens that may support the aceticlastic methanogens.

Our geochemical and isotopic analyses indicate that abiogenic hydrocarbons are not utilized by the microbial community. Instead, a chemoautotrophic ecosystem is sustained by radiolytic generation of H2 and oxidants. Abiogenic hydrocarbon gases and elevated H2 have also been reported from the Canadian and Fennoscandian Precambrian shields and from ocean floor vents. The subsurface ecosystem present in the Witwatersrand Basin, therefore, is potentially wide spread in both the continental and marine crust, yet quite distinct from continental sedimentary basins or coastal margin sea floor sediments where degradation of photosynthetically produced organic matter deposited within the strata is believed to limit the extent of the subsurface microbial ecosystem. Alternatively, organic acids, thermogenically produced ~2 by ago from the thin shale layers present in the Witwatersrand Basin, could be sustaining the microbial community observed today.

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