Capture of Planktonic Microbial Diversity in Fractures by Long-Term Monitoring of Flowing Boreholes, Evander Basin, South Africa

Abstract

The diversity of planktonic microorganisms in fluids from a group of flowing subterranean boreholes was monitored from the day they were drilled to as long as three and a half months after drilling as they drained into Evander Au mine. Geochemical analyses of the water, characterization of microbial communities by phospholipids fatty acid (PLFA) and DNA sequence analyses, and calculations of free energy flux indicated that mine-introduced microbial contaminants, dominated by β and γ Proteobacteria, Cenarchaeaceae and Candidatus Nitrososphaera, were flushed from the boreholes and replaced by fracture water derived microbial communities dominated by Firmicutes, Methanosarcinalesand Thermoproteaceaea. The fracture water was a mixture of paleometeoric water and 2.0 Ga old, diagenetically altered, hydrothermal fluid. The C and H isotopic data for C₁₋₄ indicated that the CH₄ was primarily abiogenic in origin although ∼35–50% of it might have originated from microbial methanogenesis. Noble gas analyses yielded estimated residence times of some 10 million years for the fracture water, which is estimated to represent a capture cross-section of 0.25–0.50 km². The 16S rRNA and dsrAB gene sequences indicated that the indigenous bacterial communities were predominantly comprised of sulfate reducers belonging to the genera Desulfotomaculum, Candiditus Desulforudis and Desulfofustis. The sulfur isotopic analyses of sulfate and sulfide yielded fractionation Δ³⁴S values ranging from 16 to 22% consistent with microbial sulfate reduction. Thermodynamic analyses indicate that methanogenic reactions are inhibited by the high partial pressure of abiogenic CH₄ and that sulfate-reducing reactions are more favorable, which is consistent with the abundance of 16S rRNA genes belonging to known sulfate reducing bacteria. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental files.

Keywords:

• 16S rRNAphylogeny
• dsrAB
• sulfate reduction
• methanogenesis
• isotope geochemistry

Acknowledgments

This research was supported by National Science Foundation LExEn program grant EAR-9978267 to T.C. Onstott, the NASA Astrobiology Institute grant to L.M. Pratt of Indiana University, by NSERC Discovery and Canada Council Killam Research funding to B. S. Lollar, and by the National Science Foundation Continental Dynamics program grant EAR 0409605 to Zeev Reches, University of Oklahoma. Special thanks to Harmony Gold Mine for permission to collect samples and publish these results.