In SituCultivation of Subsurface Microorganisms in a Deep Mafic Sill: Implications for SLiMEs

An in situ culturing device was incubated within a flowing borehole in a mafic sill at 1.474 km depth in Evander Au mine, South Africa. The device was designed to enrich methanogenic, Fe3 +-reducing and SO4 2 −-reducing microorganisms using acetate, formate, methanol, Fe3 +-citrate and SO4 2 − enriched agar and sand cartridges. At the end of the 33 day incubation geochemical analyses detected elevated H2, acetate, CH4 and Fe concentrations and depleted SO4 2 − concentrations. 16S rDNA sequences and PLFA analyses revealed that the microbial community composition of the substrate-bearing cartridges were distinct from that of the original borehole water and the non-substrate-bearing control cartridge. 16S rDNA and dissimilatory sulfite reductase, dsrAB, gene sequences indicated the device successfully targeted SO4 2 − reducing bacteria (SRB), which were not detected in the original borehole water. 16S rDNA sequences also revealed a shift in the microbial community from one relying on H2 based methanogenesis to one suggestive of H2 based acetogenesis supporting aceticlastic methanogenesis and SO4 2 − reduction compatible with the subsurface lithoautotrophic hypothesis.