EXTREMELY SLOW GROWTH OF SULFATE REDUCING BACTERIA IN DEEP SUBSURFACE ROCKS

The spatial distribution and metabolic activity rate of extremely small sulfate reducing bacteria (SRB) inhabiting low porosity quartzite rock cores from ~1.8km below land surface within the Witwatersrand Basin, South Africa, was determined by microautoradiography. A dry reef intersection was sampled and a succession series of cores approaching a moderately thermophilic, fluid-filled fissure were also studied, utilizing two different tracers for contamination controls. Negative controls showed little or no sulfate reduction activity in the pristine inner portions of freshly pared core-faces. Evidence for activity of SRB was observed from both sampling locations, with positive results yielding millimeter-scale ‘hotspots' of trapped radioactive sulfide on silver foils. Colony morphology of the SRB responsible for hotspot formation was determined by scanning electron microscopy (SEM) with an associated electron dispersive X-ray (EDX) on the core-face. Isolated microcolonies (~40µm diameter) of extremely small rod-shaped microorganisms and associated filamentous organic matter were observed. Cell volumes were calculated to be on the order of 1.4x10^-3µm³, barely within the lower limits necessary to harbor DNA. The intensity of silver foil hotspots was measured and correlated to a per cell metabolic activity rate: an average rate of 23 fmol_sulfide cell^-1 day^-1 was calculated. This slow rate of growth implies an in situ state of senescence or maintenance metabolism.