Abundance and Viability of the Groundwater Microbial Communities from a Borehole in the Tono Uranium Deposit Area, Central Japan.

The abundance and viability of microorganisms in groundwater were studied using a scientific borehole in the Tono uranium deposit area, central Japan. Groundwater samples were collected from scientific borehole TH-6 at four depths; 104, 132 and 153 m in sedimentary rocks and 177 m in granite rock using autoclaved geochemical water samplers. The total cell count using epifluorescence microscopy was of the order of 105 to 106 cells ml-1, showing non-systematic change in microbial parameters with depth. Viability estimated from cell membrane integrity ranged from 22.3% to almost 100%, and showed a tendency to increase with depth. In contrast, viability determined using both activities of the electron transport system (ETS) and esterase showed inconsistent depth-profiles. The ETS-active cell count corresponded to 0.57-24.7% of the total count; the minimum ETS-count was found at 153 m, just above the sediment-granite unconformity where the minimum redox potential was estimated. The esterase-active cell count corresponded to 0.4-10.3% of the total, with a maximum at 132 m, the lignite-derived organic-rich layer, and a minimum was found at 153 m, as seen for the ETS-active counts. These inconsistent profiles suggest a difference in microbial viability and activity between each depth. In addition, depth-specific microflorae may develop in response to the availability of various electron acceptors and donors in the subsurface.