Abstract
The abundance and diversity of groundwater microorganisms were studied in a geochemically defined borehole (TH-6, DH-3) in the Tono area, central Japan. Total cell counts by epifluorescence microscopy were estimated to be approximately 1.6 × 105-5.7 × 106 cells ml -1 and showed little decrease with depth. Cell viability, based on cell membrane integrity, respiration-based metabolism, and esterase activity was estimated to be 0.001% to approximately 100% of the total counts. The distribution of microbial abundance seems to be related to a variety of environmental factors, including fracture numbers, hydrological, and geochemical conditions of the groundwater. The geochemistry of the groundwater suggests that sulfate reducing bacteria play an important role in the sedimentary rock environment. It appears that microbial sulfate reduction occurs at the highest level in the upper part of the Toki Lignite-bearing Formation. This microbial reaction may play an important role in maintaining anaerobic conditions for uranium fixation, that could in turn scavenge soluble and oxidized uranium. On the other hand, in the granite groundwater, iron-oxidizing/reducing bacteria seem to play an important role in iron redox cycling. Iron-oxidizing bacteria may contribute to the formation and deposition of iron colloids in the upper part of Toki granite.
