13C AND 14C INVESTIGATIONS OF BACTERIAL CARBON CYCLING IN THE DEEP SUBSURFACE NEAR THE TONO URANIUM MINE, TOKI, JAPAN

Biogeochemical transformations of carbon in deep subsurface aquifers are important but insufficiently understood.  Carbon cycling is closely coupled to redox conversions and mobility of other elements that affect subsurface nuclear waste storage and bioremediation.  Additionally, knowledge of subsurface carbon cycling is important for current hypotheses on the evolution of terrestrial and extraterrestrial life in the subsurface.  Oligotrophic aquifers, with bacterial doubling times on the order of hundreds of years, pose challenges for the application of standard microbiological techniques.  Therefore, we conducted ¹³C and ¹⁴C studies of phospholipid fatty acids (PLFAs), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved CH₄ to investigate carbon cycling in both sedimentary and granite aquifers near the Tono Uranium Mine, Toki, Japan.  CH₄ detected in KNA-6 borehole waters has a biogenic d¹³C signature (-95‰) and, based on DIC and DOC d¹³C values, is likely formed via CO₂ reduction rather than acetoclastic methanogenesis.  PLFA d¹³C results suggest that different bacterial metabolisms occur in KNA-6 waters versus other borehole waters studied.  d¹³C values (~ -30‰) from other borehole waters are indicative of heterotophic bacterial metabolisms.  d¹³C PLFA values as low as -60‰ and the detection of the type II methanotroph PLFA biomarker 18:1w8 signify aerobic methanotrophy occurring in KNA-6 waters.  A linear correlation between PLFA d¹³C values and 18:1w8 PLFA concentrations provides an estimated range of d¹³C values for type II methanotroph PLFAs (-73 to -76‰), which compares well with that expected given the local CH₄ and DIC d¹³C values.  O₂ in KNA-6 waters is below the detection limit, but type II methanotrophs can utilize very low O₂ tensions.  Whether some O₂ survives during groundwater transport from the surface or is generated in-situ via the radiolysis of water is unknown.   Results, however, are consistent with the possibility of methanogensis and aerobic methanotrophy coexisting in the subsurface and, perhaps, driving a carbon cycle dependent on radiolysis rather than photosynthesis.  ¹⁴C investigations are ongoing, but initial results indicate that carbon assimilated by KNA-6 bacteria (33 pMC) is distinct from that assimilated by bacteria in other boreholes (60 pMC).