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).