INTRINSIC SOURCES OF INORGANIC NITROGEN AND PHOSPHORUS IN AN OLIGOTROPHIC, METHANOGENIC GROUNDWATER

In typical groundwater environments inorganic nutrients such as nitrogen (N) and phosphorus (P) are scarce and often decrease activity of the native microbial community. In near-surface or unconfined systems atmospheric or anthropogenic sources may serve as an input, but in confined environments it is assumed that these nutrients become limiting and are likely recycled through cellular material. In this study we examined dissolved inorganic and lithologic sources of N and P for a petroleum-degrading, anaerobic groundwater microbial consortium. Dissimilatory iron reducing bacteria, fermenting bacteria, and methanogens dominate the groundwater environment, which is limited with respect to P and N with no input from the atmosphere or upgradient groundwater. Using in situ field microcosms in the nutrient-limited, but carbon-rich groundwater we found that silicate minerals and glasses that contain P were preferentially colonized over similar silicates without P, while attached cells were absent on ammonium-bearing silicates. P content appears to drive preferential attachment by the microbial community and results in etching of colonized surfaces, while silicate-bound N has no discernable effect. Laboratory microcosm experiments using native consortia demonstrate that the cells not only colonize P-bearing silicates, but also utilize released P as evidenced by a significant increase in biomass. Fluorescent in situ hybridization using Bacterial and Archaeal-specific probes reveal that the surface-adherent microbial population is composed of methanogens and fermenting bacteria. The anaerobic consortium derives essential P from silicates while ammonium in silicate minerals and glasses has no influence on colonization or growth. The microbial community apparently derives N from dissolved N2; acetylene-spiked microcosms produce abundant ethylene demonstrating efficient N fixation, probably by methanogens. These results suggest that even in oligotrophic environments, native microorganisms can access adequate inorganic N and P from the immediate aquifer environment. This mode of nutrient acquisition lends insight into nutrient cycling in subsurface environments and natural attenuation.