MICROBIAL FE, MN AND AMMONIUM OXIDATION IN THE DEEP TERRESTRIAL SUBSURFACE AT HENDERSON MINE, CO

Deep subsurface crustal environments may be biomes dominated by chemolithoautotrophic microorganisms sustained by water-rock interaction. However, many of the feedbacks between rock mineralogy, aqueous geochemistry, microbial abundance, phylogeny and activity are not yet well constrained. At the Henderson Mine, CO, large co-variations in fluid chemistry and 16S rRNA gene libraries occur over small spatial scales in warm (40C), circumneutral and anoxic fluids circulating >5000 ft below the surface within fractures that intersect Precambrian granites and the Tertiary molybdenum orebody. The rock-dominated fluids are rich in sulfate (48 mM), fluorine (12 mM), dissolved metals (Fe, Mn and Al at 1-20 mM), and dissolved bicarbonate (30 mM), and micromolar quantities of methane, ammonium, nitrate and nitrite have also been detected. Thermodynamic assessments, culturing efforts and synchrotron-based investigations of Fe and Mn speciation indicate that extensive microbial metal-oxidation occurs at oxic-anoxic interfaces. In particular, we will demonstrate that Fe(II) oxidation may be a key metabolism for abundant Ralstonia sp.. We also suggest that fixed nitrogen in the Henderson fluids has a biological rather than geological source. FTIR investigations reveal little N incorporated within minerals such as biotite, whereas genes for biological N fixation (e.g. nifH) are present. A subsurface nitrification pathway may also be active at Henderson: Crenarchaea that possess the amoA gene encoding for the ammonia-oxidizing enzyme are present in high (>100 micromolar) ammonium boreholes, and Nitrospira that possess the nxrB gene capable of oxidizing nitrite to nitrate are also abundant. Altogether, the oxidation of Fe, Mn and ammonium represent widespread microbial activities not yet extensively explored in the deep subsurface.