Ralstonia species mediate Fe-oxidation in circumneutral, metal-rich subsurface fluids of Henderson mine, CO

Deep subsurface oxic/anoxic interfaces within Henderson Mine, CO were investigated for the potential to support novel metal-oxidizing microorganisms. Ralstonia sp. were isolated from Fe-oxidizing enrichments inoculated with fracture fluids released through boreholes as well as Fe-oxides mineralizing around the mouths of the boreholes. 16S rRNA clone libraries of environmental DNA revealed that closely related Ralstonia sp. were numerically-dominant in metal-rich subsurface fluids. FeCO3 gradient tubes were then utilized to demonstrate that isolate Ralstonia HM08-01 grows by oxidizing Fe(II) with O-2 at circumneutral pH. These results and the geochemical data from the borehole fluids implicate Fe-oxidation as a viable subsurface lifestyle. The differential development of Fe-oxide bands in biotic vs. abiotic gradient tubes suggests that Ralstonia HM08-01 exerts spatial control over Fe oxidation and precipitation. Geochemical profiles of Fe(II), Fe(III) and O-2 taken through the gradient tubes with voltammetric microelectrodes reveal that despite visual differences, similar total concentrations and distributions of aqueous Fe species were present in both systems. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to characterize the mineralogy of the Fe-oxides produced in biotic vs. abiotic experiments. 2L-ferrihydrite dominated the mineral fits in both systems, and SEM revealed the ferrihydrite particles to be 50-100 nm in diameter. This mineralogical identification combined with the detection of an abundant electroactive Fe( III) species are used to infer that 2L-ferrihydrite is a long-term stabilized colloidal species. The mechanism for stabilization of this phase is the presence of PO42- and Si in growth experiments. In the Henderson fluids, PO42- is below detection, but Si is at micromolar concentrations and likely influences the formation of potentially colloidal Fe-oxides in the environment. (C) 2011 Elsevier B.V. All rights reserved.