The application of HRTEM techniques and nanosims to chemically and isotopically characterize Geobacter sulfurreducens surfaces

Bioprecipitated minerals are typically at the nanometer scale, hydrous, and beam-sensitive (i.e., can recrystallize during analysis), making them difficult to characterize using standard spectroscopic or electron-beam techniques. We have combined the ion-imaging capabilities of nanoscale secondary-ion mass spectrometry (NanoSIMS) and advanced high-resolution transmission electron microscopy (HRTEM) in order to characterize the surfaces of Geobacter sulfurreducens and the bioprecipitated uranium phases. Our results reveal the association between nutrient uptake and precipitation of uranium minerals. Biosequestration of uranium is enhanced by addition of nutrients such as acetic acid, and uranium is precipitated on the surface of the bacteria as nanocrystals of uraninite (UO₂). The bioprecipitation of this anhydrous U-rich phase is significant; although UO₂ is thermodynamically stable over a range of pH values (2 to 12) and oxidizing conditions [Eh 0.2 to -0.2, or log f(O₂) of approximately -50 to -125], thermodynamic models of inorganic systems suggest that U⁶⁺ oxyhydroxide minerals should be stable. Our results suggest that the biofilm shielded the UO₂ from re-oxidation and that bacteria can immobilize uranium for extended periods, even under relatively oxidizing conditions in the subsurface.