Microbial Reduction of Chromium(VI): In-Situ Environmental Cell Transmission Electron Microscopy Study Using Electron Energy Loss Spectroscopy

Abstract The geochemistry and toxicity of chromium is controlled by its valence state. Chromium is a redox active 3d transition metal with a wide range (− to +6) of possible oxidation states; however only Cr(III) and Cr(VI) are stable. Hexavalent Cr(VI) can be readily reduced to the trivalent state by Fe2+, S2−, organic compounds, wetland plants, and several species of microorganisms. The associated mechanisms of Cr(VI) reduction are technologically and biologically important because they convert a toxic, mobile element into a less toxic, immobile form. Reduction of Cr(VI) by the bacterium, Shewanella oneidensis (previously classified Shewanella putrefaciens strain MR-1), was studied by absorption spectrophotometry and in-situ, environmental cell (EC) - transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). Shewanella oneidensis (S. oneidensis), a gram-negative, facultative bacterium is capable of respiring aerobically and anaerobically using a variety of compounds, including O2, Fe(III), Mn(IV), NO2 −, NO3 −, SO2, SO3 2−, thiosulfate (S2O3 2−), trimethyamine oxide, fumarate, U(VI), and Cr(VI) as terminal electron acceptors.