Reduction of hexavalent chromium by the thermophilic methanogen .

Despite the significant progress on iron reduction by thermophilic microorganisms, studies on their ability to reduce toxic metals are still limited, despite their common co-existence in high temperature environments (up to 70°C). In this study, , an obligate thermophilic methanogen, was used to reduce hexavalent chromium. Experiments were conducted in a growth medium with H/CO as substrate with various Cr concentrations (0.2, 0.4, 1, 3, and 5 mM) in the form of potassium dichromate (KCrO). Time-course measurements of aqueous Cr concentrations with the 1, 5-diphenylcarbazide colorimetric method showed complete reduction of the 0.2 and 0.4 mM Cr solutions by this methanogen. However, much lower reduction extents of 43.6%, 13.0%, and 3.7% were observed at higher Cr concentrations of 1, 3 and 5 mM, respectively. These lower extents of bioreduction suggest a toxic effect of aqueous Cr to cells at this concentration range. At these higher Cr concentrations, methanogenesis was inhibited and cell growth was impaired as evidenced by decreased total cellular protein production and live/dead cell ratio. Likewise, Cr bioreduction rates decreased with increased initial concentrations of Cr from 13.3 to1.9 µM h. X-ray absorption near-edge structure (XANES) spectroscopy revealed a progressive reduction of soluble Cr to insoluble Cr precipitates, which was confirmed as amorphous chromium hydroxide by X-ray diffraction and selected area electron diffraction pattern. However, a small fraction of reduced Cr occurred as aqueous Cr. Scanning and transmission electron microscope observations of cells after Cr exposure suggest both extra- and intracellular chromium reduction mechanisms. Results of this study demonstrate the ability of cells to reduce toxic Cr to less toxic Cr and its potential application in metal bioremediation, especially at high temperature subsurface radioactive waste disposal sites, where the temperature may reach ∼70°C.