Reduction of hexavalent chromium by the thermophilic methanogen Methanothermobacter thermautotrophicus

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, Methanothermobacter thermautotrophicus, an obligate thermophilic methanogen, was used to reduce hexavalent chromium. Experiments were conducted in a growth medium with H₂/CO₂ as substrate with various Cr⁶⁺ concentrations (0.2, 0.4, 1, 3, and 5 mM) in the form of potassium dichromate (K₂Cr₂O₇). Time-course measurements of aqueous Cr⁶⁺ concentrations with the 1, 5-diphenylcarbazide colorimetric method showed complete reduction of the 0.2 and 0.4 mM Cr⁶⁺ solutions by this methanogen. However, much lower reduction extents of 43.6%, 13.0%, and 3.7% were observed at higher Cr⁶⁺ concentrations of 1, 3 and 5 mM, respectively. These lower extents of bioreduction suggest a toxic effect of aqueous Cr⁶⁺ to cells at this concentration range. At these higher Cr⁶⁺ concentrations, methanogenesis was inhibited and cell growth was impaired as evidenced by decreased total cellular protein production and live/dead cell ratio. Likewise, Cr⁶⁺ bioreduction rates decreased with increased initial concentrations of Cr⁶⁺ from 13.3 to1.9 µM h^-1. X-ray absorption near-edge structure (XANES) spectroscopy revealed a progressive reduction of soluble Cr⁶⁺ to insoluble Cr³⁺ 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 Cr³⁺. Scanning and transmission electron microscope observations of M. thermautotrophicus cells after Cr⁶⁺ exposure suggest both extra- and intracellular chromium reduction mechanisms. Results of this study demonstrate the ability of M. thermautotrophicus cells to reduce toxic Cr⁶⁺ to less toxic Cr³⁺ and its potential application in metal bioremediation, especially at high temperature subsurface radioactive waste disposal sites, where the temperature may reach ∼70°C.