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Title: 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+more » 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.« less
 [1] ;  [2] ;  [3] ;  [1] ;  [4] ;  [5] ;  [4] ;  [6] ;  [1]
  1. Miami Univ., Oxford,OH (United States). Dept. of Geology and Environmental Earth Science
  2. Miami Univ., Oxford,OH (United States). Dept. of Geology and Environmental Earth Science; China Univ. of Geosciences, Beijing (China). State Key Lab. of Biogeology and Environmental Geology
  3. China Univ. of Geosciences, Wuhan (China). State Key Lab. of Biogeology and Environmental Geology
  4. Miami Univ., Oxford,OH (United States). Dept. of Chemistry and Biochemistry
  5. Case Western Reserve Univ. Center for Synchrotron Biosciences and Brookhaven National Lab, Upton, NY (United States). National Synchrotron Light Source
  6. Miami Univ., Oxford,OH (United States). Dept. of Paper and Chemical Engineering
Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 0016-7037
Grant/Contract Number:
SC00112704; SC0005333; CHE-1152755
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 148; Journal Issue: C; Journal ID: ISSN 0016-7037
The Geochemical Society; The Meteoritical Society
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States