title: Reduction of hexavalent chromium by the thermophilic methanogenMethanothermobacter thermautotrophicus.
abstract: 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/CO2as 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 ofM. thermautotrophicuscells after Cr6+exposure suggest both extra- and intracellular chromium reduction mechanisms. Results of this study demonstrate the ability ofM. thermautotrophicuscells 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.
authors.affiliation: Department of Geology and Environmental Earth Science, Miami University, Oxford, OH-45056.
authors.forename: Rajesh
authors.initials: R
authors.lastname: Singh
authors.affiliation: Department of Geology and Environmental Earth Science, Miami University, Oxford, OH-45056 ; State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing, 100083, China.
authors.forename: Hailiang
authors.initials: H
authors.lastname: Dong
authors.affiliation: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, 430074, China.
authors.forename: Deng
authors.initials: D
authors.lastname: Liu
authors.affiliation: Department of Geology and Environmental Earth Science, Miami University, Oxford, OH-45056.
authors.forename: Linduo
authors.initials: L
authors.lastname: Zhao
authors.affiliation: Department of Chemistry and Biochemistry, Miami University, Oxford, OH-45056.
authors.forename: Amy R
authors.initials: AR
authors.lastname: Marts
authors.affiliation: Case Western Reserve University Center for Synchrotron Biosciences, National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973.
authors.forename: Erik
authors.initials: E
authors.lastname: Farquhar
authors.affiliation: Department of Chemistry and Biochemistry, Miami University, Oxford, OH-45056.
authors.forename: David L
authors.initials: DL
authors.lastname: Tierney
authors.affiliation: Department of Paper and Chemical Engineering, Miami University, Oxford, OH-45056.
authors.forename: Catherine B
authors.initials: CB
authors.lastname: Almquist
authors.affiliation: Department of Geology and Environmental Earth Science, Miami University, Oxford, OH-45056.
authors.forename: Brandon R
authors.initials: BR
authors.lastname: Briggs