title: Toxicity of Cr(lll) to Shewanella sp. strain MR-4 during Cr(VI) reduction.
abstract: Bioremediation of chromium through the reduction of hexavalent chromium (as the chromate ion, CrO42-) is based on the notion that the product, trivalent chromium (Cr(III)), is less toxic than chromate. In this study, we show that soluble Cr(III), present at pH 6-8 as the Cr3+ ion and/or hydroxyl complexes (henceforth referred to as uncomplexed Cr(III)), can be found transiently in significant concentrations and has a deleterious effect on Shewanella sp. MR-4. However, Cr(lll) complexed to an organic ligand or precipitated as Cr(OH)3(s) has little or no effect on cells. Similarly, during the reduction of Cr(VI) by strain MR-4, complexation of the product, Cr(lll), results in increased cell survival and extended Cr(VI) reduction activity. These results and gene expression data obtained by qRT-PCR (quantitative reverse transcription-PCR) suggestthatthe observed toxic effect of Cr(II) formed during Cr(VI) reduction or added as an uncomplexed species is due to the interference with basic cell activities such as DNA transcription and/or replication. Important implications for the bioremediation of Cr(VI)-contaminated sites emerge from this study: Cr(VI) reduction by Shewanella sp. MR-4 is enhanced and sustained by the presence of compounds able to complex Cr(III) as it is being formed but, in turn, the complexation of Cr(III) precludes its precipitation and immobilization.
authors.affiliation: Marine Biology Research Division and Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California-San Diego, La Jolla, California 92093, USA. rizlan.bencheikh@epfl.ch
authors.forename: Rizlan
authors.initials: R
authors.lastname: Bencheikh-Latmani
authors.forename: Anna
authors.initials: A
authors.lastname: Obraztsova
authors.forename: Mason R
authors.initials: MR
authors.lastname: Mackey
authors.forename: Mark H
authors.initials: MH
authors.lastname: Ellisman
authors.forename: Bradley M
authors.initials: BM
authors.lastname: Tebo
mesh_terms: D001673:Biodegradation, Environmental; D002857:Chromium / Q000737:chemistry* / Q000633:toxicity*; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects*; D046529:Microscopy, Electron, Transmission; D010084:Oxidation-Reduction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D020592:Shewanella / Q000187:drug effects* / Q000235:genetics; D016019:Survival Analysis; D014874:Water Pollutants, Chemical; D014876:Water Pollution / Q000517:prevention & control*
chemical_list: D014874:Water Pollutants, Chemical; D002857:Chromium; C074702:chromium hexavalent ion