title: Divergent aquifer biogeochemical systems converge on similar and unexpected Cr(VI) reduction products.
abstract: In this study of reductive chromium immobilization, we found that flow-through columns constructed with homogenized aquifer sediment and continuously infused with lactate, chromate, and various native electron acceptors diverged to have very different Cr(VI)-reducing biogeochemical regimes characterized by either denitrifying or fermentative conditions (as indicated by effluent chemical data, 16S rRNA pyrotag data, and metatranscriptome data). Despite the two dramatically different biogeochemical environments that evolved in the columns, these regimes created similar Cr(III)-Fe(III) hydroxide precipitates as the predominant Cr(VI) reduction product, as characterized by micro-X-ray fluorescence and micro-X-ray absorption near-edge structure analysis. We discuss two conflicting scenarios of microbially mediated formation of Cr(III)-Fe(III) precipitates, each of which is both supported and contradicted by different lines of evidence: (1) enzymatic reduction of Cr(VI) to Cr(III) followed by coprecipitation of Cr(III) and Fe(III) and (2) both regimes generated at least small amounts of Fe(II), which abiotically reduced Cr(VI) to form a Cr-Fe precipitate. Evidence of zones with different levels of Cr(VI) reduction suggest that local heterogeneity may have confounded interpretation of processes based on bulk measurements. This study indicates that the bulk redox status and biogeochemical regime, as categorized by the dominant electron-accepting process, do not necessarily control the final product of Cr(VI) reduction.
authors.affiliation: Earth Sciences Division and ‡Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Harry R
authors.initials: HR
authors.lastname: Beller
authors.forename: Li
authors.initials: L
authors.lastname: Yang
authors.forename: Charuleka
authors.initials: C
authors.lastname: Varadharajan
authors.forename: Ruyang
authors.initials: R
authors.lastname: Han
authors.forename: Hsiao Chien
authors.initials: HC
authors.lastname: Lim
authors.forename: Ulas
authors.initials: U
authors.lastname: Karaoz
authors.forename: Sergi
authors.initials: S
authors.lastname: Molins
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Marcus
authors.forename: Eoin L
authors.initials: EL
authors.lastname: Brodie
authors.forename: Carl I
authors.initials: CI
authors.lastname: Steefel
authors.forename: Peter S
authors.initials: PS
authors.lastname: Nico
mesh_terms: D001419:Bacteria / Q000235:genetics; D002857:Chromium / Q000737:chemistry*; D016000:Cluster Analysis; D058440:Denitrification / Q000235:genetics; D005285:Fermentation / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D060587:Groundwater / Q000737:chemistry*; D007501:Iron / Q000737:chemistry; D010084:Oxidation-Reduction; D010802:Phylogeny; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D013052:Spectrometry, X-Ray Emission; D059467:Transcriptome / Q000235:genetics; D014865:Waste Disposal, Fluid; D056928:X-Ray Absorption Spectroscopy
chemical_list: D012336:RNA, Ribosomal, 16S; D002857:Chromium; C074702:chromium hexavalent ion; D007501:Iron