The reduction of structural iron in ferruginous smectite via the amino acid cysteine: Implications for an electron shuttling compound

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Microbes can reduce the structural iron (Fe(III)str) in clay minerals thus providing a potentially important terminal electron acceptor in the oxidation of organic matter. Many of these microorganisms participate in dissimilatory metal reduction with Fe(III) serving as the terminal electron acceptor either through direct contact with mineral surfaces or by way of electron shuttling compounds. Here we provide evidence for the electron shuttling capability of the amino acid cysteine with a ferruginous dioctahedral smectite (SWa-1) using infrared spectroscopy, X-ray diffraction and quantitative assay of ferric and ferrous iron. Reactions to determine the electron exchange between cysteine and SWa-1 were performed in pH 8 adjusted oxygen free solutions. Fourier transform infrared spectroscopy (FTIR) performed on self-supporting clay films reveals that cysteine has the ability to reduce Fe(III)str, as shown by the decrease in the intensity of the AlFeOH and FeFeOH deformation and stretching bands resulting from decreased hydroxyl vibrations in the octahedral sheets. X-ray diffraction of the c-oriented SWa-1 reveals that cysteine intercalated into the d00l interlayer spaces. Quantitative iron assay indicates that the SWa-1 retains its structural iron upon reduction by cysteine and reoxidation. The increased interlayer spacing due to the intercalation of cysteine implies that this electron exchange is occurring from the basal surfaces of the smectite, as opposed to edge sites. When the SWa-1 was rinsed in dialysis tubing, the AlFeOH and FeFeOH vibrations reappear in FTIR spectra and the XRD patterns reveal that the cysteine no longer occupies interlayer sites. These results are consistent with partially reversible changes in clay mineral structure resulting from the reduction of Fe(III)str. They support the hypothesis that cysteine could serve as an electron shuttling compound used by microorganisms to gain access to structural iron in clay minerals and extends the range of microbially mediated Fe redox reactions from iron oxides and oxyhydroxides to the largest pool of Fe in aquatic sediments, Fe-bearing clay minerals.