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Microbial mass-dependent fractionation of chromium isotopes |
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| Authors: | Eric R Sikora Thomas M Johnson Thomas D Bullen |
| Institution: | a Department of Geology, University of Illinois at Urbana-Champaign, 245 Natural History Building, 1301 W. Green Street, Urbana, IL 61801, USA b The Sigma Group, 1300 West Canal Street, Milwaukee, WI 53233, USA c Water Resources Division, U.S. Geological Survey, Menlo Park, CA 94025, USA |
| Abstract: | Mass-dependent fractionation of Cr isotopes occurs during dissimilatory Cr(VI) reduction by Shewanella oneidensis strain MR-1. Cells suspended in a simple buffer solution, with various concentrations of lactate or formate added as electron donor, reduced 5 or 10 μM Cr(VI) to Cr(III) over days to weeks. In all nine batch experiments, 53Cr/52Cr ratios of the unreacted Cr(VI) increased as reduction proceeded. In eight experiments covering a range of added donor concentrations up to 100 μM, isotopic fractionation factors were nearly invariant, ranging from 1.0040 to 1.0045, with a mean value somewhat larger than that previously reported for abiotic Cr(VI) reduction (1.0034). One experiment containing much greater donor concentration (10 mM lactate) reduced Cr(VI) much faster and exhibited a lesser fractionation factor (1.0018). These results indicate that 53Cr/52Cr measurements should be effective as indicators of Cr(VI) reduction, either bacterial or abiotic. However, variability in the fractionation factor is poorly constrained and should be studied for a variety of microbial and abiotic reduction pathways. |
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