Abiotic and Biotic Reduction of Iodate Driven by Shewanella oneidensis MR-1

Iodate (IO3–) can be abiotically reduced by Fe(II) or biotically reduced by the dissimilatory Fe(III)-reducing bacterium Shewanella oneidensis (MR-1) via its DmsEFAB and MtrCAB. However, the intermediates and stoichiometry between the Fe(II) and IO3– reaction and the relative contribution of abiotic and biotic IO3– reduction by biogenic Fe(II) and MR-1 in the presence of Fe(III) remain unclear. In this study, we found that abiotic reduction of IO3– by Fe(II) produced intermediates HIO and I– at a ratio of 1:2, followed by HIO disproportionation to I– and IO3–. Comparative analyses of IO3– reduction by MR-1 wild type (WT), MR-1 mutants deficient in DmsEFAB or MtrCAB, and Shewanella sp. ANA-3 in the presence of Fe(III)-citrate, Fe(III) oxides, or clay minerals showed that abiotic IO3– reduction by biogenic Fe(II) predominated under iron-rich conditions, while biotic IO3– reduction by DmsEFAB played a more dominant role under iron-poor conditions. Compared to that in the presence of Fe(III)-citrate, MR-1 WT reduced more IO3– in the presence of Fe(III) oxides and clay minerals. The observed abiotic and biotic IO3– reduction by MR-1 under Fe-rich and Fe-limited conditions suggests that Fe(III)-reducing bacteria could contribute to the transformation of iodine species and I– enrichment in natural iodine-rich environments.