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.