The successive reduction of iodate to iodide driven by iron redox cycling

Ferrous iron (Fe(II)) produced by microbial Fe(III) reduction and reactive oxygen species (ROS) generated from aerobic Fe(II) oxidation can mediate iodate (IO₃^-) reduction and iodide (I^-) oxidation, respectively. Nevertheless, how Fe redox cycling under redox fluctuating conditions drives transformation of iodine species remain unclear. In this study, Shewanella oneidensis MR-1 wildtype (WT) and its mutant △dmsEFAB, which lost the ability to enzymatically reduce IO₃^-, were chosen to conduct ferrihydrite/goethite/nontronite culture experiments under consecutive cycles of anoxic reduction of Fe(III) and re-oxidation of Fe(II) by O₂ to reveal the role of Fe redox cycling in the transformation of iodine species. The results showed that both surface-adsorbed and mineral structural Fe(II) chemically reduced IO₃^-. Chemical IO₃^- reduction by biogenic Fe(II) was slower than enzymatic IO₃^- reduction by WT. Compared to △dmsEFAB cultures, WT cultures all showed higher Fe(II) concentrations under anoxic conditions but lower cumulative •OH under oxic conditions, which imply the chemical reaction between I^- and ROS. I^- oxidation by ROS, however, did not lead to a significant production of IO₃^- compared with I^- formed under anoxic conditions. Consequently, Fe redox cycling successively reduced IO₃^- to I^-, which highlights vital roles of Fe(III)-reducing bacteria in I^- formation and mobilization in environments.