Elucidating the physiological significance of iodate reduction in Shewanella species.

The reduction of iodate by Shewanella species plays a crucial role in iodine biogeochemical cycling. However, the physiological significance of this reduction process remains poorly understood, because Shewanella species are capable of reducing only low concentrations of iodate. This study systematically examined the physiological and transcriptomic responses of Shewanella oneidensis MR-1 to iodate exposure. Our results showed that phenol-mediated elimination of hypoiodous acid did not increase the upper limit of iodate reduction achieved by S. oneidensis MR-1, and dimethyl sulfoxide was preferentially utilized over iodate by the organism. When iodate served as the sole electron acceptor, no increase in total cellular protein was observed in S. oneidensis MR-1, in contrast to marine Shewanella strains, which exhibited increased production of total cellular protein at 0.25 and 0.50 mM iodate. Furthermore, anaerobic growth of S. oneidensis MR-1 and the ΔdmsEFAB strain was dose-dependently inhibited by iodate, which induced significant oxidative stress. Transcriptomic analysis revealed that the mRNA levels of mtrCAB and several genes encoding antioxidant enzymes were elevated during iodate reduction. Deletion mutants of so1563 (glutathione peroxidase), so1659 (c-type cytochrome), and so0534 (arsenate permease) exhibited reduced capacities for iodate reduction or H2O2 degradation. Notably, supplementation of H2O2 in the medium compromised iodate reduction in both wild-type and mutant strains. Collectively, these findings provide evidence that iodate reduction in Shewanella primarily functions as a detoxification mechanism rather than a process for energy conservation. The results further highlight distinct adaptive strategies employed by Shewanella species in freshwater and marine environments with different iodine concentrations.