Iodate Reduction by Shewanella oneidensis Requires Genes Encoding an Extracellular Dimethylsulfoxide Reductase

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.

Microbial involvement in iodine cycle: mechanisms and potential applications

Stable iodine isotopes are essential for humans as they are necessary for producing thyroid gland hormones. However, there are hazardous radioactive iodine isotopes that are emitted into the environment through radioactive waste generated by nuclear power plants, nuclear weapon tests, and medical practice. Due to the biophilic character of iodine radionuclides and their enormous biomagnification potential, their elimination from contaminated environments is essential to prevent the spread of radioactive pollution in ecosystems. Since microorganisms play a vital role in controlling iodine cycling and fate in the environment, they also can be efficiently utilized in solving the issue of contamination spread. Thus, this paper summarizes all known on microbial processes that are involved in iodine transformation to highlight their prospects in remediation of the sites contaminated with radioactive iodine isotopes.

Microbial Contributions to Iodide Enrichment in Deep Groundwater in the North China Plain

Microorganisms play crucial roles in the global iodine cycling through iodine oxidation, reduction, volatilization, and deiodination. In contrast to iodate formation in radionuclide-contaminated groundwater by the iodine-oxidizing bacteria, microbial contribution to the formation of high level of iodide in geogenic high iodine groundwater is poorly understood. In this study, our results of comparative metagenomic analyses of deep groundwater with typical high iodide concentrations in the North China Plain revealed the existence of putative dissimilatory iodate-reducing idrABP1P2 gene clusters in groundwater. Heterologous expression and characterization of an identified idrABP1P2 gene cluster confirmed its functional role in iodate reduction. Thus, microbial dissimilatory iodate reduction could contribute to iodide formation in geogenic high iodine groundwater. In addition, the identified iron-reducing, sulfur-reducing, sulfur-oxidizing, and dehalogenating bacteria in the groundwater could contribute to the release and production of iodide through the reductive dissolution of iron minerals, abiotic iodate reduction of derived ferrous iron and sulfide, and dehalogenation of organic iodine, respectively. These microbially mediated iodate reduction and organic iodine dehalogenation processes may also result in the transformation among iodine species and iodide enrichment in other geogenic iodine-rich groundwater systems worldwide.

Global occurrence of the bacteria with capability for extracellular reduction of iodate

The γ-proteobacterium Shewanella oneidensis MR-1 reduces iodate to iodide extracellularly. Both dmsEFAB and mtrCAB gene clusters are involved in extracellular reduction of iodate by S. oneidensis MR-1. DmsEFAB reduces iodate to hypoiodous acid and hydrogen peroxide (H₂O₂). Subsequently, H₂O₂ is reduced by MtrCAB to facilitate DmsEFAB-mediated extracellular reduction of iodate. To investigate the distribution of bacteria with the capability for extracellular reduction of iodate, bacterial genomes were systematically searched for both dmsEFAB and mtrCAB gene clusters. The dmsEFAB and mtrCAB gene clusters were found in three Ferrimonas and 26 Shewanella species. Coexistence of both dmsEFAB and mtrCAB gene clusters in these bacteria suggests their potentials for extracellular reduction of iodate. Further analyses demonstrated that these bacteria were isolated from a variety of ecosystems, including the lakes, rivers, and subsurface rocks in East and Southeast Asia, North Africa, and North America. Importantly, most of the bacteria with both dmsEFAB and mtrCAB gene clusters were found in different marine environments, which ranged from the Arctic Ocean to Antarctic coastal marine environments as well as from the Atlantic Ocean to the Indian and Pacific Oceans. Widespread distribution of the bacteria with capability for extracellular reduction of iodate around the world suggests their significant importance in global biogeochemical cycling of iodine. The genetic organization of dmsEFAB and mtrCAB gene clusters also varied substantially. The identified mtrCAB gene clusters often contained additional genes for multiheme c-type cytochromes. The numbers of dmsEFAB gene cluster detected in a given bacterial genome ranged from one to six. In latter, duplications of dmsEFAB gene clusters occurred. These results suggest different paths for these bacteria to acquire their capability for extracellular reduction of iodate.

Bromate reduction by Shewanella oneidensis MR-1 is mediated by dimethylsulfoxide reductase

Microbial bromate reduction plays an important role in remediating bromate-contaminated waters as well as biogeochemical cycling of bromine. However, little is known about the molecular mechanism of microbial bromate reduction so far. Since the model strain Shewanella oneidensis MR-1 is capable of reducing a variety of oxyanions such as iodate, which has a high similarity to bromate, we hypothesize that S. oneidensis MR-1 can reduce bromate. Here, we conducted an experiment to investigate whether S. oneidensis MR-1 can reduce bromate, and report bromate reduction mediated by a dimethylsulfoxide reductase encoded with dmsA. S. oneidensis MR-1 is not a bromate-respiring bacterium but can reduce bromate to bromide under microaerobic conditions. When exposed to 0.15, 0.2, 0.25, 0.5, and 1 mM bromate, S. oneidensis MR-1 reduced bromate by around 100, 75, 64, 48, and 23%, respectively, within 12 h. In vivo evidence from gene deletion mutants and complemented strains of S. oneidensis MR-1 indicates that MtrB, MtrC, CymA, GspD, and DmsA are involved in bromate reduction, but not NapA, FccA, or SYE4. Based on our results as well as previous findings, a proposed molecular mechanism for bromate reduction is presented in this study. Moreover, a genomic survey indicates that 9 of the other 56 reported Shewanella species encode proteins highly homologous to CymA, GspD, and DmsA of S. oneidensis MR-1 by sequence alignment. The results of this study contribute to understanding a pathway for microbial bromate reduction.

The Roles of DmsEFAB and MtrCAB in Extracellular Reduction of Iodate by Shewanella oneidensis MR-1 with Lactate as the Sole Electron Donor

To investigate their roles in extracellular reduction of iodate (IO₃ ^- ) with lactate as an electron donor, the gene clusters of dmsEFAB, mtrCAB, mtrDEF, and so4360-4357 in Shewanella oneidensis MR-1were systematically deleted. Deletions of dmsEFAB and/or mtrCAB gene clusters diminished the bacterial ability to reduce IO₃ ^- . Furthermore, DmsEFAB and MtrCAB worked collaboratively to reduce IO₃ ^- of which DmsEFAB played a more dominant role than MtrCAB. MtrCAB was involved in detoxifying the reaction intermediate hydrogen peroxide (H₂ O₂ ). The reaction intermediate hypoiodous acid (HIO) was also found to inhibit microbial IO₃ ^- reduction. SO4360-4357 and MtrDEF, however, were not involved in IO₃ ^- reduction. Collectively, these results suggest a novel mechanism of extracellular reduction of IO₃ ^- at molecular level, in which DmsEFAB reduces IO₃ ^- to HIO and H₂ O₂ . The latter is further reduced to H₂ O by MtrCAB to facilitate the DmsEFAB-mediated IO₃ ^- reduction. The extracellular electron transfer pathway of S. oneidensis MR-1is believed to mediate electron transfer from bacterial cytoplasmic membrane, across the cell envelope to the DmsEFAB and MtrCAB on the bacterial outer membrane.