Dissimilatory\nIodate-Reducing Microorganisms Contribute\nto the Enrichment of Iodine in Groundwater

Iodate\nreduction by dissimilatory iodate-reducing microorganisms\n(DIRMs) plays a crucial role in the biogeochemical cycling of iodine\non Earth. However, the occurrence and distribution of DIRMs in iodine-rich\ngroundwater remain unclear. In this study, we isolated the dissimilatory\niodate-reducing bacteriumAzonexus hydrophilusstrain NCP973 from a geogenic high-iodine groundwater of China for\nthe first time. The analysis of genome, transcriptome, and heterologous\nexpression revealed that strain NCP973 uses the dissimilatory iodate-reducing\nenzyme IdrABP1P2 to reduce dissolved or in situ sediment-bound iodate\nto iodide. The location of IdrABP1P2 in the conjugative plasmid of\nstrain NCP973 implies that IdrABP1P2 could be spread by horizontal\ngene transfer and allow the recipient microorganisms to participate\nin the enrichment of iodide in aquifers. Based on the global iodine-rich\ngroundwater metagenomes and genomes, the identification of <i>idrA</i> showed that phylogenetically diverse DIRMs are widely\ndistributed not only in geogenic high-iodine groundwater of China\nbut also in radionuclide-contaminated groundwater of USA as well as\nin subsurface cavern waters in Germany and Italy. Moreover, the abundance\nof <i>idrA</i> was found to be higher in groundwater with\na relatively high iodine content. Collectively, these results suggest\nthat terrestrial iodine-affected groundwater systems are another important\nhabitat for DIRMs in addition to marine environments, and their activity\nin aquifers triggers the mobilization and enrichment of iodine in\ngroundwater worldwide.