Dissimilatory Iodate-Reducing Microorganisms Contribute to the Enrichment of Iodine in Groundwater

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