Iodine is essential for life, and its global biogeochemical cycle is critically important for both human and environmental health. Both iodine deficiency and excessive iodine intake can lead to thyroid disease in humans. The ocean, as Earth's largest iodine reservoir, harbors iodate (IO₃⁻), which can be microbially reduced to the more mobile iodide (I⁻). At the sea‑air boundary, I⁻ reacts with ozone to produce volatile iodine species that enter the atmosphere, where they influence ozone depletion, mercury cycling, and aerosol formation, while also supplying of terrestrial iodine through deposition.
DIRMs are bacteria that gain energy by reducing IO₃⁻ to I⁻. For years, scientists assumed that DIRMs preferentially utilize IO₃⁻ over nitrate (NO₃⁻), based on the theoretical thermodynamic calculations. That would place them in a narrow zone just above marine oxygen minimum zones (OMZs), where oxygen runs out but NO₃⁻ reduction has not yet begun.
“But our previous investigation on high-iodine groundwater told a different story,” said Professor Junxia Li of China University of Geosciences, the first author of the article. They found that Azonexus hydrophilus NCP973, a DIRM strain isolated from high‑iodine groundwater, appeared in environments where NO₃⁻ was already depleted. Moreover, a negative correlation between I⁻ and NO₃⁻ concentrations is widespread across high‑iodine groundwaters in China. These results suggest that IO₃⁻ reduction by DIRMs may not precede NO₃⁻ reduction, contradicting with thermodynamic predictions. “Thermodynamic prediction and field observation were clearly at odds,” said Li. “We were thus curious to re‑examine the ecological niche of these microorganisms.”
The reduction order of IO ₃⁻ and NO ₃⁻ by DIRMs
To resolve this discrepancy, the team conducted culture experiments with two representative DIRM strains, i.e. A. hydrophilus NCP973 from groundwater and Denitromonas iodatirespirans IR‑12 from the ocean. When both IO₃⁻ and NO₃⁻ were present, these two strains consistently reduced NO₃⁻ first, and only began reducing NO₃⁻ after IO₃⁻ was depleted . Transcriptomic analysis revealed that NO₃⁻ reductase genes ( narGHI ) were expressed first, followed by IO₃⁻ reductase genes ( idrABP1P2 ).
Why would DIRMs favor a less energy‑yielding NO₃⁻ reduction first? Because NO₃⁻ suppresses the expression of the IO₃⁻ reductase genes idrABP1P2 (similar to its suppression of perchlorate reductase). Meanwhile, IO₃⁻ itself imposes oxidative stress forcing the bacteria into a prolonged lag phase, which gives NO₃⁻ reduction a competitive advantage.
DIRMs’ true habitat: inside OMZs, not above them
Based on the finding that IO₃⁻ reduction follows NO₃⁻ reduction, the team hypothesized that DIRMs should inhabit OMZs where microbial NO₃⁻ reduction is active. To test this, they analyzed metagenomic and metatranscriptomic data from the three major OMZs (Eastern Tropical North Pacific, Eastern Tropical South Pacific, and Arabian Sea), along with two MAG datasets from global OMZs and the Tara Oceans.
The results were striking. idrA genes and transcripts were concentrated exclusively within OMZ depth profiles. Among 962 MAGs from global OMZs, 32 carried the idrABP1P2 gene, while among 2,631 MAGs from Tara Oceans, 9 carried idrABP1P2 , all of which are from OMZ samples. OMZ‑inhabiting DIRMs belonged predominantly to candidate phylum SAR324 and Alphaproteobacteria , greatly expanding known diversity of DIRMs. Notably, SAR324 MAGs carrying idrABP1P2 also contains sulfur oxidation genes, suggesting that DIRMs may couple sulfide oxidation to IO₃⁻ reduction. The IO₃⁻ reducing ability of these newly identified genes was verified experimentally through heterologous expression of two representative idrABP1P2 sequences.
Global warming accelerates marine iodine emission
Global warming reduces oxygen solubility and intensifies ocean stratification, leading to the expansion of OMZs. Over the past 60 years, the global area of OMZs has expanded from 5 % to 14 %, and this trend is expected to continue. As these oxygen‑depleted zones grow, DIRMs will have larger habitats, producing more I⁻. The I⁻ produced by DIRMs can be transported to the surface via ocean circulation, elevating surface I⁻ concentrations and marine iodine emissions. Model simulations indicate that a 1 % increase in global sea‑surface I⁻ concentration leads to an ~0.7 % rise in oceanic iodine emissions.
“Given the importance of iodine for human and environmental health, integrating this pathway into marine iodine biogeochemical models will improve our capability of understanding and predicting the future changes in oceanic iodine emissions,” Li said.
National Science Review
Experimental study