Ocean habitats of dissimilatory iodate-reducing microorganisms revealed

In the vast chemistry of the ocean, iodine occupies a peculiar position: essential for life, intimately linked to the health of the human thyroid, and increasingly recognized as a chemical actor in the atmosphere above the waves. For decades, scientists believed they knew precisely where in the water column a specialized group of microbes—the dissimilatory iodate-reducing microorganisms, or DIRMs—should make their home. A new study, published in National Science Review, upends that assumption, relocating these microbes from a razor-thin band at the edge of oxygen-starved waters to the heart of the ocean’s oxygen minimum zones, with profound implications for how much iodine the sea pumps into the atmosphere as the planet warms.

To understand why this matters, it helps to begin with the chemistry. The ocean is Earth’s largest reservoir of iodine, and most of that iodine exists as iodate, a negatively charged ion written chemically as IO₃⁻. Certain bacteria can breathe iodate the way we breathe oxygen, converting it to iodide, I⁻, and harvesting energy in the process. Iodide is far more mobile than iodate, and where it reaches the sea surface it reacts with atmospheric ozone to generate volatile iodine compounds that escape into the air. Once aloft, these species participate in ozone destruction, influence the cycling of mercury, seed the formation of aerosol particles that reflect sunlight, and eventually rain back down onto land, supplying terrestrial ecosystems with iodine. In short, where DIRMs live determines, in a very real sense, how the ocean talks to the atmosphere about iodine.

The classical prediction rested on thermodynamics. On paper, iodate reduction yields more energy than nitrate reduction, so microbial ecologists reasoned that DIRMs should gorge on iodate first, then turn to nitrate. That logic placed them in a narrow ecological window just above the ocean’s oxygen minimum zones—regions where dissolved oxygen has vanished but where denitrifying microbes have not yet exhausted the nitrate. Below that window, nitrate would be scarce; above it, oxygen would suppress anaerobic respiration altogether. Textbook-style reasoning thus confined DIRMs to a thin slice of the ocean.

But field observations from an unexpected quarter—high-iodine groundwater in China—told a stubbornly different story. Professor Junxia Li of China University of Geosciences, the first author of the new paper, had previously isolated a DIRM strain called Azonexus hydrophilus NCP973 from iodine-rich aquifers. Curiously, this organism kept turning up in waters where nitrate had already been depleted, and across high-iodine groundwaters nationwide, iodide concentrations showed a consistent negative correlation with nitrate. “Thermodynamic prediction and field observation were clearly at odds,” Li explains. “We were thus curious to re-examine the ecological niche of these microorganisms.”

The team resolved the conflict in the laboratory. Working with two representative DIRM strains—NCP973 from groundwater and Denitromonas iodatirespirans IR-12, originally isolated from seawater—they supplied cultures with both iodate and nitrate simultaneously. In every experiment, the bacteria reduced nitrate first, and only began consuming iodate once the nitrate had been exhausted. The molecular mechanism emerged from transcriptomic analysis: genes encoding nitrate reductase (narGHI) switched on first, while the iodate reductase genes (idrABP1P2) remained silent until nitrate was gone. Nitrate, it turns out, actively suppresses expression of the iodate reductase machinery, a regulatory strategy analogous to its suppression of perchlorate reductase in other bacteria. Iodate adds a further twist: on its own it imposes oxidative stress that forces the cells into a prolonged lag phase, giving nitrate reduction an unassailable head start.

The implication is striking. If iodate reduction always follows nitrate reduction, then DIRMs should not live above the oxygen minimum zones but inside them, in waters where denitrification is actively consuming nitrate. To test this, the researchers turned to environmental genomics, mining metagenomic and metatranscriptomic datasets from the world’s three major oxygen minimum zones—the Eastern Tropical North Pacific, the Eastern Tropical South Pacific, and the Arabian Sea—along with metagenome-assembled genomes (MAGs) from global OMZs and the Tara Oceans expeditions. The pattern was unambiguous. The idrA gene, the genetic signature of iodate reduction, appeared and was expressed almost exclusively within OMZ depth profiles. Among 962 MAGs recovered from global oxygen minimum zones, 32 carried the full idrABP1P2 gene cluster; among 2,631 Tara Oceans MAGs, only 9 carried it, and every single one came from OMZ samples.

The genomic survey did more than confirm the habitat—it dramatically expanded the known cast of iodate-breathing microbes. The OMZ-dwelling DIRMs belonged predominantly to the candidate phylum SAR324 and to the class Alphaproteobacteria, lineages previously not recognized as iodate reducers. Intriguingly, the SAR324 genomes carrying idrABP1P2 also harbor sulfur oxidation genes, hinting that these organisms may couple the oxidation of sulfide to the reduction of iodate—a chemoautotrophic lifestyle that would let them flourish in the chemically stratified heart of oxygen-depleted waters. To verify that the newly discovered genes actually do what they appear to do, the team expressed two representative idrABP1P2 sequences heterologously in laboratory hosts and confirmed iodate-reducing activity.

Rewriting the map of DIRM habitats is more than an exercise in microbial cartography; it changes projections of the marine iodine cycle in a warming world. Global warming lowers the solubility of oxygen in seawater and intensifies ocean stratification, both of which encourage the expansion of oxygen minimum zones. Over the past six decades, the global area covered by OMZs has grown from roughly 5 percent of the ocean to 14 percent, and the trend is expected to continue. Every square kilometer of new oxygen-depleted water is, under the revised model, potential habitat for DIRMs—more microbes converting iodate to iodide, and more iodide available for transport to the surface ocean by circulation. Model simulations cited in the study indicate that a 1 percent increase in global sea-surface iodide concentration produces approximately a 0.7 percent rise in oceanic iodine emissions to the atmosphere.

Those emissions matter on several fronts. Volatile iodine compounds destroy tropospheric ozone, alter the oxidation chemistry of the marine boundary layer, contribute to the formation of new aerosol particles that influence cloud cover and climate, and govern the atmospheric fate of mercury, a potent neurotoxin. They also deliver iodine back to land, where adequate dietary intake prevents goiter and other thyroid disorders that still afflict populations in iodine-poor regions. A microbe’s preferred address in the water column thus ripples outward to human nutrition and planetary climate chemistry alike.

The study also offers a methodological lesson that extends well beyond iodine. Thermodynamic calculations, however elegant, describe what is energetically possible, not what regulatory networks actually permit. The discovery that nitrate represses idrABP1P2 expression—inverting the presumed order of substrate use—illustrates how gene regulation can override energy-yield logic in shaping microbial niches. For biogeochemists accustomed to predicting microbial distributions from redox potentials alone, the message is clear: transcriptomics and genome-resolved surveys of the actual environment must carry at least equal weight. Ecological niches, this work reminds us, are written in regulatory circuits as much as in reaction energetics.

Li and colleagues argue that their findings should now be integrated into marine iodine biogeochemical models. Doing so would sharpen predictions of how oceanic iodine emissions will respond as oxygen minimum zones continue to expand through the coming century. Given that iodine touches everything from stratospheric ozone recovery to cloud droplet formation to human endocrine health, the stakes of getting this cycle right are considerable. The humble iodate-breathing bacteria of the deep, once relegated to a theoretical sliver of the ocean, now appear to command a far larger territory—and their expanding domain may help determine the atmospheric chemistry of a warming planet. “Given the importance of iodine for human and environmental health,” Li notes, “integrating this pathway into marine iodine biogeochemical models will improve our capability of understanding and predicting the future changes in oceanic iodine emissions.”