Dissimilatory iodate-reducing microorganisms, or DIRMs, inhabit the core depths of marine oxygen minimum zones rather than the shallower waters directly above them, according to a study published by researchers at the China University of Geosciences and international collaborators.
Revisiting the Ecological Niche of Marine Iodate-Reducing Microorganisms
Iodine is essential for human and environmental health, with both deficiency and excess leading to thyroid disease. The ocean serves as Earth’s largest iodine reservoir, storing iodate that microbes can reduce into more mobile iodide. At the sea-air boundary, this iodide reacts with ozone to generate volatile iodine species that enter the atmosphere, influencing ozone depletion, mercury cycling, and aerosol formation. DIRMs are specialized bacteria that gain metabolic energy by reducing iodate to iodide. For years, scientists relied on theoretical thermodynamic calculations to assume that DIRMs preferentially use iodate over nitrate. That assumption placed their expected habitat in a narrow zone just above marine oxygen minimum zones, where oxygen is depleted but nitrate reduction has not yet started.
Contradicting Thermodynamic Predictions Through Culture Experiments
Field observations from high-iodine groundwaters in China challenged long-held thermodynamic assumptions, according to Professor Junxia Li of the China University of Geosciences. Researchers noted a widespread negative correlation between iodide and nitrate concentrations in groundwater, alongside the presence of the DIRM strain Azonexus hydrophilus NCP973 in environments where nitrate was already depleted. To resolve this discrepancy, the research team conducted culture experiments using A. hydrophilus NCP973 from groundwater and Denitromonas iodatirespirans IR-12 from the marine environment. Transcriptomic analysis revealed that nitrate reductase genes, designated as narGHI, were expressed before iodate reductase genes, known as idrABP1P2. Nitrate suppresses the expression of these iodate reductase genes, while iodate itself induces oxidative stress that forces the bacteria into an extended lag phase, granting nitrate reduction a competitive edge.
Discovering DIRM Habitats Inside Ocean Oxygen Minimum Zones
Because iodate reduction follows nitrate reduction, researchers hypothesized that DIRMs inhabit oxygen minimum zones where microbial nitrate reduction actively occurs. To evaluate this hypothesis, the team analyzed metagenomic and metatranscriptomic data from three major oxygen minimum zones—the Eastern Tropical North Pacific, the Eastern Tropical South Pacific, and the Arabian Sea—alongside metagenome-assembled genome datasets from global oxygen minimum zones and the Tara Oceans project. The analysis revealed that idrA genes and transcripts concentrate exclusively within oxygen minimum zone depth profiles. Among 962 metagenome-assembled genomes from global oxygen minimum zones, 32 carried the idrABP1P2 gene cluster, while 9 of 2,631 genomes from Tara Oceans carried the genes, all originating from oxygen minimum zone samples. These marine DIRMs belong predominantly to the candidate phylum SAR324 and Alphaproteobacteria. Metagenomes from SAR324 also contained sulfur oxidation genes, indicating that these microorganisms may couple sulfide oxidation directly to iodate reduction.
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