The ocean, a vast reservoir of iodine, plays a crucial role in the global biogeochemical cycle of this essential element. Iodine, vital for life, is a delicate balance that can be disrupted by both deficiency and excess, leading to thyroid disease in humans. The marine environment, particularly the sea-air boundary, is a dynamic zone where iodine undergoes fascinating transformations. Iodide (I⁻), a more mobile form of iodine, reacts with ozone at the sea-air interface, influencing ozone depletion, mercury cycling, and aerosol formation. This process also contributes to the supply of terrestrial iodine through deposition.
Dissimilatory iodate-reducing microorganisms (DIRMs) are bacteria that harness energy by converting iodate (IO₃⁻) into iodide (I⁻). For years, scientists believed that DIRMs preferred iodate over nitrate (NO₃⁻) based on theoretical thermodynamic calculations, placing them in a narrow zone above marine oxygen minimum zones (OMZs). However, recent findings have challenged this assumption.
Professor Junxia Li and their team discovered that DIRMs, such as Azonexus hydrophilus NCP973, thrive in environments where nitrate is already depleted. This contradicts the previous assumption that iodate reduction precedes nitrate reduction. The team's culture experiments with two DIRM strains revealed that nitrate reduction occurs first, followed by iodate reduction. This finding is significant because it suggests that DIRMs may not be confined to the narrow zone above OMZs but instead inhabit the oxygen-depleted zones themselves.
The study's metagenomic and metatranscriptomic analysis of three major OMZs and two MAG datasets further confirmed this hypothesis. DIRMs, predominantly belonging to the candidate phylum SAR324 and Alphaproteobacteria, were found to be concentrated within OMZ depth profiles. These microorganisms possess the ability to couple sulfide oxidation with iodate reduction, as evidenced by the presence of sulfur oxidation genes in SAR324 MAGs carrying idrABP1P2. This discovery expands the known diversity of DIRMs and highlights their role in the complex marine ecosystem.
The implications of this research are far-reaching. Global warming, by reducing oxygen solubility and intensifying ocean stratification, is expected to expand OMZs. As these oxygen-depleted zones grow, DIRMs will have larger habitats, leading to increased production of iodide (I⁻). This iodide can be transported to the surface via ocean circulation, resulting in elevated surface iodide concentrations and marine iodine emissions. Model simulations suggest that a 1% increase in global sea-surface iodide concentration can lead to a 0.7% rise in oceanic iodine emissions.
This discovery has significant implications for understanding and predicting future changes in oceanic iodine emissions. By integrating this pathway into marine iodine biogeochemical models, scientists can enhance their ability to forecast the impact of global warming on iodine emissions. The ocean, as a critical component of the iodine cycle, is a dynamic and complex system that continues to reveal fascinating insights into the delicate balance of this essential element.
