The numbers reported in the review are remarkable. The highest iodine concentrations ever measured in marine primary producers exceed 1,000 micrograms per gram of dry weight, a level found in both seaweeds and phytoplankton. Some seaweeds go far beyond that, accumulating more than 10,000 micrograms per gram—meaning that in certain brown algae, iodine can account for up to a percent or more of the organism’s dry mass. Seagrasses, by contrast, are comparative lightweights, generally holding less than 400 micrograms per gram of dry weight. These differences are not trivial quirks of biology. They reflect fundamentally different strategies for dealing with an element that is abundant in seawater, where it exists mainly as iodate at concentrations of roughly 0.45 micromolar, and they have cascading consequences for how iodine moves through coastal and open-ocean ecosystems.

Seaweeds, especially brown algae such as kelps and wracks, have long been the stars of iodine research. Studies of species like Laminaria digitata and Fucus vesiculosus have shown that these organisms can take up iodide from seawater with extraordinary efficiency, concentrating it many thousands of times above ambient levels. The review emphasizes that this accumulation is not simply a matter of diffusion. Brown algae appear to store iodine largely as iodide within apoplastic spaces and cell walls, where it can serve as an antioxidant reservoir, rapidly neutralizing reactive oxygen species produced during stress such as desiccation at low tide or intense ultraviolet exposure. When the tissue is damaged or stressed, the stored iodide can be oxidized and released, feeding volatile iodine compounds into the surrounding air and water.

That release mechanism connects seaweed biology directly to the atmosphere. Volatile iodine compounds emitted from marine surfaces contribute to the atmospheric iodine budget, which in turn affects tropospheric ozone destruction and the formation of cloud condensation nuclei. In other words, the iodine metabolism of a kelp forest can, in principle, nudge the radiative properties of the sky above it. The review situates such findings within a broader body of work on marine iodine emissions in a changing world, noting that as sea ice retreats, seaweed farming expands and coastal ecosystems shift, the flux of iodine between ocean and atmosphere may change in ways that climate models have only begun to consider.

Phytoplankton, the microscopic drifters that anchor the marine food web, tell a subtler story. Laboratory studies have shown that various microalgae, including diatoms and the haptophyte Isochrysis, can assimilate iodine, and that the process is linked to the oxidation of iodide at the cell surface. But the review highlights a crucial twist: cell senescence appears to be a dominant driver of iodine transformation in the sea. As phytoplankton cells age, die or are lysed by viruses, they reduce iodate to iodide and release it back into the water. This means that the chemical speciation of iodine in surface seawater—whether it exists as the thermodynamically stable iodate or the more reactive iodide—is partly a fingerprint of the life and death cycles of the plankton community. Blooms, viral crashes and grazing all leave their signatures in the iodine chemistry of the water column.

Seagrasses occupy a middle ground and have received far less attention. The review draws on seasonal studies of the eelgrass Zostera marina on the Pacific coast of central Japan, which found that iodine concentrations in the plant fluctuate through the year in ways that track carbohydrate dynamics rather than any dedicated iodine uptake machinery. Unlike brown algae, seagrasses do not appear to possess specialized iodine-accumulating organs or antioxidant iodide pools of comparable magnitude. Instead, their iodine content seems to be an indirect consequence of survival-related physiology—growth, carbon storage and tissue turnover. This distinction matters because seagrass meadows are major blue carbon habitats, and the review suggests that their role in iodine cycling, while smaller per gram of tissue, could still be significant given the vast areas they cover and the sediments they trap.

The geochemical implications extend deep into Earth’s history. Iodine-to-calcium ratios in marine carbonates have become a widely used proxy for past ocean oxygenation, because iodate is converted to iodide under anoxic conditions and only iodate is incorporated into carbonate lattice. The review underscores that interpreting such paleo-redox archives requires a firm grasp of how modern organisms process iodine, since biological alteration of speciation can complicate the simple redox narrative. Similarly, the distribution of particulate iodine in the oceans, documented since the 1970s, reflects the biophilic nature of the element—its tendency to ride along with organic matter produced by plankton and delivered to the deep sea.

Radioactivity adds urgency to the picture. Radioactive iodine-129, released by nuclear reprocessing plants and accidents such as Fukushima, behaves chemically like its stable counterpart, and studies of seabed sediments off Japan have traced its distribution and long-term fate. Understanding how seaweeds, seagrasses and phytoplankton take up and transform iodine therefore informs radiological risk assessment in coastal waters near nuclear facilities. The same chemical knowledge cuts the other way: because brown algae concentrate iodine so effectively, they are a potent dietary source of the nutrient, and systematic reviews of human nutrition have examined the bioaccessibility and bioavailability of iodine from seaweed foods, along with the risks of excessive intake. The review’s geochemical focus thus touches human health from both directions—deficiency and excess.

What emerges from the synthesis is a unifying idea: with the partial exception of some seaweed species that have evolved dedicated iodine accumulation mechanisms, the factors governing iodine content across marine primary producers are indirect, tied to the ordinary business of staying alive. Growth rate, tissue age, carbon metabolism, oxidative stress and senescence collectively determine how much iodine an organism contains and in what chemical form it exchanges with the environment. This reframing shifts attention away from organism-specific physiology and toward a geochemical perspective in which primary producers act as converters, reservoirs and conduits within the global iodine cycle. As seaweed aquaculture expands to match the carbon burial rates of natural blue carbon habitats, and as seagrass conservation becomes central to biodiversity and climate goals, the iodine dimension of these ecosystems is likely to attract growing scientific and regulatory attention.

The review also charts where knowledge remains thin. Iodine accumulation in seagrasses and phytoplankton is still poorly quantified compared with the rich literature on macroalgae, and the molecular machinery of iodine handling in most marine producers remains only sketchily characterized, with detailed genomic and biochemical studies limited to a handful of microalgal species. Standardized, rapid analytical methods for iodine in algal tissues are improving, which should accelerate comparative work across species and regions. For now, the review stands as both a map of what is known and a call to fill the gaps—because an element that links kelp forests to cloud droplets, ancient anoxic oceans to modern nuclear monitoring, and human thyroid health to the taste of nori deserves nothing less than a fully integrated science of its own.

Subject of Research: Iodine accumulation and cycling in marine primary producers including seaweed, seagrass and phytoplankton

Article Title: Iodine in marine primary producers, seaweed, seagrass, and phytoplankton: a review

Article References: Satoh, Y. (2026). Iodine in marine primary producers, seaweed, seagrass, and phytoplankton: a review. Environmental Chemistry Letters. https://doi.org/10.1007/s10311-026-01928-w

Image Credits: AI Generated

DOI: 10.1007/s10311-026-01928-w

Keywords: iodine, seaweed, seagrass, phytoplankton, marine biogeochemistry, iodine cycle, ocean chemistry, radioiodine, climate regulation, human nutrition, blue carbon, Environmental Chemistry Letters

The central process at work is silicate weathering, a cornerstone of the long-term carbon cycle. When atmospheric carbon dioxide dissolves in rainwater and soil moisture, it forms carbonic acid, a weak acid capable of attacking the crystal lattices of silicate minerals such as feldspars and micas. As these minerals break down, the carbon carried in the acid is transformed into bicarbonate ions dissolved in water. Those ions can then travel through rivers to the ocean, where marine organisms incorporate the carbon into shells and other calcium carbonate structures that eventually settle into seafloor sediments. On geological timescales, this chain of reactions is one of the principal mechanisms by which Earth regulates atmospheric carbon dioxide and, with it, global temperature.

What makes the new analysis striking is its emphasis on drylands as an underappreciated locus for this chemistry. Arid regions receive little rain, so they are often assumed to play a minor role in weathering-driven carbon uptake compared with humid tropical belts where rainfall and vegetation accelerate mineral dissolution. Yet drylands possess distinctive advantages. Intense temperature swings between day and night physically fracture rock surfaces, expanding the reactive area available to chemical attack. Sparse vegetation means that minerals lie close to the surface, exposed to occasional but chemically aggressive runoff events. And, crucially, wind continuously supplies fresh, finely ground dust derived from distant mountain ranges, delivering new reactive material to soils that would otherwise exhaust their weathering potential.

The research team assembled this picture by reconstructing dust deposition and weathering fluxes across eolian archives reaching back to the late Pliocene, an epoch that ended approximately 2.6 million years ago. The late Pliocene is a pivotal interval in Earth’s history: global temperatures were gradually declining, ice sheets were expanding across the Northern Hemisphere, and the modern pattern of arid belts and monsoon circulation was taking shape. By examining the mineralogical and geochemical signatures preserved in dust deposits, the authors were able to track how much silicate material was delivered to dryland soils and how efficiently that material captured carbon dioxide over time.

The results point to persistence rather than volatility. Despite the dramatic climatic oscillations of the past three million years—including the repeated glacial-interglacial cycles of the Pleistocene—the carbon dioxide uptake associated with silicate weathering in eolian drylands appears to have remained remarkably stable. This stability matters because the long-term carbon cycle depends on sinks that behave predictably across changing climates. If a major weathering sink were to weaken abruptly during cold or dry intervals, the balance between volcanic carbon emissions and carbon removal would shift, amplifying climatic swings. The apparent resilience of dryland weathering suggests it has instead acted as a steadying hand, damping rather than reinforcing fluctuations in the global carbon budget.

The mechanistic explanation for this resilience lies in the interplay between supply and demand. In humid regions, weathering rates often saturate: once soils are deeply leached and vegetation covers the landscape, additional carbonic acid cannot significantly accelerate mineral dissolution. Drylands, by contrast, tend to be supply-limited rather than transport-limited. Because chemical reaction rates are slow in arid conditions, freshly deposited dust accumulates in soils awaiting reaction. Even a modest increase in moisture—a stronger monsoon season, a rare intense storm—can mobilize carbonic acid through this stored inventory of fine particles, unlocking weathering that was chemically banked during drier periods. Over thousands to millions of years, this buffering behavior smooths out climatic variability, allowing the carbon sink to persist even as individual decades and millennia fluctuate between dustier and wetter regimes.

The study also carries implications for how scientists model Earth’s climatic future. Most Earth system models represent silicate weathering through simplified parameterizations tuned primarily to temperature, runoff, and lithology, with little explicit treatment of dust supply to arid soils. If eolian drylands contribute a stable and geologically meaningful fraction of global carbon uptake, then changes in dust generation driven by land use, desertification, and shifting wind patterns could subtly alter the trajectory of natural carbon sequestration in the coming centuries. The authors’ reconstruction provides a benchmark against which such model assumptions can be tested, anchoring simulations of deep-time climate in empirical records of dust and weathering chemistry.

There is also a deeper conceptual payoff. For decades, the narrative of drylands in climate science has been dominated by their vulnerabilities: expanding deserts, degrading soils, and human populations exposed to heat and water stress. This research adds a counterpoint, portraying the same environments as engines of planetary regulation. Fine dust lofted from the Sahara, the Gobi, and the world’s other great dust sources does not simply smother ecosystems downwind; it seeds soils with reactive minerals, fertilizes distant oceans with iron, and, according to this study, sustains a chemical removal of carbon dioxide that has operated without interruption since before the Ice Ages began. In that sense, the planet’s dustiest places have been among its most dependable climate stabilizers.

The late Pliocene baseline gives the finding particular weight for understanding the modern atmosphere. Around three million years ago, carbon dioxide concentrations were comparable in broad magnitude to levels considered plausible for the coming decades, and global mean temperatures were warmer than preindustrial values. Reconstructing how weathering sinks behaved under those conditions offers a natural experiment on how the carbon cycle responds to a warmer world. The persistence of dryland silicate weathering through the Pliocene-Pleistocene transition suggests that this sink is robust to the kinds of temperature and hydrological shifts currently under way, though the authors caution that the pace of modern anthropogenic change vastly exceeds the gradual forcing of the late Cenozoic.

As with any reconstruction spanning millions of years, uncertainties remain in translating geochemical proxies into precise fluxes, and the global significance of dryland weathering relative to mountain belts and tropical basins will continue to be debated. But the study’s core message is difficult to ignore: the long-term carbon cycle is woven together by processes operating in places that rarely attract attention. Every dust storm that darkens a distant sky carries within it a shipment of silicate minerals destined to react, slowly and invisibly, with carbonic acid drawn from the air. That humble reaction, repeated across arid landscapes for millions of years, has helped keep Earth’s thermostat within the range that allows oceans, ice sheets, and life to persist. In an era when humanity is rapidly adding carbon dioxide to the atmosphere, understanding the full inventory of natural sinks—including the silent work of wind-blown dust in the world’s drylands—has never been more urgent.

Subject of Research: Long-term silicate weathering and carbon dioxide uptake in eolian drylands since the late Pliocene

Article Title: Persistent silicate weathering and carbon dioxide uptake in eolian drylands since the late Pliocene

Article References: Zhang, C., Wu, H., Hu, B., Qiao, Y., & Guo, Z. (2026). Persistent silicate weathering and carbon dioxide uptake in eolian drylands since the late Pliocene. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-04058-x

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04058-x

Keywords: silicate weathering, eolian drylands, carbon dioxide uptake, long-term carbon cycle, late Pliocene, dust deposition, climate regulation, geochemistry, carbon sequestration, Communications Earth & Environment, Persistent, silicate