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 research team, led by Anja Schroff of the Department of Medical Radiation Sciences, set out to answer a deceptively simple question with major implications for both medicine and emergency preparedness. Iodine-131 is a workhorse of nuclear medicine, used for decades to treat hyperthyroidism and certain thyroid cancers, because the thyroid gland eagerly hoards iodine to build its hormones. But the same isotope is also a signature component of nuclear accident fallout. Understanding exactly how much radiation each organ receives after exposure — and whether that dose differs between males and females, or between the young and the grown — is essential both for optimizing therapy and for refining risk estimates after a nuclear emergency.

To probe the question, the researchers gave male and female Sprague–Dawley rats a controlled internal exposure of 0.36 megabecquerels of iodine-131 at one of two life stages: five weeks of age, representing a young, still-developing animal, and seventeen weeks of age, representing adulthood. Over the following six days, they measured the activity concentration of the isotope in sixteen vital tissues at six time points, ranging from one hour to 144 hours after injection. From these time-activity data, they calculated the mean absorbed dose delivered to each organ, using both analytical methods and Monte Carlo simulations to cross-check their estimates.

The results were striking. As expected, the thyroid gland dominated the picture, showing the highest iodine-131 activity concentration of any tissue in every group — peaking at 18 hours after exposure in males and 24 hours in females, regardless of whether the animals were young or adult. Every other tissue lagged far behind, with the stomach, which also expresses iodine-transporting machinery, coming in a distant second. But the real story lay in the differences between groups, which proved to be anything but subtle.

Female rats consistently accumulated more iodine-131 in their thyroids than males, and — critically — they retained it longer. That combination of higher uptake and slower clearance translated directly into higher radiation dose. The absorbed dose to the thyroid ranged across the study groups from 23 gray per megabecquerel in adult males to a staggering 100 gray per megabecquerel in young females — a more than fourfold difference driven entirely by biological sex and age at exposure. Statistically significant sex differences emerged in all sixteen tissues examined, with the most pronounced effect in the thyroid itself.

Age mattered too, though in a sex-specific way. The most notable age-related differences in thyroid uptake and absorbed dose appeared in males: young males showed markedly higher iodine-131 uptake than adult males, resulting in a substantially greater thyroid dose. In females, whose thyroid uptake was already elevated, the age effect was less prominent. This pattern suggests that the developing thyroid — or the hormonal and metabolic milieu surrounding it — handles iodine differently depending on both its maturity and the animal’s sex, compounding rather than simply adding to the sex effect.

To understand the mechanism behind these differences, the team turned to the sodium-iodide symporter, or NIS, the membrane protein responsible for actively pumping iodide into thyroid cells. Using immunohistochemistry and western blot analysis, they quantified NIS protein expression in thyroid tissue across all groups. The result was a surprise: despite the dramatic differences in uptake and dose, NIS protein levels showed high individual variability but no clear, consistent difference between the groups. Whatever drives females and young animals to accumulate and retain more radioactive iodine, it appears not to be a simple matter of having more of the iodine transporter — hinting at deeper biological factors, perhaps involving hormone regulation, thyroid size, iodine turnover kinetics, or clearance pathways that have yet to be pinned down.

The implications reach well beyond the laboratory. After Chernobyl, epidemiological studies documented a surge in thyroid cancer among people exposed as children to iodine-131 fallout, with the increase particularly pronounced among young girls — a pattern that has long suggested some combination of biological susceptibility and dosimetric difference. This study provides the first rigorous experimental evidence that the dose itself may differ systematically by sex and age, meaning that part of the observed cancer excess could reflect the fact that young females simply received more radiation to their thyroids from the same environmental exposure. The authors are careful to note that dosimetry alone cannot explain everything — biology at the cellular level certainly contributes — but the fourfold dose range they measured is far too large to ignore in risk models.

For nuclear medicine, the findings carry a more immediate practical message. Radioiodine therapy is prescribed to patients of both sexes and all ages, yet dosing protocols have historically been built on assumptions that may not hold uniformly across the population. If sex and age influence thyroid uptake and retention as strongly in humans as they do in rats, personalized dose planning — adjusting administered activity for patient sex and age — could improve therapeutic efficacy while sparing healthy tissue. The study also underscores the value of including both sexes and multiple age groups in preclinical radiopharmaceutical research, a practice that remains inconsistent across the field.

The Gothenburg team, whose work was supported by the Swedish Radiation Safety Authority, the Swedish Research Council, and the Swedish Cancer Society, among others, emphasizes that further research is needed to clarify the underlying biological and mechanistic drivers of the observed differences. Untangling whether hormonal status, thyroid growth dynamics, renal clearance, or other factors govern the sex- and age-dependent handling of iodine will be the next step. But the core conclusion stands on its own: when it comes to radioactive iodine, who you are and how old you are when exposure happens can change the dose your thyroid receives by a factor of four — a biological reality that both radiation oncologists and emergency planners can no longer afford to overlook.

Subject of Research: How sex and age at exposure influence the biodistribution and absorbed dose of radioactive iodine-131 in rats

Article Title: Sex and age at exposure influence 131I biodistribution and dosimetry in Sprague–Dawley rats

Article References: Schroff, A., Insulander Björk, K., Rassol, N., Johansson, J., Lundberg, T., Bakr, H., Andersson, M., Spetz, J., & Forssell-Aronsson, E. (2026). Sex and age at exposure influence 131I biodistribution and dosimetry in Sprague–Dawley rats. Biology of Sex Differences. https://doi.org/10.1186/s13293-026-00989-4

Image Credits: AI Generated

DOI: 10.1186/s13293-026-00989-4

Keywords: radioiodine, iodine-131, thyroid, dosimetry, biodistribution, sex differences, Chernobyl, thyroid cancer, sodium-iodide symporter, radiation protection, Sprague-Dawley rats, nuclear medicine