Seaweed, Seagrass and Phytoplankton Shape the Ocean’s Iodine Cycle, Review Finds

Iodine is one of those elements that most people encounter only as a grain of salt on the dinner table, yet in the ocean it sits at the crossroads of climate regulation, human nutrition, air quality and even nuclear safety. A comprehensive new review published in Environmental Chemistry Letters by Yuhi Satoh of the Institute for Environmental Sciences in Aomori, Japan, pulls together decades of scattered research on how iodine behaves in the sea, with a particular focus on the marine primary producers—seaweed, seagrass and phytoplankton—that quietly govern its journey through the environment. The synthesis arrives at a striking conclusion: these organisms do not merely absorb iodine passively, they actively reshape its chemistry, supply it to food chains, sediments and the atmosphere, and in doing so influence processes ranging from cloud formation to the fate of radioactive fallout.

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