Seagrass Meadows and Seaweed Release a Surprising Chemical Cocktail Into Coastal Air

Beneath the surface of the world’s temperate coastlines, forests of seaweed and meadows of seagrass are quietly exhaling a chemical vocabulary far richer than scientists ever suspected. A new study published in the journal Biogeosciences has measured, for the first time in such detail, the biogenic volatile organic compounds (BVOCs) released underwater by three common marine plants, and the results upend the assumption that the ocean’s volatile chemistry can be captured by tracking a handful of familiar molecules. Across two contrasting marine regions, the team detected a total of 166 distinct compounds wafting from just three species, with emission profiles that differed not only between species but also, strikingly, within the same species growing on opposite sides of the North Sea.

BVOCs matter because they punch far above their molecular weight in the climate system. Once airborne, these reactive gases drive the formation of ozone and secondary organic aerosols, tiny particles that scatter sunlight and seed cloud droplets, producing a net cooling effect on the planet. They also influence air quality and can extend the atmospheric lifetime of methane by consuming the oxidants that would otherwise destroy it. Biogenic sources account for more than 80 percent of the global VOC budget, and while forests have been measured exhaustively for decades, the ocean’s contribution remains one of the largest blind spots in atmospheric science. Current atmospheric models still underestimate marine BVOC emissions, largely because the oceans are vast, biologically diverse, and expensive to sample.

Most marine BVOC research has fixated on phytoplankton and on dimethyl sulfide, or DMS, the single most emitted marine BVOC. DMS plays a central role in the global sulfur cycle: when it reaches the atmosphere it is oxidized to sulfuric acid, a key driver of new aerosol particle formation and, ultimately, cloud condensation nuclei. But that focus on plankton blooms and one headline molecule may have concealed the importance of other primary producers, particularly in nutrient-rich, highly productive coastal waters. Enter the benthic macrophytes: the macroalgae and seagrasses that carpet shallow seafloors, form the foundation of coastal ecosystems, and support services ranging from carbon sequestration to fisheries production and shoreline protection.

The scale of the potential contribution is enormous. Macroalgae cover more than six million square kilometers of the globe and seagrass meadows more than 250,000 square kilometers, and unlike ephemeral phytoplankton blooms, they support high biomass year-round while growing at shallow depths close to the sea-air interface. Yet their emission rates, especially for seagrasses, have been poorly quantified. Previous studies often ground up plant tissue to measure volatile contents, a destructive approach that reveals what a plant contains but not what it actually releases into the water, or at what rate. Others relied on gas chromatography methods that identify compounds beautifully but cannot quantify fluxes, or targeted only narrow compound groups such as halocarbons or isoprene, making integration into total budgets nearly impossible.

The new research, led by Max Gräfnings of the University of Helsinki together with colleagues in Finland and Ireland, took a different tack. The team quantified ex situ the BVOC emission rates of three temperate macrophytes: the seagrass Zostera marina, the brown alga Fucus vesiculosus, and the green alga Ulva intestinalis. Measurements were made with a Vocus proton-transfer-reaction time-of-flight mass spectrometer, a high-sensitivity instrument that generates hydronium ions to softly ionize volatile compounds with proton affinities greater than that of water, then resolves them by mass. The instrument, normally used to sniff air, was adapted to sniff seawater: plants were incubated in airtight five-liter glass flasks of filtered, pre-bubbled seawater, purged continuously with clean air, and the stripped volatiles were fed into the analyzer at six-second resolution.

Crucially, the entire experiment was repeated at two marine stations with radically different chemistry. The Irish measurements took place at the Mace Head Atmospheric Research Station on the Atlantic coast, where salinity hovers around 30 practical salinity units and tides rise and fall roughly three meters twice a day. The Finnish measurements were made at Tvärminne Zoological Station on the brackish northern Baltic Sea, where salinity sits near 5 to 6 units, tides are absent, and annual temperatures swing from freezing to over 20 degrees Celsius. By coincidence, both coastal regions were experiencing marine heatwaves during the campaigns, with shallow waters exceeding 20 degrees at both sites.

The results were striking. Zostera, the eelgrass, emitted BVOCs at significantly higher rates per unit biomass than either macroalga in both regions, averaging about 53 to 54 nanograms per gram of dry weight per hour, while the algae stayed below 20. DMS was the single most emitted compound overall, contributing roughly 30 percent of total emissions, consistent with its established dominance in the ocean. But the surprise came from Finnish eelgrass, which emitted sesquiterpenes, large fifteen-carbon terpenoids rarely measured in quantity in marine systems, at its highest rates, nearly 10 nanograms per gram per hour. The researchers speculate that seagrasses, being flowering plants more closely related to terrestrial vegetation than algae are, may simply be predisposed to strong sesquiterpene production, or that the plants were stressed by the prevailing heatwave, since sesquiterpene emissions are known to rise with stress in land plants.

Even more remarkable was the sheer chemical breadth and its variability. Only about half of the compounds emitted by each species were shared between the Finnish and Irish populations, meaning the same species produced markedly different volatile profiles depending on where it grew. The authors point to a suite of plausible drivers: differing salinity and nutrient regimes, seasonal and developmental differences, millennia of local genetic adaptation, and the distinct microbial communities living on and around the plants, which are known to both produce and consume BVOCs. Notably, DMS emissions were lower in the brackish Baltic than the fully saline Atlantic, fitting the role of DMSP, DMS’s precursor, as an osmolyte that helps cells cope with salt. Beyond DMS and terpenoids, the emissions were dominated by a wide, even spread of hydrocarbons and oxygenated hydrocarbons spanning five to fifteen carbons, a profile in stark contrast to terrestrial emissions, which are typically dominated by isoprene and monoterpenes.

That size distribution carries climate weight, because aerosol-forming yields generally scale with molecular size, and many of the newly detected compounds have ten or more carbon atoms, giving them real potential to influence aerosol formation if they reach the air. Whether they do is the critical open question: in the open ocean only a small fraction of dissolved BVOCs, roughly 10 percent of DMS, actually escapes to the atmosphere, with the rest consumed by microbes or lost to photooxidation en route. Macrophytes’ shallow habitats and, in intertidal zones, their daily exposure to air at low tide may raise that fraction, but the transfer rates remain unquantified. The authors also caution that no single analytical method captures the full picture: proton-transfer mass spectrometry cannot detect volatile halocarbons, whose proton affinities fall below water’s, while gas chromatography struggles with polar, oxygenated compounds.

The stakes rise when the study’s context is considered. Global macrophyte cover has declined sharply over the past century, with kelp forests down an estimated 40 to 60 percent and seagrass meadows down around 19 percent, a trend expected to continue. Because deforestation on land demonstrably reduces BVOC emissions and atmospheric aerosol formation, the ongoing loss of underwater vegetation may already be eroding a climate-relevant ecosystem service that has gone almost entirely unnoticed. The researchers estimate that their measured eelgrass emission rates translate to roughly 12 micrograms of BVOCs per square meter per hour using a global mean seagrass biomass, but they stress that upscaling further is premature given the high unexplained variability and scarcity of data. What the study delivers instead is a foundation: some of the most complete macrophyte emission-rate data to date, a demonstration that reliable estimates demand observations from multiple populations in contrasting environments, and a clear signal that the ocean’s coastal vegetation speaks a far more complex chemical language than the models have yet learned to hear.

Subject of Research: Biogenic volatile organic compound emissions from temperate marine macrophytes in the Baltic Sea and northeast Atlantic

Article Title: Temperate marine macrophytes are highly variable sources of Biogenic Volatile Organic Compounds – a comparative study from the Baltic Sea and NE Atlantic

Article References: Gräfnings, M., Luo, Y., Zhao, J., Cara-Ortega, C. L., Fossum, K. N., Graeffe, F., Lei, L., Stengel, D. B., Thakur, R. C., Ovadnevaite, J., Ehn, M., & Gustafsson, C. (2026). Temperate marine macrophytes are highly variable sources of Biogenic Volatile Organic Compounds – a comparative study from the Baltic Sea and NE Atlantic. Biogeosciences, 23(19), 7183-7201. https://doi.org/10.5194/bg-23-7183-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-7183-2026

Keywords: biogenic volatile organic compounds, marine macrophytes, seagrass, macroalgae, dimethyl sulfide, sesquiterpenes, Baltic Sea, northeast Atlantic, atmospheric chemistry, secondary organic aerosol, coastal biogeochemistry, Vocus PTR-TOF-MS