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

The research team, led by Jann-Gerrit T. Müller of the Leibniz Institute for Baltic Sea Research in Warnemünde, set out to answer a deceptively simple question. Contaminants of emerging concern, a category that includes pharmaceuticals, personal care products, and industrial chemicals, are routinely detected in coastal waters at concentrations ranging from nanograms to micrograms per liter. Yet their fate is not governed by inputs alone. It depends on the chemistry of the receiving water, and chief among those variables is the vast, poorly characterized pool of dissolved organic matter, or DOM, that pervades every aquatic ecosystem.

DOM is far more than background noise. This complex mixture of carbon compounds can shield pollutants from light by absorbing photons before they reach their targets, or it can quench the reactive intermediates that break contaminants down. At the same time, DOM can act as a photosensitizer, generating an arsenal of photoproduced reactive intermediates, including excited triplet states of DOM itself, hydroxyl radicals, and singlet oxygen, that attack pollutants indirectly. Which of these competing roles wins out depends on the composition of the DOM, and that composition shifts dramatically with the seasons. Spring runoff delivers humic, terrestrial compounds to the Baltic coast, while summer phytoplankton blooms flood the water with protein- and carbohydrate-like material.

To disentangle these effects, the researchers designed a factorial photodegradation experiment using water from three regionally relevant sources. North Sea water from the German Bight supplied marine DOM, a forest stream near Rostock supplied terrestrial DOM, and brackish water from the Heiligendamm pier on the southwestern Baltic coast served as the unamended background matrix. After filtering all waters through 0.22-micrometer membranes and extracting DOM by solid-phase extraction, the team prepared four matrices: ultrapure water, background water alone, and background water spiked with either terrestrial or marine DOM at roughly three times the background dissolved organic carbon concentration.

Into each matrix went a cocktail of eight contaminants selected for their high detection frequency and risk potential in a recent year-long survey of the Rostock estuary: the pharmaceuticals sulfamethoxazole, carbamazepine, and diclofenac; the natural estrogens estrone and 17β-estradiol; the synthetic estrogen 17α-ethinylestradiol; the sunscreen UV filter benzophenone-4; and caffeine. Each compound was spiked to 2 micrograms per liter, a level ten to one hundred times above typical coastal concentrations but far closer to environmental reality than the concentrations used in many comparable studies. Twenty-eight bottles were incubated for 68 days under UVA-340 lamps emitting at 300 to 450 nanometers with an integrated irradiance of 17 watts per square meter, closely matching the mean diurnal UV irradiance measured at the nearby Warnemünde coast. Identical bottles wrapped in opaque boxes served as dark controls.

Optical monitoring revealed that the two DOM sources were chemically distinct from the start. Terrestrial DOM showed the highest aromaticity and average molecular weight, indicated by elevated carbon-specific UV absorbance at 254 nanometers and lower spectral slopes. Parallel factor analysis of fluorescence spectra resolved seven components, dominated by humic-like fluorophores that accounted for 90 percent of total fluorescence in the terrestrial matrix, 80 percent in the marine matrix, and 70 percent in the unamended Baltic water. Over the irradiation period, humic-like fluorescence declined by roughly a third while protein-like fluorescence rose by more than half, and the two DOM-spiked matrices converged optically, suggesting that terrestrial DOM contained a larger pool of photodegradable, optically active material.

The pollutant results were strikingly compound-specific. Diclofenac vanished fastest, degrading below detection limits within hours in every matrix, confirming direct photolysis as its dominant removal pathway. Sulfamethoxazole degraded fastest in ultrapure water, though the authors caution that pH-dependent speciation makes that result a poor guide to natural conditions. Caffeine and carbamazepine, by contrast, degraded more slowly in the unamended background water than in the DOM-spiked matrices, indicating that higher organic matter concentrations promoted their indirect photodegradation, likely through excited triplet states of DOM. The UV filter benzophenone-4 proved stubbornly persistent, degrading by no more than 82 percent even in DOM-containing matrices, a finding consistent with its design as a stable sunscreen agent and raising concerns about its accumulation in recreational coastal waters.

The estrogens told the most consequential story. All three degraded equally fast under irradiation, confirming photodegradation as an effective sink, but their breakdown was consistently and distinctly slower in the terrestrial DOM matrix than in the marine or brackish matrices, an effect that appeared independent of dissolved organic carbon concentration. The researchers attribute this inhibition to the higher aromaticity and humic-like fluorescence of terrestrial DOM, which may preferentially quench the reactive oxygen species known to drive estrogen degradation. In the dark controls, 17β-estradiol disappeared while estrone accumulated, consistent with microbial interconversion between the two compounds, whereas the synthetic estrogen 17α-ethinylestradiol showed no dark degradation at all, suggesting it could accumulate under light-limited conditions.

Estimated half-lives in unamended coastal water, doubled to approximate the natural day-night cycle, ranged from about four hours for diclofenac to fifteen days for sulfamethoxazole, with estrone and the synthetic estrogen at roughly three days each. Translated into environmental terms, diclofenac would fall below a risk quotient of 0.1 within a day, while the synthetic estrogen would need about thirty days and sulfamethoxazole about forty-three. Caffeine, with an estimated photodegradation half-life of 158 days, and carbamazepine at 28 days, may owe their high detection frequencies in regional surveys partly to this photochemical sluggishness. The authors emphasize that these estimates assume favorable irradiation conditions in clear, shallow surface waters and that the study’s small sample size limits generalization.

The climate connection is what elevates these findings from a laboratory curiosity to an environmental warning. Projected increases in terrestrial runoff, flooding, and permafrost thaw are expected to deliver ever more terrestrial DOM to coastal waters, and the Baltic Sea is already showing signs of browning. If terrestrial organic matter systematically retards estrogen photodegradation, as this study indicates, rising riverine inputs could prolong environmental exposure to compounds that disrupt endocrine systems at sub-nanogram-per-liter concentrations. The effect appears small in absolute terms, but given the extreme ecotoxicity of estrogens, even modest delays in degradation may matter. The authors call for factorial experiments with multiple independent DOM sources at varying carbon concentrations, combined with measurements of microbial community composition and metabolic activity, to separate photochemical from biological degradation and to build more reliable predictions of how coastal seas will process the pollutant mixtures of the Anthropocene.

Subject of Research: Photodegradation of emerging organic pollutants as a function of dissolved organic matter quality in coastal Baltic Sea waters

Article Title: Dissolved organic matter quality and emerging pollutants in coastal Baltic Sea waters: Insights into photodegradation

Article References: Müller, J.-G. T., Osterholz, H., & Kanwischer, M. (2026). Dissolved organic matter quality and emerging pollutants in coastal Baltic Sea waters: Insights into photodegradation. Environmental Advances, 26, Article 100755. https://doi.org/10.1016/j.envadv.2026.100755

Image Credits: AI Generated

DOI: 10.1016/j.envadv.2026.100755

Keywords: Baltic Sea, dissolved organic matter, photodegradation, contaminants of emerging concern, estrogens, pharmaceuticals, caffeine, benzophenone-4, PARAFAC fluorescence, coastal biogeochemistry, climate change, water quality