The browning of coastal waters has emerged as one of the more visually striking signatures of global change in the marine realm. Across large stretches of the Northern Hemisphere, increasing runoff from land delivers terrigenous dissolved organic matter to the sea, staining coastal waters a tea-like amber colour and fundamentally altering the physics and chemistry of the underwater environment. This material, collectively referred to as coloured dissolved organic matter, or CDOM, originates largely from the decomposition of plant material in soils, wetlands, and forests. As precipitation patterns intensify and permafrost thaws, the flux of this terrestrial organic carbon into rivers and ultimately the coastal ocean is expected to rise substantially. While limnologists have studied lake browning for decades, its consequences for marine plankton communities have received comparatively less attention, partly because the open ocean has traditionally been viewed as too distant from terrestrial influences to be affected. The new mesocosm findings underscore that this assumption no longer holds for many coastal seas, where the interplay between land-derived organic matter and trace metal chemistry can reorganise the base of the food web.

The experimental approach behind these findings deserves particular attention because of the realism it offers. Mesocosm experiments occupy a valuable middle ground between laboratory cultures and field observations, allowing researchers to manipulate single variables while keeping an otherwise natural plankton community intact. In this study, large enclosed volumes of coastal seawater received additions of a humic substance as a source of CDOM and deferoxamine B, a strong iron-binding ligand, as a means of manipulating the speciation of dissolved iron. Deferoxamine B is a siderophore produced naturally by bacteria to acquire iron, and its use in experiments allows researchers to control how much iron remains biologically available. By combining these two treatments in a fully crossed design over 22 days, the team could separate the effects of light attenuation caused by browning from the effects of altered iron chemistry, and crucially, examine their interactions. This is important because in the real ocean these two stressors do not arrive independently; humic substances are themselves complexing agents that bind trace metals, meaning that increased terrigenous organic matter input changes not only the light field but also the chemical form and availability of iron.

The optical consequences of browning are more nuanced than simple darkening. CDOM absorbs strongly in the ultraviolet and blue portions of the spectrum, the very wavelengths that penetrate deepest in clear water and that many phytoplankton pigments exploit most efficiently. As terrigenous CDOM accumulates, the euphotic zone shoals, the spectral quality of available light shifts toward the green, and total photons reaching photosynthetic organisms decline. For large, slowly sinking phytoplankton species that depend on well-lit surface layers, these changes can be directly detrimental. Yet the experiment revealed a paradox: during the first eight days, small-sized phytoplankton actually increased their biomass under browning despite these unfavourable light conditions. The explanation lies in the dual nature of dissolved organic matter as both a screen and a substrate. Small phytoplankton and mixotrophic organisms can exploit dissolved organic compounds directly as a supplementary carbon and nutrient source, a capacity that compensates for reduced photosynthetic income. At the same time, the biomass gains suggest that growth under these conditions outpaced the grazing pressure exerted by microzooplankton, indicating that the net effect of browning on the smallest primary producers was initially positive rather than negative.

Iron chemistry sits at the heart of why this matters for ocean biogeochemistry. Iron is an essential micronutrient for phytoplankton, required for photosynthetic electron transport chains, nitrogen assimilation enzymes, and respiratory proteins. In vast regions of the ocean, iron availability limits primary production, and even in coastal waters where total iron concentrations are higher, the fraction that is truly bioavailable is governed by complexation with organic ligands. More than 99 percent of dissolved iron in seawater is typically bound to organic ligands, and the identity and origin of these ligands strongly influence whether microorganisms can access the metal. Terrestrial humic substances are increasingly recognised as important coastal ligands for iron. The experimental use of deferoxamine B to manipulate dissolved iron speciation therefore mimics a natural process: as browning intensifies, the organic ligand pool in coastal waters is augmented with terrigenous compounds that can hold iron in solution and potentially deliver it across microbial cell membranes. The finding that both phytoplankton succession and microzooplankton dynamics responded to altered iron speciation demonstrates that climate-driven changes in organic matter inputs propagate through the food web partly via trace metal chemistry, a pathway that is often overlooked in favour of purely optical or carbon-centric framing.

The temporal structure of the observed responses carries its own lessons. The benefits of browning for small phytoplankton were confined to roughly the first half of the experiment; during the second half, no changes attributable to browning were detected. This transient response pattern likely reflects the exhaustion of labile components of the added organic matter, adaptive adjustments by grazers, or the reorganisation of the community toward a new quasi-equilibrium. It also echoes a recurring theme in mesocosm research: the earliest days of a manipulation often capture the most dramatic effects, while communities buffer and reorganise over time. For ecosystem managers and modellers, this suggests that the consequences of pulsed terrigenous organic matter inputs, such as those following storm-driven runoff events, may be most pronounced in the days immediately following the pulse, even if the standing community recovers subsequently.

Perhaps the most consequential phase of the experiment came with the crash of both phytoplankton and microzooplankton populations under intense mesozooplankton grazing. This top-down control illustrates a fundamental feature of plankton ecosystems: the fate of bottom-up stimulation, whether from nutrient enrichment or organic matter subsidisation, is ultimately determined by the grazing community. Copepods and other mesozooplankton graze not only the phytoplankton directly but also, through their consumption of microzooplankton, exert cascading control over the microbial loop. The authors’ hypothesis that browning and altered iron availability could favour the vertical migration of mesozooplankton toward the surface in a darkening ocean adds a behavioural dimension to these biogeochemical and optical effects. Zooplankton vertical migration is among the largest organised animal movements on Earth, and it plays a central role in the biological carbon pump through the active transport of carbon to depth. If darker surface waters alter the trade-off between feeding opportunity in the surface and predation risk in the illuminated layer, the depth, timing, and intensity of migration could shift, with cascading consequences for carbon export, food web coupling, and the efficiency of energy transfer to fish larvae that depend on grazing zooplankton.

The broader climatic context amplifies the significance of these coastal findings. Coastal seas disproportionately matter for global biogeochemical cycling relative to their area; they host a large share of global fisheries landings, bury substantial amounts of organic carbon in sediments, and mediate the transfer of terrigenous material to the open ocean. Changes in the structure of coastal plankton communities therefore ripple outward, affecting carbon sequestration, the productivity of commercially important stocks, and the transport of organic matter and metals across the shelf. Furthermore, browning interacts with other simultaneous stressors, including ocean warming, acidification, deoxygenation, and eutrophication. Darker waters warm differently because absorbed radiation is trapped nearer the surface, stratification patterns change, and the balance between heterotrophic and autotrophic processing of carbon may tip toward respiration and CO2 outgassing. The demonstration that CDOM and iron jointly reshape phytoplankton succession and zooplankton behaviour provides an empirical foundation for incorporating these linkages into Earth system models, which historically represent coastal processes in highly simplified ways.

Finally, the study highlights the value of long-running European mesocosm infrastructure and international collaboration in addressing questions that no single nation or discipline could resolve alone. The combination of expertise in algal physiology, trace metal chemistry, optical ecology, and zooplankton behaviour was essential to disentangle the mechanisms at play. As climate change continues to intensify the hydrological cycle and mobilise terrigenous carbon from soils, experiments of this kind, conducted over ecologically meaningful timescales with realistic manipulations, will be indispensable for anticipating how the microscopic communities at the base of the marine food web will reorganise. The picture that emerges is one of an ocean whose colour is not merely an aesthetic signature of change but an active driver of ecological and biogeochemical transformation, mediated as much by metal-ligand chemistry and animal behaviour as by the amount of light reaching the cells that anchor nearly all marine food webs.

Subject of Research: …

Article Title: Coloured Dissolved Organic Matter and Iron Rewire the Marine Plankton Food Web in a Changing Climate

Article References: Segovia, M., Ramírez, L., Vázquez, V., Arnone, V., González-Santana, D., León, P., Santana-González, C., Santana-Casiano, M., González-Dávila, M., Tsagaraki, T. M., Smerdou, C., López-Parages, M., Macías, M., Nejstgaard, J. C., Cañete, S., Berger, S. A., Egge, J. K., & Larsen, A. (2026). Coloured Dissolved Organic Matter and Iron Rewire the Marine Plankton Food Web in a Changing Climate. Microbial Ecology. https://doi.org/10.1007/s00248-026-02856-6

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02856-6

Keywords: Coloured, Dissolved, Organic, Matter, Iron, Rewire, Marine, Plankton, Food, Changing, Climate, scientific research

The story begins with the plastisphere, the term coined for the communities of bacteria, algae, fungi, and other microbes that rapidly form biofilms on plastic debris once it enters aquatic environments. Diatoms are consistently among the earliest and most successful colonizers of these artificial surfaces. Because plastics are buoyant, long-lived, and chemically inert compared with natural substrates, they offer diatoms a novel and stable ecological niche in the sunlit surface ocean. Field surveys from the Black Sea to intertidal habitats worldwide, along with laboratory mesocosm experiments, have repeatedly documented genera such as Chaetoceros, Thalassiosira, Skeletonema, Navicula, and Nitzschia dominating plastic-associated biofilms, sometimes outcompeting every other photosynthetic organism on the particle.

What makes this dominance ecologically significant is that many of these same diatom genera are well known as producers of oxylipins, a family of lipid-derived defensive chemicals. When diatom cells are damaged, for example when a copepod crunches them during feeding, enzymes called lipoxygenases are released and convert membrane fatty acids into a diverse array of oxylipins, including polyunsaturated aldehydes and linear oxygenated fatty acids. Decades of laboratory and field research have shown that these compounds are teratogenic to copepods: they reduce egg production, impair hatching success, and can kill nauplii outright, sometimes with devastating effects on entire cohorts. In extreme cases documented in the literature, diets rich in oxylipin-producing diatoms such as Skeletonema marinoi have led to one hundred percent naupliar mortality.

Copepods, meanwhile, occupy a pivotal position in ocean ecology. With roughly thirteen thousand described species, they can make up seventy to ninety percent of mesozooplankton abundance in many regions, forming the critical link between phytoplankton and commercially important fish, marine invertebrates, and even cetaceans. They also play a major role in the ocean carbon cycle, packing organic material into dense fecal pellets that sink rapidly and transporting carbon through their daily vertical migrations. Because copepods passively entrain particles of suitable size into their feeding currents, they readily ingest microplastics that overlap in dimensions with their natural prey such as diatoms, dinoflagellates, and ciliates.

The consequences of this ingestion are well documented. Laboratory studies have shown that exposure to microplastic concentrations as low as fifty particles per milliliter can alter copepod feeding behavior, prey selection, reproductive output, molting cycles, and lipid production. In the copepod Acartia tonsa, polystyrene beads administered during egg formation produced smaller eggs, reduced naupliar survival, and population models projecting a thirty-fold decline over twenty generations. Molecular analyses reveal that microplastic ingestion triggers oxidative stress pathways involving MAPK and Nrf2 signaling, drains cellular energy reserves, and compromises swimming performance. Fecal pellets produced by exposed copepods become smaller and sink more slowly, potentially weakening the biological pump that carries carbon to the deep ocean.

The striking observation at the heart of the new review is that these microplastic effects closely resemble those historically attributed to oxylipin-producing diatoms. Reduced egg viability, impaired hatching, naupliar mortality, and disrupted maternal investment appear in both bodies of literature. Donnarumma and Russo propose that the overlap may not be coincidental. When a copepod ingests a plastic particle densely colonized by diatoms, the mechanical crushing of those cells during digestion could liberate lipoxygenases that then react with polyunsaturated fatty acids from any of the other organisms in the gut, whether eukaryotic or prokaryotic, generating oxylipins on the spot. In effect, the plastic particle would function as a chemical weapon factory inside the grazer.

Biofouling makes this scenario more plausible rather than less. Experiments consistently show that copepods ingest aged, biofilm-coated microplastics more readily than pristine particles, apparently because microbial colonization makes the plastic smell and behave more like food. Biofilm-derived infochemicals can even act as foraging cues that attract grazers. Meanwhile, nutrient limitation, a condition that increases oxylipin production in diatoms, also drives diatoms to over-secrete adhesive extracellular polymeric substances, making their biofilms stickier and more robust on plastic surfaces. The result is a particle that is simultaneously more attractive to copepods and more chemically loaded with potential toxin producers.

The authors are careful to stress that no study has yet directly demonstrated oxylipin synthesis by epiplastic diatoms on plastic debris. Their framework remains a hypothesis, albeit one grounded in converging lines of indirect evidence. To test it, they propose a clear experimental roadmap: culture an oxylipin-producing species such as Skeletonema marinoi on plastic debris, extract and characterize the resulting metabolites using liquid chromatography with tandem mass spectrometry, and compare copepod responses to sterile versus diatom-colonized particles. Adult females would be exposed to four treatments, healthy diets, free-living diatoms, sterile microplastics, and colonized microplastics, with hatching success and gene expression serving as key endpoints. Field surveys using fine neuston nets, rather than the standard manta nets that miss the relevant size fractions, would then establish whether the mechanism operates in nature.

The stakes extend well beyond copepod physiology. If plastic-borne oxylipins compound the reproductive failures already observed in laboratory studies, population-level consequences could ripple upward through marine food webs, reducing prey availability for fish larvae and other consumers. Disruption of copepod-mediated carbon export could further alter the efficiency of the biological pump, with implications for climate-relevant biogeochemical cycles. The authors also highlight a glaring geographic bias in existing research toward the Northern Hemisphere, leaving the Southern Ocean and much of the tropics as data deserts in which the combined stress of microplastics and epiplastic chemistry remains entirely unquantified.

For now, the review reframes microplastic pollution as a dual threat: a physical and nutritional burden on grazers, and a potential vector for chemical stress generated by the living communities that plastics carry with them. Whether that second threat is real in the ocean will depend on the experimental and field campaigns the authors now call for, but the convergence of evidence they assemble makes the hypothesis one of the most intriguing new directions in marine chemical ecology.

Subject of Research: Microplastics as potential vectors of diatom-derived oxylipins affecting copepod physiology and reproduction

Article Title: Microplastics as potential vectors of diatom oxylipins and possible effects on copepods

Article References: Donnarumma, V., & Russo, E. (2026). Microplastics as potential vectors of diatom oxylipins and possible effects on copepods. Discover Ecology, 2(1), Article 11. https://doi.org/10.1007/s44396-026-00029-w

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00029-w

Keywords: microplastics, plastisphere, diatoms, oxylipins, copepods, marine chemical ecology, biofilms, copepod reproduction, biological pump, marine pollution, Skeletonema marinoi, zooplankton

The study, published in the journal Ocean Microbiology, drew on the metaPR2 database, a curated collection of processed 18S rRNA gene metabarcodes from more than forty studies spanning thousands of samples worldwide. The researchers classified roughly 47,000 marine protist amplicon sequence variants, or ASVs, into functional categories using the Mixoplankton Database, a resource that catalogues known mixotrophic species and their trophic strategies. Four mixoplankton types emerged as the focus of the analysis: constitutive mixoplankton, which build their own chloroplasts; generalist non-constitutive mixoplankton, which steal chloroplasts from a broad range of prey; plastidic specialist non-constitutive mixoplankton, which selectively retain plastids and even nuclear material from specific prey for weeks or months; and endosymbiotic specialist non-constitutive mixoplankton, which harbor long-term photosynthetic symbionts inside their cells.

These distinctions matter because each strategy carries different physiological costs and ecological consequences. Constitutive mixoplankton, which include familiar dinoflagellate and haptophyte genera such as Alexandrium, Karlodinium, and Karenia, can photosynthesize continuously while opportunistically consuming prey. Generalists of the non-constitutive kind, such as ciliates in the genera Strombidium and Laboea, must feed frequently, on scales of hours to days, because their stolen plastids degrade quickly. Plastidic specialists such as Mesodinium and Dinophysis can maintain sequestered photosynthetic machinery for extended periods, while endosymbiotic specialists like Ornithocercus and the green form of Noctiluca scintillans sustain stable partnerships with algal cells that contribute sugars and recycle nutrients derived from prey digestion.

After filtering the data to euphotic-zone samples, those from the upper 200 meters where light penetrates, the final dataset comprised nearly 44,000 ASVs and more than 366 million reads from 4,190 samples reaching latitudes from roughly 74 degrees south to 89 degrees north. Mixoplankton appeared in 94 percent of the samples, confirming their global ubiquity. Overall, mixoplankton accounted for about 7 percent of protist ASVs, corresponding to 3,537 sequence variants and 192 species, which represents some 44 percent of the species listed in the Mixoplankton Database. Protozooplankton and parasites dominated the ASV counts at 38 and 24 percent respectively, while diatoms made up 8 percent and other phytoplankton 22 percent.

To untangle the patterns hidden within this enormous dataset, the team turned to self-organizing maps, an unsupervised machine learning technique that condenses thousands of ASV abundance profiles into two-dimensional neuronal grids that can then be hierarchically clustered. Because sequencing methodology, particularly the choice between the V4 and V9 hypervariable regions of the 18S rRNA gene and the seawater filtration strategy, strongly shapes recovered community composition, the researchers deliberately analyzed separate subdatasets defined by consistent methods rather than pooling everything together. The clustering, applied independently to three subdatasets, each explained between 75 and 78 percent of total variance and produced community groupings that aligned with four major oceanic biomes: polar, subpolar to temperate, temperate to subtropical, and subtropical to tropical.

Those biome assignments were far from arbitrary. Principal component ordination and temperature-salinity-nitrate diagrams showed that the machine learning clusters ordered themselves consistently along environmental gradients of temperature, salinity, and nitrate concentration, with statistical tests confirming significant differences among clusters. Polar and subpolar communities were associated with the coldest waters and highest nitrate levels, while temperate, subtropical, and tropical communities corresponded with warmer, nutrient-poor conditions. Salinity played a comparatively weaker structural role, likely because it varies over a relatively narrow range in marine waters. The resulting biogeography matched classical oceanographic regions described in earlier plankton studies, lending confidence to the approach.

Within this framework, each mixoplankton functional type revealed its own ecological signature. Constitutive mixoplankton were broadly distributed across all biomes and showed distributional patterns closely paralleling those of non-diatom phytoplankton, suggesting either functional overlap or shared resource use between the two groups. Generalized additive models, which can capture non-linear relationships, showed that constitutive mixoplankton reached high relative abundances across a wide temperature span from near zero to 30 degrees Celsius, typical oceanic salinities, and low nitrate concentrations, consistent with the idea that mixotrophy confers a competitive advantage when dissolved nutrients are scarce. Endosymbiotic specialists, by contrast, were restricted to warmer subpolar through tropical waters and were largely absent from polar regions, with more than 88 percent of their sequence variants in one subdataset belonging to Collodaria, radiolarian colonies characteristic of oligotrophic open oceans.

The remaining two mixotypes were scarcer but ecologically revealing. Generalist non-constitutive mixoplankton, the least abundant group, consistently co-occurred with diatoms and extended into nitrate-rich regimes of 20 to 30 micromolar, echoing their dependence on frequent ingestion of phototrophic prey that flourish in productive waters; they were also detected in upwelling zones such as the equatorial Pacific and the Agulhas Current. Plastidic specialists spanned all biomes but were sparse, and in this study appeared in lower-nutrient conditions than previously reported, a shift the authors attribute to seasonal sampling differences and the capacity of retained plastids to sustain photosynthesis across varying nutrient regimes. Diatoms themselves, the only protists confidently confirmed as strictly phototrophic, were predominantly tied to cold, nitrate-rich waters, while protozooplankton and parasites displayed trends generally inverse to those of the mixoplankton, hinting at partitioned consumer niches and host-driven distributions.

The analysis also exposed how profoundly methodological choices shape what scientists see. Mixoplankton richness and relative abundance were, respectively, threefold and sixfold higher in the V9 dataset than in the V4 dataset, largely because the Tara Oceans V9 data captured radiolarians whose exceptionally high rRNA gene copy numbers are differentially amplified by the two marker regions. Comparisons of samples sequenced with both markers showed roughly 60 percent species overlap and significantly correlated abundances, yet one endosymbiotic radiolarian, Collozoum amoeboides, appeared three orders of magnitude more abundant in V9 than in V4. Filtration strategy added further complications, since fragile cells can be disrupted during size fractionation while unfractionated samples can mask rarer groups. The authors stress that these discrepancies do not undermine the conclusions but underscore the need for careful, method-aware interpretation.

By placing mixoplankton within the full context of marine protistan communities at a global scale, the study delivers the first community-level assessment of mixoplankton biogeography relative to co-occurring functional types, and it establishes an empirical foundation for incorporating these organisms into predictive models of marine ecosystem dynamics. The researchers argue that future work should prioritize targeted detection of the underrepresented generalist and plastidic specialist types, whose sparse detection partly reflects their small numbers of known species and the fragility of their cells, and should embrace emerging transcriptomic machine learning methods that can infer trophic mode from gene expression in field communities. As oceans warm and nutrient cycles shift, knowing which mixoplankton strategies dominate where, and under what environmental conditions, may prove essential for forecasting how marine food webs and biogeochemical cycles will respond.

The recognition of mixoplankton as a distinct ecological category represents a relatively recent shift in plankton science. For much of the twentieth century, marine protists were sorted into a simple dichotomy of phytoplankton and zooplankton, an arrangement that implicitly assumed photosynthesis and phagotrophy were mutually exclusive trophic modes. Observations of planktonic ciliates carrying algal plastids and dinoflagellates consuming prey date back more than a century, but only with the development of trait-based frameworks and curated databases has the full diversity of these strategies become systematically catalogued.

The ecological stakes of this reclassification are considerable. Because mixoplankton can acquire nutrients through both dissolved uptake and prey ingestion, they occupy a flexible position in microbial food webs, capable of acting as primary producers when inorganic nutrients are scarce and as grazers when prey are abundant. This dual capacity influences how carbon and nitrogen move through planktonic communities, and models that omit mixotrophy risk misallocating energy flow and nutrient recycling pathways.

The global niche patterns documented in the study also carry implications for a changing ocean. As surface waters warm and stratification intensifies, nutrient supply to the euphotic zone is expected to decline in many regions, conditions that favor organisms able to supplement photosynthesis with feeding. The observed affinity of constitutive mixoplankton for warm, oligotrophic waters, and of endosymbiotic specialists for tropical and subtropical biomes, suggests that these groups may expand as such conditions become more widespread, potentially reshaping community composition and the efficiency of biological carbon export.

Equally important is the methodological legacy of the work. By demonstrating that marker gene choice and sample processing measurably alter perceived mixoplankton abundance, the analysis provides a cautionary benchmark for future metabarcoding surveys and underscores the value of standardized, method-aware databases for tracking marine biodiversity over time.

Subject of Research: Environmental structuring of mixoplankton functional types within marine protist communities: a global analysis

Article Title: Environmental structuring of mixoplankton functional types within marine protist communities: a global analysis

Article References: Larsson, M. E., Leles, S. G., Mitra, A., Faure, E., Vaulot, D., & Santoferrera, L. (2026). Environmental structuring of mixoplankton functional types within marine protist communities: a global analysis. Ocean Microbiology, 2(1), Article 1. https://doi.org/10.1186/s44375-026-00007-3

Image Credits: AI Generated

DOI: 10.1186/s44375-026-00007-3

Keywords: Environmental, structuring, mixoplankton, functional, types, within, marine, protist, communities, global, analysis, scientific research