The research team hand-gathered mussels from sublittoral rocks and other hard substrates at depths of one to six meters, sampling sixteen sites along the coast in June and September across 2017 and 2018. Back in the laboratory, the animals were dissected and three organs were analyzed separately: the gills, the digestive gland and the foot. Each organ was frozen in liquid nitrogen and stored at minus 80 degrees Celsius before being homogenized in potassium phosphate buffer under strictly controlled cold conditions. The team then measured a battery of biochemical indicators spectrophotometrically, including lipid peroxidation, glutathione levels, and the activities of six key antioxidant enzymes: catalase, superoxide dismutase, glutathione peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase and glutathione-S-transferase.

The logic behind the study rests on a well-established concept in ecotoxicology: oxidative stress. When marine organisms encounter metals, polycyclic aromatic hydrocarbons or the effects of eutrophication, their cells generate reactive oxygen species that can damage lipids, proteins and DNA. In response, a suite of antioxidant enzymes and molecules mobilizes to neutralize these threats. Because this cellular alarm system reacts before damage becomes visible at the level of whole organisms or populations, oxidative stress biomarkers are regarded as an early warning of environmental deterioration. Mussels, as sessile filter-feeders, integrate the chemical history of their surroundings in their tissues, which is why they are increasingly deployed in environmental monitoring and conservation programs worldwide.

The researchers hypothesized that the intensified environmental pressure of summer, driven by a combination of biogenic, abiogenic and anthropogenic factors, would weaken the antioxidant protection of mussels by the beginning of autumn. What they actually found was far more nuanced. The response of the mussels proved to be highly specific to the organ examined and to the type of biomarker measured, and the seasonal patterns frequently ran in opposite directions at northern versus southern locations along the coast.

Lipid peroxidation, a classic marker of oxidative damage to cell membranes, showed remarkably few seasonal differences overall. The sole significant exception came from mussels collected near the quay on the western side of St. Ivan Island, where September samples displayed higher lipid peroxidation than June samples. The authors attribute this anomaly to intense boat traffic or a momentary pollution event at that particular spot, a reminder that local human activity can leave a biochemical fingerprint even when the broader seasonal picture remains stable.

The clearest seasonal signals emerged in the glutathione system. Glutathione is a small but abundant cellular molecule capable of neutralizing a wide range of reactive oxygen species, including singlet oxygen, hydroxyl radicals, superoxide and hydrogen peroxide, and it also serves as a co-substrate for the enzymes glutathione peroxidase and glutathione-S-transferase. In the digestive gland, glutathione concentrations were significantly higher in autumn samples at almost every location, with exceptions only at Kavarna in the north and at Sozopol port and the Sozopol coastal rocks in the south. Meanwhile, glutathione-S-transferase activity, which participates in binding xenobiotic compounds, rose significantly in autumn across the foot tissue of mussels from all coastal locations and in the gills and digestive gland of mussels from all southern sites. The researchers suggest this autumn surge reflects the cumulative pollution pressure of the summer tourist season, when intensified visitor flows and municipal wastewater discharges activate the detoxification machinery of the mussels.

Glutathione peroxidase told an even more revealing story of geographic divergence. In mussels from all northern locations, the activity of this enzyme in September was significantly lower than in June, in the gills and digestive gland alike, while in mussels from the southern locations the opposite pattern held, with autumn activity generally exceeding summer levels. Glutathione reductase followed a similar split: foot tissue from northern mussels showed significantly increased activity in September, whereas southern mussels displayed a significant autumn decrease in both gills and digestive gland. Glucose-6-phosphate dehydrogenase, which supplies the reducing power that keeps glutathione in its active form, dropped significantly in the gills and digestive gland of all northern September samples but showed no significant seasonal change in southern animals. The authors propose that these opposing north-south trajectories may stem from multiple stressor effects, including the pollutant inflow carried by the Danube River into the northwestern Black Sea.

Superoxide dismutase and catalase, the front-line enzymes that dismantle superoxide radicals and hydrogen peroxide respectively, showed more restrained seasonal behavior. Superoxide dismutase activity remained largely stable between seasons in the foot and gills, with a single exception at cape Chervenka, but the digestive gland told a different tale: all northern locations showed significant seasonal differences in this organ, while in the south only the mussels from St. Ivan Island west and the Sozopol coastal rocks did. Catalase activity in the foot declined from June to September at five locations spanning both regions, yet in the digestive gland of southern mussels the enzyme actually rose in September, while northern digestive glands showed no seasonal shift at all. The digestive gland, which constantly receives substances from the environment including various xenobiotics, proved to be the most responsive organ overall, with statistically significant seasonal differences present in every biomarker measured there.

To make sense of this dense matrix of measurements, the team applied multidimensional scaling, a statistical technique that projects complex multivariate data onto an interpretable two-dimensional plane. The analysis confirmed what the raw numbers suggested: mussels from the northern locations clustered separately from those in the south along the first dimension, demonstrating genuine dissimilarities in pro/antioxidant status between the two coastal regions. Along the second dimension, samples with elevated glutathione and related detoxification enzymes grouped apart from samples characterized by higher lipid peroxidation and activated catalase, indicating distinct physiological states of antioxidant mobilization versus oxidative damage.

The study’s authors conclude that the mussel’s reaction to seasonal abiotic, biotic and anthropogenic stress factors is specific to the target organ and the type of biomarker, and that the animals continuously respond to local fluctuations in temperature, hypoxia, the hydrological cycle and their own metabolic condition. For the growing field of biomonitoring, the message is clear: a single-season, single-organ snapshot risks misreading the health of a coastal ecosystem. Seasonal baselines must be established for each locality before oxidative stress biomarkers can be interpreted with confidence. The researchers emphasize that further work is needed to confirm these findings and to build a complete picture of seasonal and spatial variation in the antioxidant defenses of Bulgarian Black Sea mussels, so that this hardy filter-feeder can serve as a reliable sentinel for one of Europe’s most environmentally pressured semi-enclosed seas.

Subject of Research: Seasonal variation in oxidative stress biomarkers of mussels from Bulgarian Black Sea coastal habitats

Article Title: Seasonal changes in the pro/antioxidant status of mussels Mytilus galloprovincialis (Lamarck, 1819) from Bulgarian Black Sea coastal habitats

Article References: Tsvetanova, E., Georgieva, A., Chipev, N., & Alexandrova, A. (2022). Seasonal changes in the pro/antioxidant status of mussels Mytilus galloprovincialis (Lamarck, 1819) from Bulgarian Black Sea coastal habitats. BioRisk, 17, 241-251. https://doi.org/10.3897/biorisk.17.77279

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.77279

Keywords: Mytilus galloprovincialis, oxidative stress, antioxidant enzymes, Black Sea, biomonitoring, glutathione, lipid peroxidation, marine bivalves, environmental monitoring, Bulgarian coast, digestive gland, seasonal variation

The study, led by Raffaele Guastella and Joana C. Prata, drew on publicly available records from Portugal’s official control program for bivalve molluscs, covering the period from 2013 to 2025. Rather than collecting new samples, the researchers performed a historical analysis, re-examining the surveillance data that regulators already gather to classify harvesting areas and decide when shellfish beds can be opened or must be closed. This approach allowed the team to detect seasonal and spatial trends that are invisible in any single year of monitoring. Mussels of the genus Mytilus, primarily the Mediterranean mussel Mytilus galloprovincialis, dominate Portuguese production, and the monitoring data spanned estuaries, lagoons, and open coastal waters across the country’s northern, central, and southern regions.

The headline finding is, on balance, reassuring for consumers. Most of the harvested mussels complied with the strictest microbiological standard, known as Class A, in 75.5 percent of samples, meaning they could legally be sold for direct human consumption without prior depuration, the controlled purification process in which shellfish are held in clean water to purge contaminants. Heavy metals, a perennial worry in shellfish because elements such as cadmium, lead, and mercury bind to tissues and resist breakdown, breached regulatory limits only once across the entire dataset. In other words, the industrial and chemical contamination that dominated twentieth-century concerns about coastal seafood appears to be largely under control in Portuguese waters, a testament to decades of environmental regulation and improved wastewater treatment.

But the picture changes dramatically when the researchers turned to marine biotoxins. Toxins of the okadaic acid group, which cause diarrhetic shellfish poisoning in humans, exceeded legal limits in 19.5 percent of samples, nearly one in five. These compounds are not produced by the mussels themselves but by microscopic algae of the genus Dinophysis, which bloom in coastal waters and are concentrated by filter-feeding shellfish. Okadaic acid and its analogues inhibit protein phosphatases, enzymes essential for cellular regulation, and ingestion of contaminated shellfish causes severe gastrointestinal illness, including diarrhea, nausea, and vomiting. Although rarely fatal, diarrhetic shellfish poisoning is a significant public health burden and a persistent economic headache for the shellfish industry, because toxin-laden mussels must be withheld from market until the algae subside and the animals depurate the compounds.

The seasonal geography of contamination turned out to be strikingly divergent. Microbiological contamination, measured through the indicator bacterium Escherichia coli, peaked in winter and was most pronounced in estuaries and coastal lagoons, the semi-enclosed water bodies where freshwater runoff converges with the sea. Biotoxin exceedances, by contrast, peaked in summer and were most frequent in open coastal areas, where warm, stratified waters and nutrient dynamics favor the proliferation of toxic dinoflagellates. This seasonal split matters for regulators and producers alike: the two hazards respond to different environmental drivers, peak at different times of year, and therefore demand different monitoring strategies rather than a one-size-fits-all surveillance calendar.

Geography told a parallel story. Both microbiological failures and biotoxin exceedances occurred most often in Portugal’s North and Center regions, a pattern the researchers attribute largely to rainfall-related inputs. Heavy rain flushes fecal microorganisms from agricultural land, urban drainage, and inadequate sewage systems into rivers and estuaries, delivering E. coli and other microbes directly to the shellfish beds. The same hydrological pulses also carry nutrients that can fuel algal growth, linking the two contamination pathways to a shared meteorological trigger. Similar associations between rainfall and E. coli levels in bivalves have been documented elsewhere in Europe, including Sardinia, reinforcing the conclusion that land-based runoff is a dominant driver of shellfish microbiological quality.

Here the study acquires an urgent contemporary relevance. Climate projections for the Iberian Peninsula, including the Portuguese RNA2100 climate assessment cited by the authors, anticipate more frequent and more intense extreme weather events, including torrential rainfall episodes punctuating prolonged droughts. Each such event can deliver a concentrated pulse of contaminants to coastal waters, overwhelming treatment infrastructure and triggering both bacterial spikes and, in the aftermath, nutrient-driven algal blooms. The researchers warn that the contamination patterns documented over the past decade may intensify under these changing conditions, meaning that today’s monitoring baselines could understate tomorrow’s risk. Drought and wildfire, too, have been linked to shifts in diarrhetic shellfish toxin trends in northwestern Portugal, suggesting that the full cascade of climate impacts on shellfish safety is only beginning to come into focus.

One of the study’s most important conceptual contributions is its reframing of what harvesting restrictions actually mean. When regulators close a shellfish bed because of E. coli or biotoxin exceedances, the closure is a food safety measure, but the researchers argue it should also be read as an ecosystem health signal. A mussel bed contaminated with fecal bacteria is a watershed with a waste management problem; a bed laden with okadaic acid is a coastal zone experiencing harmful algal blooms, often a symptom of eutrophication and shifting ocean conditions. In this sense, the humble mussel functions as an integrated biological sensor, and the official monitoring program, designed primarily to protect consumers, doubles as an unintentional but remarkably comprehensive survey of coastal environmental quality.

The authors call for strengthened monitoring and mitigation measures to safeguard both ecosystem integrity and public health. Practical implications include sharpened surveillance during winter rainfall events in estuarine production areas of the North and Center, enhanced summer monitoring for Dinophysis toxins along the coast, and investment in the upstream infrastructure, from wastewater treatment to agricultural runoff management, that determines what reaches the shellfish beds in the first place. Advances in forecasting toxic harmful algae for the northeast Atlantic, an active research frontier, could eventually give producers advance warning of bloom events, while continued reliance on mussels as sentinel organisms ensures that the biological record of coastal contamination keeps accumulating. For now, Portuguese consumers can take genuine comfort in the findings: the mussels on their plates have, overwhelmingly, been safe. But the same data that certify that safety are quietly chronicling a changing coastline, one filtered mouthful of seawater at a time, and the decade ahead will test whether monitoring programs can evolve as fast as the climate that increasingly drives the risk.

Subject of Research: Historical analysis of microbiological, metal, and marine biotoxin contamination in harvested mussels in Portugal

Article Title: Contamination of mussels with microorganisms, metals, and biotoxins: a historical analysis from Portugal

Article References: Guastella, R., Nuvoloni, R., Teixeira, A., Pérez-Mongiovi, D., Freitas, A. R., & Prata, J. C. (2026). Contamination of mussels with microorganisms, metals, and biotoxins: a historical analysis from Portugal. Environmental Monitoring and Assessment, 198(10), Article 1072. https://doi.org/10.1007/s10661-026-15890-1

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15890-1

Keywords: mussels, Mytilus galloprovincialis, marine biotoxins, okadaic acid, Escherichia coli, heavy metals, shellfish safety, harmful algal blooms, Portugal, coastal monitoring, climate change, food safety

The research, led by Marino Korlević of the Ruđer Bošković Institute in Rovinj, Croatia, together with colleagues from several Croatian marine research institutions, set out to answer a deceptively simple question: which microbes genuinely live on mussel gills, and which are merely passing through? This distinction matters because resident taxa may perform functions that benefit the host, such as contributing to nutrition or defense, while transient taxa simply reflect what the animal has recently filtered from the water column. Separating the two requires sampling that spans both space and time, which is precisely what the team undertook.

The researchers sampled mussels at two locations on the eastern coast of the Adriatic Sea, Lim Bay on the Istrian peninsula and Mali Ston Bay in Dalmatia, two of Croatia’s most important shellfish farming areas. Crucially, they did not stop at the mussels themselves. At the same time and at the same locations, they also characterized the microbial communities in the surrounding seawater and in the sediment. This three-way comparison is what gives the study its analytical power: if a microbe found on the gills is also abundant in seawater, it may simply be a hitchhiker; if it appears on the gills regardless of its abundance in the environment, it is more likely a true associate of the host.

To characterize the communities, the team used Illumina MiSeq sequencing of the V4 region of the 16S rRNA gene, the standard workhorse of modern microbiome research. This approach amplifies and reads a short, highly conserved stretch of bacterial DNA that allows researchers to identify which taxa are present and in what relative proportions. Sequences were grouped into operational taxonomic units, or OTUs, providing a fine-grained picture of community structure that could be compared across individuals, sites, and months.

The first major finding was that the gill microbiome is genuinely distinct. When the researchers compared the structure of the gill communities, measured at the OTU level, with the microbial communities of the surrounding seawater and sediment, the gills stood apart from both. This is not a trivial result. Mussels pump enormous volumes of water across their gills every day, and if the gill community were simply a snapshot of the planktonic bacteria in that water, the two communities would look statistically indistinguishable. The fact that they do not suggests that the gill environment actively selects for particular microbes, whether through the gill’s epithelial surfaces, its mucus chemistry, the immune defenses of the host, or the microconditions of oxygen and nutrients that prevail there.

Equally striking was what the comparison between the two bays showed, or rather did not show. The structure of the gill microbiome did not differ significantly between Lim Bay and Mali Ston Bay, despite the geographic separation and the environmental differences between the sites. In other words, whatever forces shape the mussel gill community, they appear to operate consistently across the Adriatic farming grounds studied. The gill microbiome, in this sense, behaves like a host-associated community rather than a local environmental artifact. What did differ was time: the gill communities showed clear differences between months, indicating temporal dynamics in their composition over the course of the year-long sampling program.

The taxonomic analysis then delivered the study’s central insight. Within the family Endozoicomonadaceae, a group of Gammaproteobacteria well known from associations with marine invertebrates, the researchers found members without known relatives at the genus level, so-called unclassified Endozoicomonadaceae, present in the gills of mussels from both locations and throughout the entire study period. This persistence across space and time marks them as resident members of the gill microbiome, microbes that maintain a stable association with the host rather than drifting in and out with the seasons. Their consistent presence hints at a functional role, though the study, based on 16S rRNA gene profiles, does not directly demonstrate what that role might be.

In contrast, other members of the same family behaved very differently. Endozoicomonas and the taxon provisionally known as Candidatus Endoecteinascidia appeared in the gill microbiome of mussels from both locations only in certain months, marking them as transient taxa whose presence ebbs and flows with the seasons. The picture became even more site-specific with the Vibrionaceae. Vibrio and unclassified members of the Vibrionaceae were also transient, appearing only in certain months, but with a telling distinction: they were specific to Lim Bay. That a well-known group of bacteria, which includes both harmless marine organisms and significant pathogens, showed up as a seasonal, location-limited component of the gill community is likely to attract attention from aquaculture researchers, since Vibrio dynamics in farmed bivalves are a persistent concern for the industry.

The conceptual framework the study reinforces is one that microbiome science has been converging on for years: host-associated communities are mixtures of core and casual members, and only long-term, repeated sampling can tell them apart. A single snapshot of a gill microbiome might have recorded the unclassified Endozoicomonadaceae and concluded they were the whole story, or it might have caught a Vibrio bloom and drawn alarming conclusions. By sampling monthly across a year and pairing the mussel data with seawater and sediment references, the Croatian team was able to show that the gill community combines a stable backbone with a shifting overlay of seasonal and local arrivals. The temporal dynamics they observed, with community composition differing between months even as the overall gill signature remained distinct from the environment, suggest that both the host and the changing conditions of the Adriatic play a role in shaping who lives on the gills.

For the aquaculture sector, the implications are practical as well as conceptual. Mytilus galloprovincialis is farmed intensively in the Mediterranean, and the health of farmed mussels is increasingly understood in terms of their microbial associations. Knowing that a stable, resident core of Endozoicomonadaceae persists on the gills across sites and seasons provides a baseline against which future perturbations, whether from warming waters, disease outbreaks, or changes in farming practice, can be measured. Knowing that Vibrio appears only transiently and only at one of the two studied bays provides a template for monitoring programs that distinguish routine seasonal variation from genuine warning signs. And methodologically, the study demonstrates the value of the three-compartment design, comparing host, water, and sediment simultaneously, as a way to separate true host association from environmental background. As microbiome research moves from description toward prediction, studies of this kind, grounded in a full year of observation at real farming sites, provide the empirical foundation on which that transition will depend. The gills of the Mediterranean mussel, it turns out, are not just a filter but a curated habitat, one whose permanent residents and temporary guests can now be told apart.

Subject of Research: Resident and transient bacterial taxa in the gill microbiome of the Mediterranean mussel Mytilus galloprovincialis in the Adriatic Sea

Article Title: Resident and Transient Microbial Taxa in the Gill Microbiome of the Mussel Mytilus galloprovincialis

Article References: Korlević, M., Markovski, M., Kapetanović, D., Vardić Smrzlić, I., Orlić, K., Bolotin, J., Kožul, V., Bobanović-Ćolić, S., Nerlović, V., & Perić, L. (2026). Resident and Transient Microbial Taxa in the Gill Microbiome of the Mussel Mytilus galloprovincialis. Microbial Ecology. https://doi.org/10.1007/s00248-026-02888-y

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02888-y

Keywords: gill microbiome, Mytilus galloprovincialis, Mediterranean mussel, Endozoicomonadaceae, Vibrionaceae, 16S rRNA gene sequencing, Adriatic Sea, aquaculture, marine microbiology, temporal dynamics, Gammaproteobacteria, microbial ecology

The research, published in the journal Ecotoxicology, was carried out by a team at Çanakkale Onsekiz Mart University in Türkiye led by Emircan Aksoy and corresponding author Selin Ertürk Gürkan. The investigators focused on two of the four internationally recognised caramel colour classes: Ammonia Caramel, known as Caramel III, and Sulfite-Ammonia Caramel, known as Caramel IV. Both are produced by heating carbohydrates with ammonia-containing reagents, and both are chemically far more complex than the word caramel might suggest, containing a heterogeneous mixture of polymeric colour bodies and, in some cases, nitrogen-containing by-products such as 4-methylimidazole that have drawn toxicological attention in the past.

What makes the study unusual is its exposure route. Safety assessments of caramel colourants have overwhelmingly assumed that humans encounter them in food and drink, so comparative data on the two ammonia-based classes under waterborne conditions have been scarce. To fill that gap, the researchers exposed mussels for 96 hours to three concentrations of each caramel class: 0.05, 0.5 and 5 milligrams per litre. The top concentration was anchored to available ecotoxicological information for Caramel IV, while the lower doses allowed a controlled comparison of Caramel III under identical aquatic conditions. Mussels, which filter enormous volumes of water and are notoriously sensitive to dissolved contaminants, are considered ideal sentinels for this kind of experiment because their responses often foreshadow broader ecosystem effects.

After the exposure period, the team examined two organs with very different jobs: the gills, which handle respiration and particle capture, and the digestive gland, the mussel’s equivalent of a liver and primary site of detoxification. Under the microscope, the differences between exposed and control animals were striking. In gill tissue, the caramel treatments produced epithelial deformation, hyperplasia, an abnormal thickening of the tissue lining, fusion of the delicate gill filaments known as lamellae, infiltration by hemocytes, the molluscan immune cells, and accumulation of lipofuscin, a pigment that marks cellular wear and oxidative damage. Each of these changes can impair the gill’s ability to exchange gases and filter food, and filament fusion in particular reduces the effective surface area available for respiration.

The digestive gland told a parallel but distinct story. There, the researchers documented progressive tubular disorganisation, dilation of the tubule lumina, vacuolisation of the digestive cells and eventual epithelial atrophy. In a healthy mussel, the digestive gland tubules are tightly organised structures where intracellular digestion takes place; when they unravel and their lining cells swell with vacuoles and then waste away, the animal’s capacity to process nutrients and detoxify harmful compounds is directly compromised. The pattern of damage intensified with concentration in both organs, and the two caramel classes were not interchangeable in their effects.

Indeed, one of the study’s clearest findings was that Caramel III, the plain ammonia caramel, generally provoked stronger structural and biochemical alterations than Caramel IV at comparable concentrations. That result is notable because Caramel IV, the sulfite-ammonia variant used in dark soft drinks, has historically received more regulatory and public attention, partly because of its 4-methylimidazole content. The new data suggest that the less-studied Caramel III deserves ecotoxicological scrutiny in its own right, and the authors frame their work as the first ecotoxicological evidence for this class in a marine invertebrate model.

Structural damage was only half the picture. The team also measured a battery of oxidative stress biomarkers in both tissues, focusing on three antioxidant enzymes: superoxide dismutase, or SOD, which converts the superoxide radical into hydrogen peroxide; catalase, or CAT, which breaks hydrogen peroxide down into water; and glutathione S-transferase, or GST, a versatile detoxification enzyme that conjugates reactive electrophilic compounds to glutathione. Alongside enzyme activities, the researchers quantified lipid peroxidation, the oxidative degradation of membrane lipids that serves as a classic fingerprint of free-radical damage to cells.

The biochemical data revealed a familiar but sobering dynamic. Antioxidant enzymes were activated in parallel with rising lipid peroxidation, meaning the mussels’ cellular defence systems did respond to the caramel-induced oxidative challenge, but the compensatory response was insufficient to fully prevent oxidative damage. In other words, the animals were fighting back biochemically and still losing ground. Multivariate statistical analyses of the combined biomarker dataset confirmed that the shifts were integrated and coherent rather than random noise, producing a clear separation between control and exposed groups and reinforcing the conclusion that both caramel classes impose genuine physiological stress rather than merely cosmetic discolouration.

The tissue-specific nature of the responses adds an important layer of interpretation. Gills are the first point of contact for dissolved substances in seawater, so epithelial deformation, hyperplasia and hemocyte infiltration there reflect direct contact toxicity and an inflammatory-type defence at the interface with the environment. The digestive gland, by contrast, is where absorbed compounds are processed and stored, so tubular disorganisation and vacuolisation suggest that caramel constituents or their metabolites reach internal detoxification machinery and overwhelm it. This kind of organ-by-organ fingerprinting is precisely why bivalve histopathology has become a standard tool in environmental quality assessment, and the study’s design follows well-established biomarker methodology used previously for mussels exposed to pharmaceuticals, tar, nanoparticles and other emerging contaminants.

The broader implications reach beyond the laboratory tank. Caramel colourant production volumes continue to climb with global processed food consumption, and effluents from food and beverage manufacturing can carry colourants and their degradation products into coastal waters where filter feeders abound. The concentrations tested here were selected from limited existing ecotoxicological data rather than from measured environmental levels, so the authors are careful not to claim that wild mussels are routinely exposed at these doses. What the study does establish is that ammonia-based caramel classes are not biologically inert in seawater, that their effects are concentration-dependent and tissue-specific, and that the two classes differ in potency, with Caramel III emerging as the more aggressive of the pair across most endpoints. As food additives face growing scrutiny under the lens of whole-life-cycle environmental impact, the humble brown dye that colours the world’s favourite drinks may turn out to have a footprint that extends well beyond the glass. The researchers suggest their findings provide a foundation for future work on chronic exposures, mixtures with other effluent components, and environmental monitoring of food-colourant residues in marine ecosystems, questions that will matter as production volumes and coastal pressures continue to rise together.

Subject of Research: Histopathological and oxidative stress effects of ammonia-based caramel food colourants on the Mediterranean mussel Mytilus galloprovincialis

Article Title: Tissue-specific histopathological and oxidative stress responses of Mytilus galloprovincialis to ammonia-based caramel colourants

Article References: Aksoy, E., Gürkan, M., Güzel, E. C., Can, İ., & Ertürk Gürkan, S. (2026). Tissue-specific histopathological and oxidative stress responses of Mytilus galloprovincialis to ammonia-based caramel colourants. Ecotoxicology, 35(8), Article 169. https://doi.org/10.1007/s10646-026-03153-1

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03153-1

Keywords: caramel colourants, Mytilus galloprovincialis, ecotoxicology, oxidative stress, histopathology, marine mussels, food additives, E150c, E150d, antioxidant enzymes, gill damage, digestive gland

The research, led by scientists at the Stazione Zoologica Anton Dohrn in Naples, Italy, set out to address a glaring gap in comparative physiology. Intertidal bivalves are famous for their tolerance of hypoxia and reoxygenation, but the cellular and molecular mechanisms behind that tolerance remain largely unexplored, particularly in the nervous system. This is a critical blind spot, because neural tissue is among the most metabolically active in any animal, demanding a continuous supply of oxygen to sustain ion gradients, neurotransmitter cycling, and the electrical signaling that underpins behavior. In humans, even brief interruptions of cerebral oxygen flow cause devastating damage, which makes the mussel’s nonchalance all the more intriguing.

To build a foundation, the team first performed a detailed histochemical characterization of each ganglion of the mussel’s nervous system, mapping out the architecture of the cerebro-pleural ganglia, pedal ganglia, and visceral ganglia, along with the commissures and connectives that link them. Using molecular markers, they charted the distribution of catecholaminergic, peptidergic, and serotonergic neurons, identifying populations of cells that produce dopamine-like catecholamines, the neuropeptide FMRFamide, and serotonin. This anatomical atlas provided the necessary roadmap for asking which parts of the mussel brain respond when oxygen disappears and returns.

The pedunculate pedal ganglia emerged as a focal point of the response. Histochemical analysis showed that catecholaminergic, peptidergic, and serotonergic neurons distributed within the pedal ganglia contribute to the homeostatic response to hypoxia and the subsequent reoxygenation phase. In practical terms, this means that the mussel’s nervous system does not simply shut down and ride out the stress passively; instead, specific neurochemical signaling systems modulate their activity in a coordinated way as conditions change. Neuroplasticity of this kind, the authors conclude, represents one important component of a broader, integrated set of strategies that the animal deploys to adapt to fluctuating oxygen.

But neurochemical remodeling is only part of the story. To probe deeper, the researchers turned to transcriptomics, sequencing the messenger RNA landscape of the pedal ganglia under normoxic, hypoxic, and reoxygenation conditions. This unbiased approach generated a catalog of candidate adaptive genes whose expression shifts in response to oxygen stress, spanning multiple biological processes. Among the most prominent functional categories were the regulation of cellular homeostasis, the control of programmed cell death, the regulation of cell proliferation, and, perhaps unsurprisingly, mitochondrial biology. Mitochondria are the cellular power plants where oxygen is consumed, and they are also the primary source of the reactive oxygen species that spike dangerously when oxygen suddenly returns after a period of scarcity.

Several of the genes flagged by the analysis belong to pathways that will be familiar to anyone who studies hypoxia biology. The hypoxia-inducible factor pathway, centered on the transcription factor HIF-1α and its regulators, including prolyl hydroxylase domain-containing protein 2 and the von Hippel–Lindau tumor suppressor, is the canonical oxygen-sensing machinery of animal cells. The transcriptomic data also pointed to genes involved in antioxidant defense, mitochondrial electron transport, and even an alternative oxidase, a mitochondrial enzyme that allows some invertebrates to keep electrons flowing through their respiratory chains in ways that sidestep the reactive oxygen bursts typical of standard respiration. Genes connected to apoptosis regulation, including components of tumor necrosis factor receptor signaling, suggest that the mussel brain actively manages the decision between cell survival and cell death during stress.

Markers of cell proliferation, such as proliferating cell nuclear antigen, added another dimension. Their presence hints that the mussel nervous system may maintain or even replenish its cellular components in response to stress, a form of structural plasticity that complements the neurochemical changes. Taken together, the transcriptomic results paint a picture of a nervous system that engages a coordinated transcriptional program, adjusting everything from energy metabolism to life-and-death decisions at the single-cell level, in order to preserve function through cycles of oxygen withdrawal and restoration.

The team then asked whether microRNAs, the small regulatory RNAs that fine-tune gene expression after transcription, might orchestrate these responses. MicroRNAs are powerful post-transcriptional regulators in virtually every animal lineage, and they have been implicated in hypoxic responses in mammals. Using small RNA sequencing, the researchers identified a suite of mature microRNAs expressed in the pedal ganglia, including microRNAs that appear to be entirely novel to science. However, when they compared the miRNA profiles across hypoxia and reoxygenation conditions, they found no significant changes. This negative result is itself informative: it suggests that post-transcriptional regulation mediated by microRNAs plays, at most, a secondary role in how the mussel nervous system copes with fluctuating oxygen, leaving transcriptional regulation and neurochemical plasticity as the primary levers of adaptation.

The study represents the first comprehensive look at the molecular and cellular regulatory mechanisms that allow the nervous system of M. galloprovincialis to adapt to natural oxygen fluctuations. Its significance extends well beyond molluscan curiosity. As climate change warms the oceans and coastal waters experience increasingly frequent and severe hypoxic events, understanding how resilient species maintain neural function under oxygen stress becomes a matter of ecological urgency. Mussels are ecosystem engineers, forming reefs that shelter entire communities, and their capacity to withstand deoxygenation shapes the fate of these habitats. Moreover, the conserved pathways uncovered in this work, from HIF signaling to mitochondrial alternative oxidases, offer comparative insights that may one day illuminate how mammalian brains, including our own, could be better protected against ischemia and reperfusion injury.

There is also a humbling evolutionary lesson embedded in the data. The molecular tools the mussel deploys, including oxygen sensors, mitochondrial safeguards, apoptotic gatekeepers, and modulatory neurotransmitters, are not exotic inventions. They are variations on themes shared across the animal kingdom, tuned by natural selection in an environment where oxygen comes and goes with the tides. What the Naples team has shown is that resilience is not a single trait but an orchestra: neurochemical flexibility in defined ganglia, a shifting transcriptomic program governing homeostasis, death, proliferation, and mitochondria, and a microRNA layer that, in this context, appears to hold steady while the rest of the system adapts. As hypoxic waters spread across the world’s coastlines, the lowly Mediterranean mussel may prove to be one of the best teachers we have about how nervous systems endure the thin line between suffocation and recovery.

Subject of Research: Neural and molecular adaptations of the Mediterranean mussel Mytilus galloprovincialis to hypoxia and reoxygenation stress.

Article Title: Effect of hypoxia and reoxygenation on the nervous system of the Mediterranean mussel Mytilus galloprovincialis

Article References: van Gelderen, T. A., D’Aniello, E., Macina, A., D’Aniello, S., Gallo, A., Carrella, S., & Mirra, S. (2026). Effect of hypoxia and reoxygenation on the nervous system of the Mediterranean mussel Mytilus galloprovincialis. BMC Biology. https://doi.org/10.1186/s12915-026-02731-3

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02731-3

Keywords: hypoxia, reoxygenation, Mytilus galloprovincialis, nervous system, neuroplasticity, pedal ganglia, transcriptomics, microRNAs, mitochondria, HIF-1alpha, intertidal bivalves, marine biology