The study was conducted at the Department of Biology of the University of Shumen and was described by the authors as the first pilot assessment of the composition of microplastic particles in different anatomical structures of key fish species from the Bulgarian Black Sea area. After the fish were transported to the laboratory at 4 °C, they were dissected and samples were collected from the skin, muscle, gills, intestinal tract and caviar.

To reduce the risk of introducing microplastics during laboratory work, the investigators used glass and metal tools, rinsed equipment with bidistilled water, filtered the glassware and tested reagents for particles through a blank-sample procedure. Minced tissue was treated with 5 ml of 1 M sodium hydroxide at 50 °C for 15 minutes. The researchers then added 17.5 ml of 49% nitric acid and 2.5 ml of ultrapure water, heated the mixture at 50 °C for another 15 minutes and then applied further heating for 15 minutes at up to 80 °C.

Following filtration through cellulose nitrate filters with an 8 µm pore size, the filters were dried for 24 hours at 37 °C and examined by stereomicroscopy at 1,000× magnification. Particles were classified by shape as pellets, fibres or irregular fragments and by their largest cross-section into size groups of no larger than 25 µm, 25–100 µm and 100–200 µm. Fourier-transform infrared spectroscopy was used to identify polymer composition, with the analyses performed in triplicate.

Pellets were generally the most abundant morphological type, followed by fibres, although their distribution differed among species and tissues. Garfish contained small pellets in the gills, muscle and skin and small fibres in the gills, while irregular particles occurred in the skin and intestinal tract. No particles in the 100–200 µm group were reported for this species. Mullet contained predominantly small pellets, especially in the intestinal tract, gills and skin, and small fibres in the skin, muscle and intestinal tract. Pellets in the 100–200 µm group were detected in the muscle, gills and intestinal tract, while irregular particles occurred in every sampled structure except the caviar.

Knout goby also contained mostly small pellets across the skin, gills, intestinal tract and muscle. Small fibres were recovered from muscle and skin, and fibres in the 100–200 µm group appeared in the gills. No microplastics were detected in the sampled knout goby or Pontic shad caviar. Pontic shad contained small pellets in the skin and gills, small fibres in the gills and fibres in the 25–100 µm group in both skin and gills; irregular particles were isolated from muscle. Mediterranean horse mackerel contained small pellets in the intestinal tract, gills, muscle and skin. Fibres in the two smallest size groups occurred in the intestinal tract, while 25–100 µm particles also appeared in the gills and skin.

The largest species-specific count in the intestinal tract was 36 microplastic particles in mullet. This total represented the three sampled mullet gastrointestinal-tract samples combined, rather than a count from a single fish. The investigators noted that particle counts lower than those reported in some previous investigations could reflect differences in analytical methods and pollution levels. They also observed higher proportions in the pelagic garfish and Mediterranean horse mackerel, while noting that literature links potentially greater uptake in bottom-dwelling fish to feeding behaviour and sediment deposition.

Fourier-transform infrared analysis identified low-density polyethylene, polyamide, polyethylene terephthalate, polypropylene, polystyrene, expanded polystyrene and plasticised polyvinyl chloride among the isolated particles. Spectral characteristics were also reported for individual fibres, including peaks consistent with polyamide, polyethylene terephthalate and acrylates, as well as a material spectrally similar to siloxanes. The study did not quantify the contribution of every polymer across all samples, so its results establish diversity among detected polymers rather than a precise basin-wide polymer profile.

The authors proposed domestic wastewater, particularly the washing of synthetic fabrics, as a possible principal source of the particles, while emphasizing that detailed studies would be needed to establish that origin. Because the pilot included only 15 fish from one locality and month and examined tissue contamination rather than toxicological outcomes, it cannot establish food-web biomagnification, population-wide exposure, adverse effects in fish or consequences for human health. The authors called for further characterisation of microplastics and research into possible toxicological effects in benthic and pelagic fish communities.

Subject of Research: Pilot assessment of microplastic contamination in five Black Sea fish species

Article Title: The assessment of the bioaccumulation of microplastics in key fish species from the Bulgarian aquatory of the Black Sea

Article References: Toschkova, S., Ibryamova, S., Bachvarova, D. C., Koynova, T., Stanachkova, E., Ivanov, R., Natchev, N., & Ignatova-Ivanova, T. (2024). The assessment of the bioaccumulation of microplastics in key fish species from the Bulgarian aquatory of the Black Sea. BioRisk, 22, 17-31. https://doi.org/10.3897/biorisk.22.117668

Image Credits: AI Generated

DOI: 10.3897/biorisk.22.117668

Keywords: microplastics, Black Sea, fish contamination, Bulgaria, Sozopol, polymer analysis, marine pollution, bioaccumulation, assessment, fish, species, Bulgarian

The research combined environmental sampling with taxonomic identification and statistical analysis of surface-active soil invertebrates collected in eight gallery-forest sites within the 10,500-hectare ecological station in the Federal District. Each 100-square-metre sampling unit contained 10 pitfall traps, whose combined catches formed one composite sample. Traps were deployed from May through July 2021 and collected weekly during seven campaigns. The organisms were identified, counted, measured, freeze-dried and homogenised, while soil and litter were collected from the centre of each sampling unit. Total mercury was measured by atomic absorption spectrometry, with certified reference materials used to verify analytical accuracy and recovery rates of 81% to 103%.

Mercury was significantly more concentrated in litter than in soil, a difference associated with paired statistical testing that yielded t = 4.402, seven degrees of freedom and p = 0.0032. The authors regarded the forest canopy as a plausible interception surface for atmospheric mercury, allowing mercury captured by vegetation to enter the soil compartment through litterfall. Sampling took place in gallery forests, a Cerrado formation that follows watercourses. Their dense canopies limit direct deposition to the ground, while prolonged leaf exposure, reduced litter leaching and shelter from sunlight can help retain deposited mercury. These mechanisms are consistent with the observed litter enrichment, but the comparison does not by itself identify the precise source or chemical pathway responsible.

Soil organic matter was a significant predictor of mercury concentration, accounting for 45.5% of the observed variation in the regression analysis, with p = 0.039. The estimated slope indicated that soil mercury increased by 0.537 units for each unit increase in organic matter content. Soil pH showed no significant relationship with total mercury. The authors connected the organic-matter association with mercury’s affinity for sulfur-bearing functional groups, including thiol and disulfide sites, which can promote binding of inorganic mercury in soil. They also noted that litter decomposition can transfer captured mercury into the underlying soil. Because only eight paired sampling units supplied the environmental comparison, the regression and soil–litter contrast should be interpreted within the scope of this small, site-specific field survey.

The invertebrate collections spanned 15 orders and included ants, termites, spiders, scorpions, beetles, flies, wasps, crickets and other epigeic fauna. Among orders with enough observations for comparison, termites had the highest mean mercury concentration at 458.11 ± 357.9 nanograms per gram, followed by scorpions at 457.48 ± 264.06 nanograms per gram and spiders at 340.70 ± 184.72 nanograms per gram. Variation was often substantial: for several groups, including harvestmen, flies, Vespidae wasps and ants, the standard deviation exceeded the mean. These broad spreads may reflect genuine differences among taxa, but the study’s relatively coarse taxonomic resolution, together with the analysis of whole animals without prior removal of gut contents, likely added variability.

Bioaccumulation factors were calculated by comparing mercury in each organism with its dry-weight concentration in soil or litter. A factor below 1 was classified as no significant bioaccumulation, 1 to 2 as microaccumulation, and above 2 as macroaccumulation. Overall, 83.75% of the analysed samples had a bioaccumulation factor of at least 1 relative to soil, and 91.4% of those accumulators met that threshold relative to litter. Among samples showing accumulation potential, 80.38% were macroaccumulators when compared with soil and 68.53% when compared with litter. Spiders were macroaccumulators in both comparisons, while ants showed a stronger tendency to exceed the macroaccumulation threshold relative to soil than to litter. The divergence between the two reference compartments reinforces that the surrounding soil may be an important exposure pathway for some animals.

Four mercury concentration classes were then compared with the feeding habits assigned to the organisms. Predator frequency rose from 9.1% in the lowest class, no more than 43.50 nanograms per gram, to 30.1% in the next class, 38.1% in the third and 45.5% in the highest class, above 273.81 nanograms per gram. Phytophagous organisms moved in the opposite direction, declining from 26.1% of the lowest class to 15.0%, 8.2% and 2.3% across the progressively higher classes. Omnivores predominated in the second group, while necrophagous organisms were proportionally more frequent in the highest. This shift is consistent with mercury transfer through feeding relationships, particularly the ingestion of contaminated prey, but the study characterized the inference as a local trophic-network pattern rather than a direct measurement of every trophic pathway.

Multivariate analysis provided additional support for an association between ecological function and mercury burden. The analysis combined organism mercury concentration, the bioaccumulation factor based on litter and mean body size. The first two discriminant functions explained 96.69% of the separation among feeding groups, and their profiles differed significantly at p < 0.0001. Xylophagous and sap-sucking organisms occupied similar positions, whereas detritivores, necrophages, parasitoids and predators showed partially overlapping profiles. Predatory and omnivorous groups lay closer to the centre. The overlap demonstrates that organisms with broadly similar feeding strategies did not all accumulate identical amounts of mercury, indicating that diet is not the only relevant variable. Exposure through soil contact, habitat use, body size, taxonomy and other physiological or ecological traits may also influence tissue concentrations.

The authors identified several important limitations, including the inability to identify many specimens to genus or species, large within-group variation and possible contributions from undigested material in whole-body samples. They recommended species-level identification and gastrointestinal depuration in future work. The study also took place in gallery forests inside a peri-urban reserve bordered by urban and agricultural landscapes, where previous research found elevated mercury near roads and attributed it to traffic-related emissions. Regional atmospheric deposition, wildfires and natural geological contributions are also plausible influences on the reserve’s mercury cycle. Accordingly, the findings describe bioaccumulation and patterns consistent with biomagnification in one Brazilian savanna system; they do not establish clinical effects in people, prove a specific mercury source or show that the same concentration gradients occur throughout the Cerrado. Nevertheless, the data demonstrate how litter, soil properties, organism–environment contact and feeding relationships can jointly structure terrestrial mercury transfer in a relatively under-studied soil food web.

Subject of Research: Terrestrial mercury bioaccumulation and trophic transfer in Brazilian Cerrado soil invertebrates

Article Title: Mercury dynamics in surface-active soil invertebrates from an ecological reserve in the Brazilian savanna

Article References: Fernandes, I. O., Monteiro, L. C., de Miranda, V. L., de Souza, J. P. R., Rodrigues, Y. O. S., de Castro Paes, É., Júnior, W. F. A., & Bernardi, J. V. E. (2026). Mercury dynamics in surface-active soil invertebrates from an ecological reserve in the Brazilian savanna. Environmental Monitoring and Assessment, 198(11), Article 1230. https://doi.org/10.1007/s10661-026-16051-0

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16051-0

Keywords: mercury, bioaccumulation, biomagnification, soil invertebrates, Brazilian Cerrado, terrestrial food webs, leaf litter, soil contamination, ecological monitoring, dynamics, surface-active, soil

The research, published in the journal BioRisk, set out to carry out an initial screening of trace metal bioaccumulation in the soft tissues of wedge clams collected from different localities of the Bulgarian Black Sea coastal area, and to evaluate whether the species could serve as a bioindicator of metal pollution. Trace metals constitute a significant proportion of the pollutants in the Black Sea, where the highest concentrations are typically locked in marine sediments from which they can be released back into the water column. Because metals are persistent and bioaccumulative, their discharge into the Black Sea can have profound effects on the entire marine environment, and unlike many other pollutants they remain in soils and waters for hundreds to thousands of years.

Bivalves are particularly well suited to pollution monitoring because they are filter feeders and primary consumers in the Black Sea food chain, accumulating metals and other contaminants in their tissues and shells. Mussels of the species Mytilus galloprovincialis have traditionally been used as biomonitors of Black Sea metal pollution, but data on the spatial and temporal distribution of metal accumulation in benthic clam species from the Bulgarian coast has been quite scarce. Although the spatial variation of metal concentrations in sediments of the Bulgarian Black Sea coastal areas is well documented, no comprehensive studies on bioaccumulation of metal elements in clams from these habitats had been carried out before, despite the growing economic importance of clams for Bulgaria.

To fill this gap, the researchers collected wedge clams measuring 23 to 35 millimetres in length, either manually or from commercial providers, from sublittoral sandy habitats at representative localities along the Bulgarian coast. Sampling took place in June and September 2020, before and after the intensive touristic season, allowing the team to compare conditions on either side of the summer influx of visitors. The soft tissues of individual clams were digested with nitric acid and then analysed by Inductively Coupled Plasma Optical Emission Spectrometry, a technique known as ICP-OES, for the determination of seven elements: cadmium, chromium, copper, iron, nickel, lead and zinc. The measurements were carried out in the certified chemical laboratory at the Department of Chemistry of the Medical University-Varna, and differences between sites were tested statistically while patterns of similarity were explored with cluster analysis.

The results revealed that the content of trace metals in the wedge clams differed significantly amongst localities. Higher metal loads were present in clams from Sveti Vlas, Shkorpilovtsi, Slanchev Bryag, Ahtopol and Kranevo, several of them popular resort beaches. The highest values of lead, at 2.51 milligrams per kilogram wet weight, and cadmium, at 0.32 milligrams per kilogram, were found in samples from Sveti Vlas, together with high concentrations of chromium and copper. The highest concentration of copper, 34.12 milligrams per kilogram, along with the peak values of iron at 269.52 milligrams per kilogram and nickel at 0.32 milligrams per kilogram, were detected in wedge clams from Shkorpilovtsi. Maximum chromium content, 0.58 milligrams per kilogram, was present in samples from Slanchev Bryag, accompanied by high iron values, while the highest zinc concentration, 18.04 milligrams per kilogram, together with elevated chromium and iron, was measured in clams from Irakli. Varna Bay clams, meanwhile, recorded the highest June zinc value of 15.98 milligrams per kilogram among the tabulated sites.

Cluster analysis using Euclidean distance as a measure grouped the localities into two main clusters based on the similarity of metal concentrations in the clams. The first cluster comprised localities in the northern coastal area, namely Shkorpilovtsi, Sveti Vlas, Kranevo and Slanchev Bryag, with Shkorpilovtsi and Sveti Vlas showing the highest similarity, evidently due to the high accumulated levels of lead, cadmium and copper in clams sampled there. The second cluster included Primorsko, Arkutino, Sozopol and Ahtopol from the southern coastal area, with Varna Bay grouping together with Ahtopol because of the similarity of metal concentrations between them. As a whole, the levels of metal bioaccumulation in wedge clams from the northern localities were significantly higher than in clams from the southern localities, indicating different metal pollution levels in the northern and southern Bulgarian Black Sea coastal areas. Amongst the southern sites, Primorsko and Arkutino were most similar, and Ahtopol stood out as an exception, showing relatively higher values of copper, cadmium and nickel that linked it to Varna Bay instead.

The seasonal comparison, based on four localities sampled in both June and September 2020, found little variation overall. In wedge clams from Kranevo, however, the concentrations of copper, chromium and iron were significantly lower in September samples compared to June, and the content of chromium in clams from Varna Bay and Primorsko was also significantly lower in September. The researchers suggest that the observed seasonal variations were probably connected with local changes in the hydrological parameters of the ecosystems rather than with tourism alone, although the sampling design, spanning the period before and after the intensive touristic season, was chosen precisely to capture such pressures.

The study’s conclusions carry a double significance. On one hand, the wedge clams from localities known to have higher coastal inflows and touristic pressures, namely Varna Bay, Shkorpilovtsi, Sveti Vlas, Slanchev Bryag and Ahtopol, accumulated significantly higher metal elements in their tissues, confirming that the species registers the human footprint along the coast. On the other hand, the data showed that concentrations of accumulated metals in wedge clams from the studied localities were, as a whole, below the maximum residual levels prescribed by local and international regulations for seafood. Recent studies on commercial wedge clams from the Bulgarian Black Sea coast had already indicated that the species is generally safe for human consumption, and the new findings broadly support that assessment.

There was one notable exception. The high concentration of lead, 2.51 milligrams per kilogram, in wedge clams from Sveti Vlas in June significantly exceeded the national and European regulations for seafood, which set a limit of 1.5 milligrams per kilogram. This single exceedance at a busy resort beach underscores how localised metal contamination can be, and how a sentinel species can flag a problem that might otherwise go unnoticed in routine water or sediment sampling. Lead is among the trace metals of greatest toxicological concern, and its persistence in the marine environment makes such a signal particularly worth following up.

Taken together, the study establishes the wedge clam Donax trunculus as a suitable bioindicator for the assessment and monitoring of metal pollution in the Bulgarian Black Sea environment. As a filter feeder that plays a significant role in maintaining the functionality of marine ecosystems, the species can accumulate considerable amounts of trace metals while remaining abundant and accessible along sandy shores. For a coastline facing growing tourism and development pressure, the small clam buried in the sand may prove to be one of the most informative witnesses to the state of the Black Sea environment, turning every handful of beach sediment into a record of the metals that flow through it.

Subject of Research: Trace metal bioaccumulation in wedge clams as a bioindicator of pollution on the Bulgarian Black Sea coast

Article Title: Trace metal accumulation in tissues of wedge clams from sandy habitats of the Bulgarian Black Sea coast

Article References: Bachvarova, D., Stancheva, M., Ignatova-Ivanova, T., Ibryamova, S., Chipev, N., & Alexandrova, A. (2022). Trace metal accumulation in tissues of wedge clams from sandy habitats of the Bulgarian Black Sea coast. BioRisk, 17, 73-81. https://doi.org/10.3897/biorisk.17.77290

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.77290

Keywords: wedge clam, Donax trunculus, trace metals, Black Sea, bioindicator, metal pollution, Bulgaria, ICP-OES, marine monitoring, bioaccumulation, Trace, metal

The formulation in question is nanoatrazine encapsulated in poly(ε-caprolactone), or nATZ-PCL, a nanocarrier system designed to make herbicide application more efficient while reducing the total amount of active ingredient released into the environment. Polymer nanoencapsulation is one of the flagship ideas of agricultural nanotechnology: by controlling the release of the herbicide, improving its stability and solubility, and limiting off-target drift and leaching, such formulations promise more sustainable weed control. But the very nanoscale properties that make these carriers attractive in the field also change how they behave once they enter an aquatic ecosystem. Particle size, surface chemistry, and polymer composition all influence how a nanopesticide interacts with water, dissolved organic matter, and living cells, which means its environmental fate cannot simply be inferred from decades of data on conventional atrazine.

To probe that fate, researchers at Embrapa Environment and São Paulo State University designed a two-step experiment tracing nATZ-PCL through a simplified freshwater food chain, from the microalga Raphidocelis subcapitata, a standard ecotoxicological test species and primary producer, to the water flea Daphnia magna, a grazing zooplankter that feeds on algae. The team first characterized the nanoparticles using dynamic light scattering, finding a mean hydrodynamic diameter of 226 nanometers, a low polydispersity index of 0.12, and a negative zeta potential of about minus 25 millivolts, all signs of a relatively homogeneous and physically stable suspension. Encapsulation efficiency, established in earlier work, was high at roughly 86.7 percent, meaning most of the atrazine was locked inside the polymer matrix rather than floating free in solution.

Stability, however, proved to be matrix-dependent. When the nanoparticles were suspended in the algal culture medium, their hydrodynamic diameter and polydispersity index climbed steadily over seven days, a signature of aggregation or surface interactions driven by the ionic composition of the medium and possibly by the formation of an abiotic eco-corona, a coating of natural organic molecules that adsorbs onto nanoparticle surfaces and reshapes their biological identity. Natural organic matter, or NOM, is ubiquitous in real water bodies, and the researchers included it at an environmentally relevant concentration of 10 milligrams per liter using Suwannee River humic substances. In most other test media the formulation remained comparatively stable, but the progressive growth of particle size in the algal medium hinted that clumping could alter how much herbicide actually reaches algal cells.

Exposure concentrations were anchored to sublethal effect levels determined in previous toxicity work: the EC10 and EC25, the concentrations causing 10 and 25 percent growth inhibition in the alga. Without NOM these were 0.040 and 0.103 milligrams per liter; with NOM they dropped to 0.020 and 0.043 milligrams per liter, indicating that organic matter itself modulates toxicity. Algae were exposed for seven days under controlled conditions, then harvested, washed, freeze-dried, and analyzed for atrazine using a micro-QuEChERS extraction protocol coupled to high-performance liquid chromatography with diode-array detection. Because each two-liter culture yielded only about 100 to 150 milligrams of dry biomass, the team had to repeat the cultures twelve times to accumulate enough material for reliable chemical quantification, a logistical feat that underscores how demanding nanopesticide bioaccumulation studies can be.

The results were clear: Raphidocelis subcapitata did accumulate atrazine. Measured concentrations in exposed algal biomass ranged from 0.25 to 0.89 micrograms per gram, with the highest value, roughly 0.89 micrograms per gram, recorded under the EC25 condition without natural organic matter, significantly above all other treatments. When NOM was present at EC25, accumulation fell to about 0.33 micrograms per gram, consistent with the idea that organic molecules adsorbing onto nanoparticle surfaces form an eco-corona that alters colloidal behavior and reduces bioavailability to algal cells. A control centrifugation assay ruled out a mundane artifact: no visible nanoparticle pellet formed when clean nATZ-PCL suspensions were spun under the same conditions used to harvest algae, so the detected atrazine was not simply co-sedimenting nanoparticles masquerading as cellular uptake, although the authors note the measured residue may include both internalized herbicide and strongly associated particles that washing could not remove.

The critical question came next: would that algal burden travel up the food web? To find out, the researchers fed Daphnia magna with the nATZ-PCL-exposed algae at a standardized density of four million cells per milliliter for 48 hours, followed by a 24-hour depuration period in clean medium. In a parallel experiment, daphnids were exposed directly to nATZ-PCL in reconstituted water at their own species-specific EC10 and EC25 values of 2.05 and 5.69 milligrams per liter. Pooled daphnid samples were analyzed first by liquid chromatography and then, to push sensitivity further, by high-resolution LC-QTOF mass spectrometry targeting the protonated atrazine ion at mass-to-charge ratio 216.1016. The verdict was unambiguous: atrazine was not detected above the method detection limit of 0.373 micrograms per gram in any dietary or direct exposure treatment.

Several non-exclusive mechanisms could explain this apparent decoupling of bioaccumulation from trophic transfer. The aggregation observed in the algal medium may have reduced the bioavailability of the nanoparticles to both producer and consumer. Daphnia’s limited assimilation efficiency for engineered nanomaterials, documented in earlier studies, may have prevented meaningful uptake from the gut. Rapid depuration is another candidate; previous work on conventional atrazine shows that uptake steady states in algae and Daphnia are reached within hours, while elimination occurs within minutes for algae and hours for microcrustaceans, leaving little time for residues to build up. The researchers also suggest that nATZ-PCL particles may simply have adhered to the daphnids’ carapace and swimming appendages rather than being absorbed, and that the small daphnid biomass available limited analytical sensitivity, meaning trace transfer below the detection limit cannot be excluded. The authors are careful to stress that their experiments identify the absence of measurable transfer without pinpointing which mechanism dominates.

The broader significance of the study lies in what it says about how we assess nanopesticide risk. For conventional hydrophobic pollutants and metals, bioaccumulation at the base of a food web is often a red flag for biomagnification up the chain. This work demonstrates that the same logic cannot be automatically applied to nanostructured formulations: accumulation in primary producers does not necessarily translate into efficient transfer to consumers, because aggregation state, eco-corona formation, bioavailability, assimilation efficiency, and toxicokinetics all intervene at each step. Earlier work by some of the same group had already found no significant adverse effects in Daphnia following trophic exposure to nanoatrazine, and the new data reinforce that picture. At the same time, the team cautions that real freshwater food webs involve far more species, greater ecological complexity, and concomitant exposure to transformed nanoparticles and other pollutants, all of which limit generalization from a single producer-consumer pair tested in the laboratory.

There is also a regulatory gap. Despite the rapid growth of nanopesticide products, no internationally accepted framework yet exists for evaluating their environmental risks, and laboratory toxicity data from a handful of model species remain a fragile basis for protecting the enormous diversity of freshwater life. The authors call for longer, more elaborate trophic-chain protocols, in situ physicochemical characterization of nanoparticles throughout exposure, and toxicokinetic studies across multiple trophic levels to determine whether the apparent containment of nanoatrazine at the producer level holds under field-realistic conditions. For now, the message is twofold: nanoencapsulation may genuinely curb the movement of a persistent herbicide through aquatic food webs, but proving that promise will require watching not just where the particles go, but what they become along the way.

Subject of Research: Bioaccumulation and trophic transfer of polymeric nanoencapsulated atrazine in a freshwater algae-zooplankton food chain

Article Title: Decoupling bioaccumulation and trophic transfer: nanoatrazine accumulates in microalgae but shows limited transfer to zooplankton

Article References: Braga, A. P. A., Vallim, J. H., Mendes, M. D. L., Levanteza, V. O., Assalin, M. R., Tavares, M. M., Fraceto, L. F., do Espírito Santo Pereira, A., & de Castro, V. L. S. S. (2026). Decoupling bioaccumulation and trophic transfer: nanoatrazine accumulates in microalgae but shows limited transfer to zooplankton. Environmental Monitoring and Assessment, 198(9), Article 1021. https://doi.org/10.1007/s10661-026-15831-y

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15831-y

Keywords: nanopesticides, atrazine, bioaccumulation, trophic transfer, Raphidocelis subcapitata, Daphnia magna, aquatic food web, nanoecotoxicology, polymer nanoparticles, natural organic matter, freshwater ecosystems, environmental monitoring

The zebra mussel is no ordinary freshwater mollusc. Classified among the top 100 most harmful invasive species, it inflicts costly biofouling damage on industrial infrastructure worldwide, clogging pipes, screens and cooling systems wherever it establishes populations. Its native range spans eastern Bulgaria, southern Romania and Ukraine, where it inhabits temperate lakes, rivers and reservoirs along the Golyama Kamchia River system. The species lives an average of three to five years, though some specimens survive up to fifteen, and individuals grow from roughly one to 1.6 centimetres in their first year, with a maximum total length of about five centimetres. Reproduction, development and growth depend on a delicate balance of water temperature, pH, calcium concentration, dissolved oxygen, turbidity and salinity, with breeding occurring at water temperatures between 12 and 28 degrees Celsius.

Calcium stands at the heart of shell construction, and the zebra mussel demands more of it than most bivalves. According to the researchers, adult specimens require calcium concentrations between 12 and 15 milligrams per litre to grow and survive. Values below 15 milligrams per litre signal a risk to reproduction and growth, while concentrations of 15 to 30 milligrams per litre are considered moderate and 30 to 80 milligrams per litre high. Yet while several earlier studies documented element concentrations in the soft tissues of the species, analyses of the shell itself remained scarce. This gap motivated the present investigation, which sought to determine the chemical composition of shells from mussels in their native areal and to correlate those findings with the elemental content of the surrounding water and sediments.

The PIXE technique offered the analytical muscle to do this. The method is highly sensitive to elements with atomic numbers above 10, thanks to the thin beryllium window of the detector most commonly employed. It is non-destructive, meaning samples are preserved for further analysis, and it enables the examination of large numbers of small-mass samples in short time, with irradiation of a single sample lasting less than one minute. Its sensitivity is an order of magnitude higher than comparable methods, and the detection limit for elements in biological samples falls below one milligram per gram, benefiting even trace element detection. PIXE has found applications across chemistry, medicine, biology, archaeology, agriculture, materials science, geology, environmental research and pollution monitoring, and has previously been used to determine elemental composition in the bodies of various aquatic inhabitants.

Sampling took place in the spring of 2009 at the Ticha Dam, built along the Golyama Kamchia River approximately 18 kilometres from the town of Varbitsa and 40 kilometres from Shumen. With a water volume of 311,103 cubic metres and a circumference exceeding 100 kilometres, it is the dam with the largest circumference in Bulgaria. Water temperature at collection ranged from 6 to 9 degrees Celsius. The mussels were sorted into three size groups: small shells up to one centimetre, medium shells between one and two centimetres, and large shells above two centimetres. Water samples were taken from 20 centimetres below the surface and sediments from the uppermost layer. All samples were dried to constant weight, finely ground into powder with a porcelain mortar, and pressed into 13-millimetre pellets using a five-tonne hydraulic press. The pellets were coated with a thin carbon layer to make them electrically conductive and dissipate accumulated charge during measurement.

The irradiation itself was carried out at the PIXE camera of the 2.5 MV AN2000 Van de Graaff accelerator at the National Laboratories of Legnaro, part of the National Institute of Nuclear Physics in Italy. Shell samples were bombarded with a proton beam of 1.8 MeV energy and 4-millimetre diameter. Ionisation of the atoms in the samples resulted from irradiation with these accelerated protons, and the characteristic X-ray emission was detected by a high-purity germanium detector with an energy resolution of 160 eV at the 5.89 keV manganese K-line. The accelerator current was kept below 5 nanoamperes, with total charge deposited on targets on the order of 2 microcoulombs. Spectra were processed with the GUPIX WIN program, and calibration relied on standards of known elemental content prepared from the International Atomic Energy Agency’s Soil-7 reference material under geometry similar to that of the samples.

To quantify the relationship between shell chemistry and environmental conditions, the team calculated the Metal Pollution Index, or MPI, a geometric mean of metal concentrations in each sample. Values above 1 indicate a relationship between chemical elements and the environment, while values below 1 suggest no such relationship. The results were unambiguous. The MPI values for small, medium and large shells were 0.61, 2.86 and 4.13 respectively, demonstrating a strong environmental dependence during the mussel’s growth and development. As the mussels grew, they accumulated chemical elements from their surroundings in their shells, with the accumulation intensifying dramatically across ontogenetic stages.

The element-specific results revealed striking patterns. Calcium, potassium, iron and manganese were detected in all shell samples. Calcium appeared in approximately the same amount across all three size groups, within experimental error, confirmed by a coefficient of variation of 18.43 percent. Potassium concentrations differed across all three samples, with the highest values in the largest mussels and the lowest in medium-sized ones, yielding a coefficient of variation of 108.06 percent for the shells. Iron told an even more dramatic story: a significantly higher amount of 13 milligrams per gram was measured in the shells of the largest mussels, compared with 1.2 milligrams per gram in smaller and 1.7 milligrams per gram in medium representatives, producing a coefficient of variation of 125.91 percent. Manganese followed the same trend, with a coefficient of variation of 134.07 percent. Titanium, chlorine and zinc appeared in only two of the three groups, with titanium detected in medium and large specimens at twice the concentration in the largest mussels compared with the medium ones. Chromium showed the lowest coefficient of variation of all, at 6.73 percent.

Multivariate statistical analysis using IBM SPSS Statistics 19 brought the elemental relationships into sharp focus. A Pearson test across nine variables produced 36 correlation coefficients, of which 15 were negative and 21 positive. Potassium correlated positively with all other elements, with its highest coefficients against titanium at 0.704 and iron at 0.689. The highest positive correlation of all appeared between sulphur and chlorine at 0.911. Calcium, by contrast, correlated positively only with sulphur and chlorine, and negatively with all other elements, with its largest negative correlations against titanium at -0.932 and iron at -0.929, the very elements most strongly correlated with potassium. The team concluded that iron, titanium and potassium share the same sources, accumulating in the shells from the water and sediments of the mussel’s habitat. Cluster analysis confirmed these relationships, grouping potassium, manganese, iron, titanium and chromium together as the elements most interconnected and most strongly linearly correlated with one another, falling into the shells and influencing growth as concentrations changed with shell size.

The growth-rate findings may be the most consequential of all. The tendency of shell growth was associated with an increasing iron-to-calcium ratio: 0.0041 in small shells, 0.0043 in medium shells and 0.043 in large shells, a tenfold increase from juvenile to adult. The manganese-to-calcium ratio followed a similar trajectory, rising from 0.0003 in small mussels to 0.004 in large ones. Iron in the shells accumulated mainly from water, with a correlation coefficient of 0.872, while manganese accumulated from both water and sediments, with correlation coefficients of 0.9845 and 0.9541 respectively. The correlation between iron and manganese in the shells themselves reached 0.9998. The researchers propose that the accumulation of manganese, and especially iron, is crucial for the development of the species, and that the absence of these two metals in a habitat may affect populations and limit the ability of mussels to reach the reproductive ontogenetic stage. The iron concentration measured in adult mussels, 10.3 micrograms per gram, proved comparable to that of 72 marine bivalve species, though younger specimens deviated significantly despite high iron levels in the surrounding water and superficial sediment layers. The team notes that the accumulation of these elements in the individual white and black stripes of the zebra-like shell pattern would be of considerable interest, but that this forms the topic of their next study. For now, the work provides valuable baseline data on the shell chemistry of the species in its native range, information that may ultimately inform methods for mitigating its impact as an invasive species far beyond Bulgarian waters.

Subject of Research: The application of PIXE analysis in the identification of key elements for the ontogenetic development of Zebra mussels in their native areal

Article Title: The application of PIXE analysis in the identification of key elements for the ontogenetic development of Zebra mussels in their native areal

Article References: Arhangelova, N., Salim, S., Valkanov, K., Kolev, N., Marinova, P., Veleva, L., & Natchev, N. (2026). The application of PIXE analysis in the identification of key elements for the ontogenetic development of Zebra mussels in their native areal. One Ecosystem, 11, Article e167875. https://doi.org/10.3897/oneeco.11.e167875

Image Credits: AI Generated

DOI: 10.3897/oneeco.11.e167875

Keywords: zebra mussel, PIXE, Dreissena polymorpha, invasive species, elemental analysis, freshwater ecology, bioaccumulation, application, analysis, identification, elements, ontogenetic

The research team sampled prawns, water and bottom sediments from four sites across three distinct water bodies: Sukhyi Lyman, an estuary of the Akkarzhanka and Dalnyk rivers; the Cuciurgan Reservoir in the Dniester River drainage; and two locations in the Ukrainian sector of the Danube Delta, a canal in the city of Vylkove and the Ochakiv Branch. All sampling took place in the warm season of May and June 2024, at depths of one to 1.5 metres. At each site the researchers collected one litre of water and samples of bottom sediment in sterile containers, and captured prawns using six-millimetre-mesh fyke nets. Five individuals were analysed per locality, with the Danube Delta contributing two sets of five prawns so that metal loads could be compared between two hydrologically different delta channels.

The analytical work was carried out in the certified Water Hygiene and Ecology laboratory of the Ukrainian Research Institute of Transport Medicine using atomic absorption spectrophotometry. Prawn abdominal muscle tissue was digested in concentrated nitric acid at 135 degrees Celsius for four hours, followed by the addition of hydrogen peroxide and perchloric acid until the solution cleared. Every measurement was performed in five analytical replicates, and all glassware was acid-washed to avoid contamination. Calculations of metal concentrations relied on wet weight, and results were benchmarked against maximum permissible concentrations set by Ukraine’s Ministry of Health and by the Food and Agriculture Organisation for crustacean products destined for human consumption.

The water itself told a story of uneven and sometimes dramatic pollution. Mercury in surface water was roughly twice the permissible level of 0.01 micrograms per litre at both Sukhyi Lyman and the Cuciurgan Reservoir. Cadmium at Sukhyi Lyman reached 24.5 micrograms per litre, nearly five times the fish-farm quality standard of five micrograms per litre, while at the Cuciurgan Reservoir and in the Danube Delta cadmium remained below the regulatory limit. Copper exceeded the fisheries limit more than threefold in the Cuciurgan Reservoir, reaching 3.6 micrograms per litre, and lead stayed within permissible bounds everywhere, though it was highest at Sukhyi Lyman and the reservoir. A clear salinity gradient ran from nearly fresh water in the Danube Delta to brackish conditions in the estuary and reservoir, and dissolved oxygen remained above four milligrams per litre throughout, indicating that the pollution signal was chemical rather than a symptom of oxygen collapse.

Bottom sediments proved to be the real reservoirs of contamination. At Sukhyi Lyman, mercury in the black silt reached 8.35 micrograms per kilogram, an order of magnitude above levels in the overlying water. Lead in the same sediments climbed to 7,260 micrograms per kilogram, and copper to 5,709 micrograms per kilogram. In the Danube Delta, copper in the sediment reached an extraordinary 18,189.5 micrograms per kilogram, the highest sediment value recorded in the study. The authors attribute this persistent copper loading in the delta catchment to the long-term use of copper compounds as plant-protection products on the region’s extensive vineyards. Because sediments accumulate and store metals, they can act as long-term secondary sources, releasing stored pollutants back into the water column when storms, floods, dredging or construction disturb the silt.

To quantify how metals move from the environment into living tissue, the researchers calculated two key indicators. The bottom concentration factor, or BCF, compares metal levels in sediment with those in water and captures the sediment’s capacity to concentrate pollutants. The bioaccumulation factor, or BAF, compares metal levels in prawn tissue with those in water and therefore reflects the total real-world exposure of the animal, including uptake from its surroundings and diet. BCF values generally exceeded BAF values across the sites, confirming that trace metals accumulate far more intensively in the sediments than in the biota. For copper and lead in particular, the team obtained some of the highest BCF figures, with copper in the Danube channels at Vylkove reaching a BCF of 12,126 and a BAF of 7,172.

Inside the prawns themselves, copper dominated the metal burden, consistently and greatly exceeding the concentrations of the other three metals and often far surpassing the levels found dissolved in the water. The highest tissue copper occurred in prawns from the Cuciurgan Reservoir, at roughly 18,282 micrograms per kilogram. This pattern has a biological logic: copper is an essential element for crustaceans, forming the oxygen-binding centre of the respiratory protein haemocyanin in their haemolymph and supporting metabolism, enzyme synthesis and antioxidant defence. Mercury, by contrast, was consistently the lowest of the four metals in prawn tissue, although one striking exception emerged in the Ochakiv Branch of the Danube, where prawns carried 10.2 micrograms per kilogram of mercury, far above the 0.04 to 0.47 micrograms per kilogram measured elsewhere.

Crucially for food safety, none of the measured tissue concentrations crossed the regulatory thresholds. Under FAO and World Health Organisation guidelines, and under Ukraine’s national regulations on contaminants in food, lead, cadmium and mercury in edible crustacean meat must each remain below 0.5 milligrams per kilogram, and the prawns fell comfortably short of those limits. The measured values were also lower than those reported for related decapods elsewhere, including the green tiger prawn of the Persian Gulf, and broadly comparable to or below figures recorded for the species within its native Chinese range. The study’s results support a simple underlying premise: the absorption of trace quantities of metals from water by aquatic organisms depends primarily on the relative dissolved concentration of each element, so the more of a metal that is dissolved, the more the prawn absorbs.

Yet the authors urge strong caution rather than celebration. Each of the sampled water bodies bears a distinctive anthropogenic fingerprint. Sukhyi Lyman has been transformed into a large maritime complex, with trade and fishing ports and a railway junction, a transformation accelerated by the Russia-Ukraine War, and its waters now show very high technogenic cadmium pollution. The Cuciurgan Reservoir serves as the cooling pond for the Moldovan State District Power Plant, whose discharges of warm water and acid reagents drive hypoxia, acidification, eutrophication and hydrogen sulphide accumulation, processes that promote the reverse diffusion of metals from the benthic sludge back into the water. The Danube Delta, meanwhile, integrates pollutants arriving from an enormous international catchment, with complex exchanges that appear to be intensifying under wartime conditions. The war also restricts scientists’ access to many critical sites, leaving gaps in the monitoring record.

The commercial stakes are considerable. In 2025 the commercial stock of the Oriental river prawn in the Ukrainian sector of the Danube was estimated at 650 tonnes and is predicted to grow, and the species is prized both as wild-caught food and as a candidate for aquaculture in pens or flow-through systems using locally adapted brood stock. The study concludes that, for now, all four water areas meet present-day Ukrainian and international guidelines for prawn aquaculture or commercial harvesting. But given the current levels of contamination, the ongoing potential for new pollutant inputs, the unassessed interactions among chemicals, biology and ecosystems, and the unpredictable pressures of an ongoing war, the researchers recommend extreme caution and continued, expanded monitoring of trace metals and other regulated pollutants in Ukraine’s waters for decades to come.

Subject of Research: Bioaccumulation of trace metals in the invasive Oriental river prawn in southwestern Ukrainian waters

Article Title: Accumulation of trace metals in tissues of the Oriental river prawn Macrobrachium nipponense ( Arthropoda , Crustacea , Decapoda , Palaemonidae ) from polluted waters in southwestern Ukraine

Article References: Lepekha, A., Koshelev, O., Stepien, C. A., & Kvach, Y. (2025). Accumulation of trace metals in tissues of the Oriental river prawn Macrobrachium nipponense (Arthropoda, Crustacea, Decapoda, Palaemonidae) from polluted waters in southwestern Ukraine. Estuarine Management and Technologies, 2, 185-204. https://doi.org/10.3897/emt.2.176866

Image Credits: AI Generated

DOI: 10.3897/emt.2.176866

Keywords: Macrobrachium nipponense, trace metals, bioaccumulation, heavy metal pollution, invasive species, Ukraine, Danube Delta, aquaculture, bottom sediments, food safety, mercury, copper

Synthetic pyrethroids are synthetic derivatives of pyrethrins, the natural insecticides found in chrysanthemum flowers, and they are prized in agriculture and aquaculture for their potency against pests and relatively short environmental persistence. Yet they are also highly lipophilic, meaning they dissolve readily into fats rather than water, and they act on the nervous systems of fish by disrupting sodium channels in nerve membranes. Because of their chemical affinity for organic surfaces, pyrethroids bind tightly to plastic debris, and the review argues that this binding turns microplastics into mobile reservoirs of pesticide contamination within aquaculture systems.

The physicochemical mechanics of this partnership hinge on sorption and desorption dynamics, the processes by which chemicals attach to and detach from particle surfaces. Polymer crystallinity, or the degree to which plastic chains are ordered rather than amorphous, governs how much chemical a given particle can hold. Weathering, the combined effect of sunlight, abrasion, and microbial activity, alters surface chemistry and roughness, often increasing the available binding sites over time. Shifting water parameters such as salinity, pH, and temperature further modulate how strongly pyrethroids cling to plastic or release back into the water column, meaning that vector efficiency is not a fixed property but a moving target that responds to the environment.

Once contaminated particles are ingested, the classic assumption has been the so-called Trojan Horse effect, in which microplastics smuggle their chemical cargo deep into animal tissues and release it under digestive conditions, amplifying internal exposure beyond what the water alone would deliver. The review challenges this paradigm, pointing out that the picture is more nuanced than the label suggests. Some studies indicate that microplastics can dilute rather than concentrate chemical exposure under certain conditions, and the authors emphasize significant gaps in commercially relevant in vitro digestion models that attempt to simulate what actually happens inside a fish gut. Without better models, the true magnitude of the vector effect remains uncertain.

What is clearer is that vulnerability is not evenly distributed across farmed species. Benthic organisms, those living on or near the pond bottom, and species that are consumed whole rather than filleted face disproportionate exposure because they ingest particles directly from sediments or from the water column without the partial filtering that larger, selectively feeding animals perform. Tissue-specific bioaccumulation patterns suggest that pyrethroids carried on plastic surfaces may concentrate in particular organs rather than distributing uniformly, complicating both toxicological assessment and food safety monitoring.

The review also dissects how different aquaculture architectures shape exposure pathways. Recirculating aquaculture systems, or RAS, which reuse water through mechanical and biological filtration, concentrate particles and dissolved chemicals in ways that open-sea cages do not, since the latter rely on natural currents to dilute and disperse contaminants. Semi-intensive ponds, which blend natural productivity with supplemental feeding, sit somewhere in between, with hydrodynamics driven largely by wind, aeration, and fish movement rather than engineered flow. Each configuration produces a distinct profile of particle residence time, chemical partitioning, and trophic structure, meaning that risk cannot be generalized across the industry.

Trophic transfer, the movement of contaminants through food webs as predators consume prey, adds another layer of concern. When smaller organisms ingest contaminated particles and are then eaten by larger fish, the associated pyrethroids can move upward through the food chain, potentially reaching concentrations that exceed what direct water exposure alone would produce. The review highlights evidence that this pathway matters in aquaculture settings, particularly where natural forage organisms supplement formulated feeds, and it underscores the vulnerability of whole-consumed species whose digestive tracts, and therefore particle burdens, are eaten along with the muscle tissue.

The toxicological consequences compound across multiple biological systems. Pyrethroids are neurotoxic, and co-exposure with microplastics has been linked in experimental studies to heightened neuro-stress, disrupted behavior, and altered neurotransmitter activity in fish. Beyond the nervous system, the combination appears to compromise immune function, leaving animals more susceptible to disease, and to disturb gut microbiota, the microbial community that supports digestion and immune regulation. These sublethal effects translate directly into economic consequences for farm performance, including reduced growth rates, elevated mortality, and increased treatment costs, all of which erode the margins on which aquaculture operations depend.

Against this backdrop, the review identifies a significant regulatory blind spot. Maximum Residue Limits, or MRLs, the legal ceilings on pesticide residues in food products, are typically set for individual chemicals assessed in isolation, without accounting for the possibility that microplastic carriage may alter bioavailability, tissue distribution, or persistence of the residue in edible tissue. The authors argue that current frameworks do not capture the synergistic threat posed by the convergence of plastic pollution and routine pyrethroid application, leaving a gap between what regulations assume and what aquaculture ecosystems actually deliver to the consumer.

Looking forward, the review proposes a set of mitigation strategies aimed at closing this gap. Targeted green pharmacology, the development of plant-derived or biologically based alternatives to synthetic pesticides, could reduce the chemical burden entering farm systems in the first place, and the authors point to growing evidence that certain botanical feed additives can help fish cope with xenobiotic stress. Advanced filtration technologies, including improved mechanical separation and possibly biofilm-based capture, offer engineering routes to reduce particle loads before they accumulate. Ultimately, the authors call for a holistic paradigm shift in aquaculture risk assessment, one that treats microplastics and pesticides not as separate problems but as an interacting system whose combined behavior determines the real exposure profile of farmed fish and, by extension, the safety of the seafood that reaches the market.

Subject of Research: Microplastics as vectors for synthetic pyrethroid pesticides in aquaculture ecosystems

Article Title: The accelerating role of microplastics as vectors for synthetic pyrethroids: a critical review of bioaccumulation and trophic transfer in aquaculture ecosystems

Article References: Üstüner, E., Öz, M., İleri, E., & Dikel, S. (2026). The accelerating role of microplastics as vectors for synthetic pyrethroids: a critical review of bioaccumulation and trophic transfer in aquaculture ecosystems. Environmental Monitoring and Assessment, 198(9), Article 999. https://doi.org/10.1007/s10661-026-15850-9

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15850-9

Keywords: microplastics, synthetic pyrethroids, aquaculture, bioaccumulation, trophic transfer, Trojan horse effect, food safety, neurotoxicity, pesticide vectors, recirculating aquaculture systems, maximum residue limits, green pharmacology

The research team, led by Mohammed Amine Ansel of Chadli Bendjedid University in El Tarf, together with Raouf Zaidi and Amina Tania Dahel, focused on four wild species from the Sparidae family: the gilthead seabream (Sparus aurata), the red porgy (Pagrus pagrus), the common pandora (Pagellus erythrinus), and the axillary seabream (Pagellus acarne). These bottom- and mid-water feeders are staples of Mediterranean fisheries and are widely consumed along the North African coast, making them ideal sentinels for both environmental contamination and human exposure. Rather than sampling whole fish, the researchers concentrated on muscle tissue, the part of the animal that actually reaches the consumer, which makes their measurements directly relevant to dietary risk assessment.

Three elements took center stage in the analysis: mercury (Hg), arsenic (As), and chromium (Cr). Each has a distinct toxicological profile. Mercury, particularly in its organic methylmercury form, is a potent neurotoxin that binds strongly to muscle proteins and biomagnifies through marine food webs. Arsenic in seafood occurs mostly as organic compounds that are generally less hazardous than inorganic arsenic, though total arsenic levels remain a standard monitoring benchmark. Chromium’s toxicity depends heavily on its oxidation state, with hexavalent chromium being far more dangerous than the trivalent form, yet total chromium measurements still serve as a useful indicator of industrial influence on coastal waters. By quantifying all three in the edible portion of the fish, the team built a picture of contamination that speaks directly to public health.

The results revealed a remarkably uniform hierarchy. Across all four species, mean concentrations followed the same descending order: arsenic highest, mercury intermediate, and chromium lowest. Concentration ranges, expressed in milligrams per kilogram of wet weight, spanned 1.323 to 3.396 for arsenic, 0.102 to 2.004 for mercury, and 0.232 to 0.915 for chromium. This consistent pattern suggests that the underlying drivers of accumulation, from uptake pathways in the marine environment to the chemical affinity of each element for muscle tissue, operate similarly across these closely related species, a finding that simplifies the task of predicting contamination in sparids generally.

Species-level differences added a second layer of insight. The gilthead seabream recorded the highest mean chromium concentration, at 0.65 plus or minus 0.02 milligrams per kilogram, while the red porgy emerged as the species of greatest concern for the two more notorious contaminants, posting the highest mean levels of both mercury (0.98 plus or minus 0.10 milligrams per kilogram) and arsenic (2.70 plus or minus 0.07 milligrams per kilogram). These interspecies contrasts likely reflect differences in diet, habitat depth, trophic position, and metabolic handling of each element. Red porgy, which feeds on hard-shelled benthic prey, may encounter arsenic-rich invertebrates more frequently, while its position in the food web could favor mercury accumulation over its lifetime.

Perhaps the most technically compelling result of the study is the strength of the size-metal relationship. When the researchers plotted muscle metal concentrations against fork length and total weight, they found strong positive linear correlations in every case, with correlation coefficients ranging from 0.92 to 0.99 and p-values below 0.01. In practical terms, this means that fish size alone explains the vast majority of the variation in metal content. The mechanism is well understood in ecotoxicology: metals such as mercury are absorbed and retained faster than they can be excreted, so concentrations rise steadily with age as fish continue to feed in contaminated waters. Larger, older individuals have simply had more time to bank the metals passing through their diets.

This size effect carries immediate implications for how consumers and regulators think about seafood. A large red porgy landed off the Algerian coast will, on average, carry roughly an order of magnitude more mercury in its flesh than the smallest individuals of the same species, given the range of 0.102 to 2.004 milligrams per kilogram observed in the study. Choosing smaller fish within a species can therefore meaningfully reduce dietary exposure without requiring any change in the type of seafood consumed. The finding also aligns with a broader body of Mediterranean research, including studies from the Adriatic, the Aegean, and the French coast, which have repeatedly documented mercury rising with fish size and have used that relationship to refine consumption advisories for pregnant women and children, the groups most vulnerable to methylmercury’s neurodevelopmental effects.

The Algerian coastline provides a particularly interesting backdrop for this work. The central coast receives inputs from urban centers, ports, and industrial activity, and previous studies in the region have examined trace elements in tuna and other commercial catches. The Mediterranean Sea as a whole is a semi-enclosed basin in which contaminants can persist, and regional monitoring programs have long flagged mercury as a persistent concern. By anchoring their measurements in a specific stretch of coastline and a commercially vital group of species, the authors have produced a baseline against which future changes, whether from new industrial development, shifting ocean chemistry, or climate-driven changes in fish distribution, can be judged. Such baselines are rare and valuable in the southern Mediterranean, where monitoring infrastructure is often thinner than on the northern shore.

Crucially, the study’s risk assessment delivers a reassuring verdict. The mean metal concentrations measured in all four sparid species were within the safe limits established by international organizations for human consumption, including benchmarks maintained under European Commission regulations and the Codex Alimentarius standards of the World Health Organization and the Food and Agriculture Organization. That does not mean the metals are absent, but rather that typical dietary exposure from these fish, at the concentrations observed, does not exceed the thresholds that health authorities consider protective. The researchers’ choice to measure the edible muscle rather than organs such as liver or gonads, which typically concentrate metals more heavily, makes this conclusion directly applicable to the seafood that consumers actually buy.

The study also underscores a broader scientific principle: fish are not passive containers of pollution but dynamic systems whose chemistry reflects their age, diet, and habitat. The near-perfect linear correlations between size and metal burden observed here suggest that even simple measurements, a fork length or a weight taken at the fish market, could serve as a proxy for contamination risk, offering a low-cost screening tool for regions without sophisticated laboratory infrastructure. As coastal development intensifies across the southern Mediterranean and demand for local seafood grows, studies of this kind will become increasingly essential. For now, Algerian consumers can take comfort that their beloved sea bream remain within safety limits, while researchers and regulators gain a precise, quantified map of how mercury, arsenic, and chromium move through one of the region’s most important groups of food fish, and a clear warning that the biggest specimens deserve the closest scrutiny.

Subject of Research: Heavy metal bioaccumulation and its relationship to fish size in four wild sparid species from the central Algerian Mediterranean coast

Article Title: Assessment of heavy metals in four wild sparid species from the central coast of Algeria: effects of fish size on metal bioaccumulation

Article References: Ansel, M. A., Zaidi, R., & Dahel, A. T. (2026). Assessment of heavy metals in four wild sparid species from the central coast of Algeria: effects of fish size on metal bioaccumulation. Environmental Monitoring and Assessment, 198(9), Article 1033. https://doi.org/10.1007/s10661-026-15896-9

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15896-9

Keywords: heavy metals, sparid fish, bioaccumulation, mercury, arsenic, chromium, Mediterranean Sea, Algeria, fish size, food safety, marine pollution, Environmental Monitoring and Assessment

Anticoagulant rodenticides work by interfering with the vitamin K cycle, effectively preventing blood from clotting and causing fatal internal hemorrhage in the target rodents. The problem for wildlife is that these compounds do not stay where they are deployed. Rodents that consume poisoned bait often die slowly, sometimes days later, wandering into the open where predators and scavengers encounter them. In the process, the poison moves up the food chain. Second-generation anticoagulants, developed after rodents evolved resistance to earlier formulas, are particularly insidious because they are far more potent and persist in liver tissue for months. A single poisoned rodent can deliver a lethal or sublethal dose to a fox, an owl, a fisher, or a bobcat, and repeated exposure compounds the risk through bioaccumulation.

The research team, led by Jennifer A. Grauer and colleagues from institutions across the Northeast, took advantage of an unusual but powerful resource: opportunistic samples from bobcats harvested by hunters and trappers, along with animals recovered as roadkill, collected across New Hampshire, Vermont, New York, and New Jersey. Liver tissue, where anticoagulant residues concentrate, was analyzed for the presence of seven rodenticide compounds. Because the samples came from animals already dead, the study avoided the ethical and logistical challenges of trapping and sampling live carnivores, while still providing a statistically rich picture of exposure across a large region and a long time span. The approach also allowed the researchers to compare exposure between animals taken by hunters and those killed on roads, a distinction that turned out to matter.

The spatial results were striking. Exposure was not uniform across the landscape. New Hampshire and Vermont recorded the highest levels of contamination, and a spatial clustering algorithm identified localized hot spots of rodenticide detection in those two states as well as in New Jersey. New York, by contrast, emerged as comparatively cold in the analysis, with fewer clusters of positive detections. This patchiness matters because it suggests that rodenticide exposure is driven by local conditions, likely the density of human development, pest-control practices, and the structure of the food web connecting poisoned rodents to wild predators, rather than by broad regional factors alone. The hot spots provide concrete targets for mitigation efforts, from restrictions on consumer-grade poisons to public education campaigns in the affected areas.

Within individual animals, the picture was one of multiple, accumulating exposures. Bobcats that tested positive carried between one and four different rodenticide compounds in their systems, with a mean of 1.07 compounds among exposed individuals and a standard deviation of 1.17. Co-exposure to several compounds is particularly concerning from a toxicological standpoint, because different anticoagulants act through the same physiological pathway and their effects can be additive. An animal carrying multiple compounds may face a substantially higher risk of hemorrhage than one carrying a single residue, especially under stressors such as injury, disease, or starvation. The presence of several compounds in a single bobcat also indicates repeated encounters with poisoned prey over time, painting a picture of chronic, ongoing contamination rather than a one-off accident.

Age emerged as one of the clearest predictors of exposure. The researchers’ statistical models showed that the predicted odds of anticoagulant rodenticide exposure increased by approximately 35 percent, and the predicted number of compounds in an animal’s system rose by roughly 17 percent, for each additional year of bobcat age. This pattern is consistent with bioaccumulation: older animals have had more years to accumulate persistent residues from contaminated prey, and they may also occupy higher positions in the food web, taking larger prey that themselves carry accumulated burdens. Exposure rates varied by state, by age, and by sampling source, with harvested animals differing from roadkill in their likelihood of testing positive, but the models found no difference between male and female bobcats. That absence of a sex effect suggests that both sexes face similar dietary exposure risks across their ranges.

Perhaps the most sobering finding is the temporal trend. Second-generation anticoagulant rodenticides, the most potent and persistent class, have faced federal restrictions in the United States that went into effect during the course of the study, limiting their availability to professional applicators and removing them from many consumer products. Yet detection rates in bobcats did not decline. Instead, they increased steadily at a rate of approximately 25 percent per year across the 2013 to 2024 sampling window. The researchers are careful not to over-interpret the mechanism behind this rise, but the pattern suggests that current regulations have not yet translated into reduced contamination of wild predators. Possible explanations include lag effects from the persistence of residues in the environment, continued legal use by licensed professionals, illegal or off-label use, and the movement of regulated compounds across jurisdictions where rules differ.

The study gains additional weight from a comparison with another northeastern carnivore. The spatial patterns of exposure observed in bobcats closely mirrored those previously documented in fisher (Pekania pennanti), a mustelid predator with very different habitat preferences and hunting behavior. Fishers tend to occupy deep forest interiors, while bobcats range across a broader mosaic of forest edge, farmland, and suburban development. If two predators with such divergent ecologies show similar geographic fingerprints of contamination, the implication is that rodenticides have become ubiquitous in the northeastern food web, reaching animals regardless of how or where they hunt. The poisoned rodent, in effect, has become a common item on the menu of the region’s mesocarnivores.

For wildlife managers and conservationists, the study provides something that has been lacking: a long-term, region-wide baseline for a species of growing public interest. Bobcats have been recovering across the Northeast in recent decades, expanding back into areas where they were once eliminated, and their return has been celebrated as a conservation success. That recovery now carries a hidden cost, as expanding populations intersect with landscapes saturated with rodenticides. Sublethal exposure can impair health, reduce survival, and potentially affect reproduction, even when it does not kill outright, and the cumulative burden across a population remains difficult to quantify. The baseline established by this research will allow future monitoring to track whether regulatory changes eventually bend the exposure curve downward.

The broader lesson is one about the reach of human chemicals into ecosystems that seem, on the surface, far removed from suburban garages and city basements. Rodenticides are deployed to solve a narrowly human problem, yet their residues now appear in more than half of the bobcats tested across a four-state region, in hot spots that map onto the interface of development and wild land, and in concentrations that grow with every passing year of an animal’s life. As evidence accumulates across species, from fishers to bobcats to birds of prey, the case for rethinking how these compounds are used, regulated, and replaced grows stronger. The bobcats of the Northeast, silently carrying seven different poisons in their livers, are delivering a message that regulators and consumers alike cannot afford to ignore.

Subject of Research: Anticoagulant rodenticide exposure in bobcats across the northeastern United States

Article Title: Spatiotemporal patterns of anticoagulant rodenticide exposure in bobcats across the northeastern United States

Article References: Grauer, J. A., Needle, D. B., Schuler, K. L., Connelly, P. J., Fowles, G., Miller, E. A., Furfey, B., Royar, K., Watson, M. B., DeBow, J., & Frair, J. L. (2026). Spatiotemporal patterns of anticoagulant rodenticide exposure in bobcats across the northeastern United States. PLOS One, 21(10), e0360055. https://doi.org/10.1371/journal.pone.0360055

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360055

Keywords: bobcats, anticoagulant rodenticides, wildlife toxicology, Lynx rufus, northeastern United States, second-generation rodenticides, bioaccumulation, mesocarnivores, PLOS One, contamination hot spots, food web contamination, wildlife health

The researchers followed the PRISMA guidelines, the gold standard for systematic reviews, to screen studies from three major databases: Scopus, PubMed, and Web of Science. From that search, 55 studies made the final cut, each one dissected for data on soil lead concentrations, plant species, experimental conditions, enhancement strategies, lead uptake and translocation, and the physiological responses of the plants themselves. By standardizing information across such a heterogeneous literature, the review exposes patterns that no single experiment could reveal, and some of those patterns challenge long-held assumptions about how to measure success in soil cleanup.

The first surprise is geographic and methodological. More than three-quarters of the studies, 76.36 percent, were conducted in Asia, and nearly all of them relied on pot experiments using soils that had been artificially spiked with lead salts rather than soils contaminated by real-world pollution. That matters because freshly dissolved lead behaves very differently from the aged, mineral-bound lead found at an abandoned smelter or a shooting range. Artificially spiked soils tend to overestimate the metal’s bioavailability, meaning the plants in these experiments often had access to more lead than they would in a genuine contaminated field. The review’s authors caution that this skew makes it difficult to extrapolate laboratory triumphs to the messy, heterogeneous conditions of actual remediation sites.

The second, more fundamental finding concerns plant biology itself. Across the analyzed studies, lead accumulation was consistently higher in roots than in shoots, confirming what plant physiologists have long suspected: lead is notoriously poor at making the journey from root to stem to leaf. The metal binds strongly to cell walls in the root epidermis, gets trapped in the endodermis by the Casparian strip, and is sequestered in vacuoles before it can enter the xylem, the plant’s long-distance transport highway. This root-bound behavior is a double-edged sword. For phytoextraction, the goal of removing lead from soil by harvesting contaminated biomass, limited translocation is a serious bottleneck, because the harvestable aboveground parts carry only a fraction of the metal the roots have taken up. For phytostabilization, however, keeping lead locked in roots and rhizosphere soil is exactly what you want, since it prevents the metal from entering the food chain.

To overcome the translocation bottleneck, researchers have tested a pharmacopoeia of chemical assistants, and the review delivers a clear verdict on the most famous of them: chelating agents such as EDTA. These synthetic molecules grab lead ions from soil particles and hold them in soluble complexes, dramatically increasing the metal’s mobility and its availability for root uptake. The studies analyzed confirmed that chelators can boost lead accumulation in plant tissues. But the boost comes at a cost. Chelate-assisted treatments frequently reduced plant biomass and intensified physiological stress, undermining the very growth that makes phytoextraction viable in the first place. Worse, mobilized lead does not simply wait politely for a plant root to absorb it; it can leach downward through the soil profile toward groundwater, converting a localized contamination problem into a potential water quality crisis. The review flags these leaching risks as a central concern that has too often been sidelined in the enthusiasm for higher uptake numbers.

Organic acids, the gentler cousins of synthetic chelators, showed more variable behavior. Compounds such as citric acid, acetic acid, oxalic acid, and tartaric acid occur naturally in root exudates and are far less persistent in soil than EDTA. Yet their effectiveness depended heavily on dosage and soil properties. At the right concentration and in the right soil chemistry, organic acids enhanced lead uptake and even improved photosynthetic performance in crops like mung bean and sunflower. At other doses or in other soils, they did little or shifted the balance toward stress. This context-dependence, the review argues, is not a footnote but the central lesson of two decades of assisted phytoremediation research: there is no universal additive, no magic molecule that works everywhere.

A very different strategy has been quietly gaining ground: making lead less available rather than more. Biochar, the carbon-rich charcoal-like material produced by heating biomass in low oxygen, along with other soil amendments such as compost, lime, phosphate compounds, and iron-based materials, promoted phytostabilization by decreasing lead’s bioavailability. These amendments work through adsorption, precipitation, and pH shifts that bind lead into insoluble forms, effectively tucking the metal into chemical pockets where roots and leaching water cannot reach it. The bonus is that biochar and compost also improve soil structure, water retention, and nutrient supply, so plants grow better even as the metal is immobilized. Studies combining biochar with compost, iron materials, or intercropping systems showed improved plant growth alongside reduced lead mobility, a combination that makes phytostabilization attractive for sites where complete removal is unrealistic, such as large urban areas, roadsides, and former farmland.

Perhaps the most biologically intriguing assistants are microscopic. Microbial inoculants, including plant-growth-promoting rhizobacteria, endophytic bacteria, and mycorrhizal fungi, consistently enhanced plant biomass and antioxidant activity in the reviewed studies. These microbes act through several mechanisms: they produce siderophores and organic acids that modulate metal solubility, they secrete enzymes and hormones that stimulate root development, and they buffer the oxidative stress that lead inflicts on plant cells. Arbuscular mycorrhizal fungi, which form symbiotic partnerships with the majority of land plants, extended the effective root network and helped hosts like poplar, black locust, and licorice tolerate lead exposure. Some bacterial strains, such as Pseudomonas species applied to sunflower, improved nutrient uptake and antioxidant defenses simultaneously. Importantly, the microbes often supported remediation indirectly, by keeping plants healthy enough to accumulate or stabilize metal, rather than by dramatically increasing lead translocation. That indirect route, the review suggests, may be more sustainable than forcing more metal into harvestable tissue.

All of this leads to the review’s most consequential methodological argument: judging phytoremediation by total soil lead concentration alone is inadequate. A treatment can lower the total lead number on a lab report while leaving the metal just as dangerous, or it can leave the total unchanged while drastically reducing the fraction plants and children can actually absorb. The authors call for comprehensive evaluation frameworks that integrate the translocation factor, which measures how much metal moves from root to shoot, bioaccumulation indices, which capture how much metal the plant gathers relative to the soil, and plant health indicators such as biomass, photosynthesis, and antioxidant enzyme activity. Only by combining these metrics, they argue, can researchers and practitioners tell whether a given strategy is extracting lead, stabilizing it, or merely shuffling it around while stressing the vegetation.

The bottom line from Pisa is a call for realism and precision. Assisted phytoremediation works, but its effectiveness is highly context-dependent, shaped by soil chemistry, contaminant age, plant species, microbial communities, and the specific goal of the project. A chelate-washed hyperaccumulator may make sense in a small, contained plot with impermeable subsoil and careful leachate capture; a biochar-amended grass cover may be the wiser choice for a sprawling urban brownfield where the priority is preventing exposure rather than achieving total cleanup. What the 55 studies collectively demonstrate is that the era of one-size-fits-all soil remediation is over. The future of cleaning lead from the earth’s skin lies not in a single miracle plant or additive, but in matched pairs of strategy and site, guided by measurements that reflect what actually matters: how much lead is moving, where it is going, and whether the living soil is getting healthier in the process.

Subject of Research: Assisted phytoremediation of lead-contaminated soils through plant uptake, translocation, and physiological responses

Article Title: Assisted phytoremediation of lead-contaminated soils: a systematic review of plant uptake, translocation, and physiological responses

Article References: D’Asaro, L., & Landi, M. (2026). Assisted phytoremediation of lead-contaminated soils: a systematic review of plant uptake, translocation, and physiological responses. Plant and Soil. https://doi.org/10.1007/s11104-026-09072-8

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09072-8

Keywords: phytoremediation, lead contamination, soil remediation, chelating agents, biochar, phytostabilization, phytoextraction, microbial inoculants, translocation factor, bioaccumulation, plant physiology, heavy metals