Benzo[a]pyrene, often abbreviated BaP, belongs to the polycyclic aromatic hydrocarbons, a family of compounds assembled from fused aromatic rings and generated mainly by the combustion of fossil fuels, plant biomass, and petroleum derivatives. These molecules are persistent, highly lipophilic, and notorious for their affinity for particulate surfaces. In aquatic organisms they are associated with narcosis, cardiac dysfunction, mutagenicity, and carcinogenicity, and BaP in particular ranks among the most potent genotoxic and bioaccumulative agents known in aquatic toxicology. Microplastics, meanwhile, have become so widespread that measuring their effects in isolation increasingly misses the point. Contaminants in rivers and lakes rarely arrive alone, and microplastic particles, with their vast hydrophobic surface areas, are prime candidates to interact chemically with hydrocarbons dissolved in the water.

The research team, led by Yuuri Gabriel de Souza Santos of Universidade Santa Cecília together with colleagues from Universidade Santa Cecília and the Universidade Federal de São Paulo, designed a full-factorial exposure experiment to disentangle these interactions. Cultures of Raphidocelis subcapitata were exposed for 72 hours under tightly controlled conditions of temperature, light, and agitation to low-density polyethylene microplastics at three concentrations: 5, 50, and 500 milligrams per liter. The lowest of these reflects concentrations actually reported in freshwater environments, while the higher doses were intended to simulate more extreme pollution scenarios. Each microplastic treatment was tested alone and in combination with the median effective concentration of BaP, the dose that inhibits algal growth by fifty percent, which the team first determined experimentally for this species.

That determination produced a sobering baseline. The calculated EC50 for BaP in R. subcapitata was approximately 21 micrograms per liter, with a 95 percent confidence interval of 20 to 25 micrograms per liter. Even the lowest concentration tested, 3 micrograms per liter, caused statistically significant growth inhibition, marking it as the lowest observed effect concentration under the study’s conditions. At the highest dose of 39 micrograms per liter, growth inhibition approached 90 percent. The estimated EC50 falls squarely within the range of BaP concentrations documented in contaminated freshwater systems worldwide, where water column values typically span from less than a hundredth of a microgram to roughly 2.4 micrograms per liter, and sediments can hold far higher burdens. In other words, the toxicological benchmark established in the laboratory is uncomfortably close to what polluted rivers actually deliver.

The mechanism behind BaP’s damage to algae is thought to involve both photosynthetic disruption and direct membrane interactions. As a lipophilic compound, BaP binds readily to the lipid bilayers of algal cells, and polycyclic aromatic hydrocarbons can drive excessive production of reactive oxygen species through interference with cellular electron transport. Prior work has also shown that high hydrocarbon loads impair chlorophyll production, starving cells of photosynthetic capacity. Intriguingly, during routine microscopic examination the researchers occasionally observed clumps of lipids forming on the outer surfaces of BaP-exposed cells, a response consistent with previous reports that green algae increase membrane lipids as a defensive strategy against cytotoxic hydrocarbon damage.

When the microplastics were tested on their own, the results defied simple dose-response logic. At 5 and 50 milligrams per liter, the polyethylene particles significantly reduced algal density, an effect the authors attribute largely to chemical additives incorporated into the polymer, such as plasticizers, antioxidants, UV stabilizers, lubricants, and pigments. Because these additives are not covalently bound to the plastic matrix, they can leach into the surrounding water and exert their own toxicity; benzotriazole UV stabilizers, for example, have previously been shown to harm the freshwater alga Chlamydomonas reinhardtii. Yet at 500 milligrams per liter, toxicity vanished entirely and algal biomass actually exceeded the control by 3.7 percent, surpassing the lower-dose treatments by more than 40 percent. The most plausible explanation is that the enormous particle surface area at high concentrations provided an ideal substrate for biofilm formation, effectively turning the plastic into a growth platform for the microorganisms it would otherwise harm.

The combined exposures produced the study’s most consequential findings. BaP alone at 21 micrograms per liter inhibited roughly 57.6 percent of algal growth. But when the same BaP concentration was paired with 50 or 500 milligrams per liter of microplastics, inhibition dropped sharply to about 34.9 and 22.8 percent respectively, both statistically distinct from BaP alone. The interpretation is that at these higher particle densities, the hydrophobic hydrocarbon preferentially sorbs onto plastic surfaces, sequestering it away from the water column and lowering its effective bioavailability to the algae. This protective sorption effect mirrors previous observations in gammarids, cladocerans, sea urchins, and mysids, and echoes a study of juvenile common gobies in which microplastics delayed pyrene-induced mortality from 48 to 60 hours. Plastic, in these scenarios, acts as a temporary chemical sponge.

At the environmentally relevant concentration, however, the picture reversed into genuine concern. When just 5 milligrams per liter of microplastics accompanied the BaP, inhibition remained essentially unchanged from BaP alone, at about 58.3 percent, and both treatments were significantly more toxic than controls. This dose of plastic was evidently too low to strip a meaningful fraction of BaP from solution, leaving the hydrocarbon free to attack cell membranes. Since 5 milligrams per liter reflects concentrations documented in real freshwater systems, the message for regulators is troubling: at realistic pollution levels, microplastics do nothing to buffer hydrocarbon toxicity, while the plastic itself independently inhibits growth at that same dose.

The authors emphasize that this is the first study to evaluate combined BaP and microplastic effects in Raphidocelis subcapitata, and that their generalized linear model analyses confirmed a statistically significant interaction between the two contaminants, meaning the impact of each depends on the level of the other. The relationship is explicitly nonlinear: microplastics can either enhance or mitigate BaP toxicity depending on their concentration, rendering simple additive assumptions obsolete. Fourier transform infrared spectroscopy confirmed the test particles were linear low-density polyethylene, the polymer characteristically identified by split methylene peaks and methyl-group bands in its spectrum, matching the manufacturer’s specification.

The ecological stakes extend well beyond a single algal species. As primary producers, freshwater microalgae drive oxygen production, nutrient cycling, and energy transfer through food webs, and both microplastics and hydrocarbons can bioaccumulate in algal cells before moving to higher trophic levels. Green algae such as R. subcapitata are also valued agents of PAH bioremediation, and the new results raise questions about whether plastic contamination could undermine that cleanup capacity by altering hydrocarbon bioavailability. The authors acknowledge limitations: BaP adsorption onto the particles was not directly quantified, and responses such as chlorophyll content, oxidative stress, and gene expression were not measured. Notably, prior work on marine invertebrates found that even when survival appeared unaffected by combined exposures, surviving organisms carried DNA damage and elevated lipid peroxidation, hinting that growth-based endpoints may understate harm. The team calls for future research on chronic effects, trophic transfer, and additional freshwater species under environmentally realistic conditions, and concludes that microplastics cannot be treated as inert particles in aquatic systems. Their concentration, not merely their presence, may determine whether they worsen or mask the toxicity of the organic pollutants they travel with.

Subject of Research: Interactive toxicity of low-density polyethylene microplastics and benzo[a]pyrene in the freshwater microalga Raphidocelis subcapitata

Article Title: Interactive effects of low-density polyethylene microplastics and benzo[a]pyrene on the growth of the freshwater microalgae Raphidocelis subcapitata

Article References: de Souza Santos, Y. G., Choueri, R. B., Nobre, C. R., Simões, F. R., & Gusso-Choueri, P. K. (2026). Interactive effects of low-density polyethylene microplastics and benzo[a]pyrene on the growth of the freshwater microalgae Raphidocelis subcapitata. Ecotoxicology, 35(7), Article 161. https://doi.org/10.1007/s10646-026-03143-3

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03143-3

Keywords: microplastics, benzo[a]pyrene, Raphidocelis subcapitata, freshwater ecotoxicology, polycyclic aromatic hydrocarbons, low-density polyethylene, algal growth inhibition, bioavailability, sorption, additive leaching, biofilm formation, ecological risk assessment

The research team, led by scientists at the University of São Paulo and the University of Campinas in Brazil, set out to answer a question that has lingered in ecotoxicology for years. Plastic debris floating in rivers and lakes is not a static pollutant. Sunlight, heat, and oxygen steadily break polymer chains apart, and this weathering alters everything about the particle, from its surface chemistry to its roughness and its appetite for other contaminants. Yet most laboratory toxicity studies still rely on pristine, factory-fresh plastic, potentially painting a misleading picture of what actually drifts through natural waters.

To simulate years of environmental exposure in the laboratory, the researchers bombarded commercial polyethylene microparticles with UV-C radiation for 94 days in an accelerated aging chamber, a dose they calculated to be equivalent to roughly 643 days of natural sunlight. The transformation was striking. Scanning electron microscopy showed that smooth, homogeneous particles emerged covered in cracks, cavities, and pitted surfaces, evidence of a progressively weakening polymer matrix. Fourier-transform infrared spectroscopy confirmed the chemical counterpart of this physical decay: new absorption bands at 1710 and 1181 inverse centimeters revealed the formation of carbonyl and carbon-oxygen groups, the classic fingerprints of polyethylene photo-oxidation.

These chemical changes matter far beyond materials science. Oxygenated functional groups make the plastic surface more polar and more reactive, increasing its capacity to adsorb organic pollutants such as pesticides. The aging process also nudged the particle size distribution upward slightly, with degraded particles averaging 30 micrometers compared with 27 for pristine ones, and produced a more porous structure that could offer additional binding sites for dissolved chemicals. In other words, every day a plastic particle spends in the sun converts it into a more chemically active platform for carrying other contaminants into the food web.

The ecotoxicological centerpiece of the study was the water flea Daphnia similis, a tiny filter-feeding crustacean and a standard sentinel of freshwater health. In 21-day chronic exposure tests conducted under OECD guidelines, control animals thrived, with 90 percent survival and healthy production of around 66 offspring per female. But exposure to pristine polyethylene microplastics proved unexpectedly lethal. At 80 milligrams per liter, mortality reached 90 percent, and at 140 milligrams per liter it reached 70 percent, concentrations the authors note correspond to just a few times the highest levels of microplastics reported in natural freshwaters.

Curiously, the dose-response was not neatly linear, with the strongest lethal effect appearing at the intermediate concentration rather than the highest one. The researchers attribute the mortality to physical mechanisms: daphnids readily ingest particles in this size range, and accumulated microplastics can create a false sense of satiety, starve the animals of real nutrition, abrade the gut lining, and even impair oxygen diffusion through the filtering apparatus. The observed particles were also smaller on average than the manufacturer’s specification, and prior work suggests finer particles are substantially more harmful to daphnids than larger ones.

Photodegraded particles told a different and arguably more insidious story. While they killed fewer animals outright, they delivered sharper sublethal blows, particularly to reproduction. At 140 milligrams per liter, aged microplastics cut neonate production by nearly 70 percent. Reproduction is the demographic engine of any population, and a generation of daphnids that fails to replace itself cannot sustain the base of freshwater food webs that fish and other predators depend upon. The study demonstrates that the toxicity profile of a microplastic particle is not fixed; it is rewritten by every hour of sunlight it absorbs.

The most provocative results emerged when imidacloprid entered the picture. This neonicotinoid insecticide, which acts on the nervous systems of insects by overstimulating nicotinic acetylcholine receptors, is highly water soluble, persistent to hydrolysis, and routinely detected in surface waters worldwide at concentrations ranging from nanograms to hundreds of micrograms per liter. Because polyethylene is known to adsorb imidacloprid efficiently, the researchers expected the plastic to act as a vector, concentrating the pesticide and amplifying its harm.

Instead, the mixture data revealed an antagonistic interaction. In combination with 1.5 milligrams per liter of imidacloprid, pristine microplastics caused no lethal effects at any concentration, and a model deviation ratio analysis confirmed that the observed toxicity at 80 and 140 milligrams per liter fell significantly below what additive behavior would predict. The likely explanation lies in feeding behavior: daphnid feeding depends on the rhythmic beating of filtering appendages under nervous control, and imidacloprid’s neurotoxic action disrupts this neuromotor coordination. Animals exposed to the insecticide simply ingest fewer plastic particles, indirectly shielding them from the plastic’s physical harms, even as reproduction still dropped by up to 75.7 percent in some mixture treatments.

The authors caution that this apparent mercy is temporary. Previous research shows that prolonged imidacloprid exposure starves daphnids by suppressing feeding until energy reserves are exhausted, and the long-term ecological cost of a contaminated, underfed zooplankton community could be severe. Meanwhile, mixtures involving photodegraded particles followed a largely additive pattern, with progressive, dose-dependent reproductive losses up to fourfold below controls, likely reflecting hydrogen bonding between the polymer’s new oxygenated groups and the insecticide’s amine moieties. Taken together, the study delivers a clear message for regulators and risk assessors: the ecological risk of microplastics cannot be evaluated in isolation from their weathering state or their chemical traveling companions, and realistic assessments must embrace chronic exposures, aged materials, and contaminant mixtures if they are to protect the integrity of freshwater ecosystems.

Subject of Research: Photodegradation of polyethylene microplastics and its combined chronic toxicity with the insecticide imidacloprid to the freshwater cladoceran Daphnia similis

Article Title: Integrated Study of Polyethylene Microplastic Degradation and Its Interaction with the Insecticide Imidacloprid: Chronic Toxicity to Daphnia similis

Article References: Kiihl, M. E., da Silva Pinto, T. J., Dias, M. A., Montagner, C. C., Espíndola, E. L. G., & Alexandre, D. S. (2026). Integrated Study of Polyethylene Microplastic Degradation and Its Interaction with the Insecticide Imidacloprid: Chronic Toxicity to Daphnia similis. Archives of Environmental Contamination and Toxicology, 91(2), Article 16. https://doi.org/10.1007/s00244-026-01213-5

Image Credits: AI Generated

DOI: 10.1007/s00244-026-01213-5

Keywords: microplastics, polyethylene, photodegradation, imidacloprid, Daphnia similis, freshwater ecotoxicology, chronic toxicity, contaminant mixtures, UV aging, reproduction, neonicotinoids, water pollution