Aged Plastic Particles Turn a Common Herbicide Into a Far Deadlier Threat to Freshwater Life

Microplastics rarely travel alone. In rivers, lakes, and agricultural runoff, the tiny fragments of degraded plastic drift alongside pesticides, heavy metals, and industrial chemicals, and scientists have long suspected that these encounters are more than coincidental. A new study published in Discover Toxicology provides some of the clearest evidence yet that weathered polyethylene microplastics can act as carriers for one of the world’s most widely used herbicides, dramatically amplifying its toxicity to a humble but ecologically important freshwater flatworm.

The research, led by Rayane Reis Sousa and colleagues at the Federal University of Tocantins in Brazil, focused on the interaction between polyethylene microplastics and 2,4-dichlorophenoxyacetic acid, better known as 2,4-D. This herbicide has been in continuous use since the 1940s and ranks third among the most heavily applied herbicides globally, behind only glyphosate and inhibitors of acetolactate synthase. Because 2,4-D dissolves readily in water, it is routinely detected in surface waters, groundwater, and even drinking water supplies around the world, with estimated environmental concentrations in water bodies ranging from 4 to 24 micrograms per liter and reaching as high as 4,000 micrograms per liter in agricultural fields depending on application regimes.

What makes the new work distinctive is its insistence on realism. Most laboratory studies of microplastic toxicity use pristine, factory-fresh plastic beads that bear little resemblance to the battered, sun-bleached fragments found in nature. The Brazilian team instead subjected pure polyethylene particles, measuring 34 to 50 micrometers, to a month-long chemical oxidation treatment using potassium persulfate at 70 degrees Celsius, followed by eight months of exposure to natural solar radiation in river water at intensities between 800 and 900 watts per square meter. This two-stage regimen was designed to simulate the natural aging and weathering that plastic undergoes in the environment before it ever encounters a pollutant.

Spectroscopic analysis revealed that this aging process fundamentally altered the polymer. Raman spectroscopy showed changes in the spatial conformation of the polymer chains, with bands at 1,462 and 1,130 inverse centimeters shifting in intensity in ways that indicate amorphous chains converting into trans conformations. Infrared spectroscopy confirmed the trend, revealing an increase in the band at 730 inverse centimeters and a decrease between the 1,472 and 1,460 bands, both signatures of rising crystallinity. The researchers note that increased crystallinity makes polyethylene more resistant yet more brittle, a combination that can promote fragmentation into ever smaller particles. Crucially, the aged plastic proved capable of adsorbing the herbicide, taking up 0.3432 milligrams of 2,4-D per gram of treated microplastic in equilibrium tests quantified by ultraviolet-visible spectroscopy.

To test the biological consequences, the team turned to Girardia tigrina, a freshwater planarian flatworm that has become a workhorse of ecotoxicology. Planarians occupy an intermediate position in freshwater food webs, serving as both predator and prey, and they possess a remarkable suite of traits that make them ideal sentinels: they regenerate lost body parts with extraordinary speed, they reproduce both sexually and asexually, and their locomotor behavior is easily quantified in the laboratory. Previous work by the same group had already shown that pure polyethylene microplastics of the same size range cause measurable harm to these animals, providing a baseline against which the herbicide-laced particles could be compared.

The experimental design was elegantly simple. Planarians were fed bovine liver homogenized with either nothing, aged polyethylene microplastics, or aged microplastics loaded with 2,4-D, delivering 100 micrograms of particles per gram of food. Over seven days for behavioral and regeneration assays and thirty days for reproduction tests, the researchers tracked three sensitive endpoints: how fast the worms moved, how quickly they regrew severed heads, and how many offspring they produced. Statistical analysis using analysis of variance with Dunnett’s post hoc comparisons, supplemented by non-parametric Kruskal-Wallis tests where data did not meet parametric assumptions, revealed differences that were significant at p values below 0.0001.

The results were striking across every endpoint, and in every case the herbicide-coated particles were worse than the plastic alone. Locomotor speed increased by 52.77 percent in worms fed aged microplastics and by 79.55 percent in those fed the microplastic-herbicide combination, compared with controls. The researchers suggest this hyperactivity may reflect stress responses, including increased mucus production, obstruction and inflammation of the digestive tract, or peristaltic movements as the animals attempt to expel the indigestible particles. Unlike the directed movement of control animals, the exposed worms darted in random directions, a disorganized pattern that could impair foraging and predator avoidance in the wild.

Regeneration, the planarian’s signature ability, was also compromised. When worms were decapitated behind the auricles, the length of the regenerating blastema, the mass of stem cells that rebuilds the head, was delayed by 42.85 percent at 24 hours and 23.07 percent at 48 hours in the microplastic group, but by 57.14 percent and 46.15 percent respectively in the combination group. The emergence of photoreceptors, the simple eyes that allow planarians to sense light, was delayed by 22.22 percent with microplastics alone and 41.66 percent with the mixture, while auricle regeneration lagged by 15.38 percent and 46.15 percent respectively. Because auricles carry chemotactic receptors that planarians use to locate prey, and photoreceptors govern their negative phototactic responses, these delays translate directly into impaired survival skills. In two specimens exposed to the combination treatment, the researchers even observed malformed photoreceptors of unequal size and irregular shape, a visible deformity that emerged by the end of the regeneration period.

Reproduction suffered the most dramatic damage. Fecundity, measured as the number of egg cocoons produced over three weeks, fell by 54.2 percent in the microplastic group and 61.6 percent in the combination group. Fertility, the hatching success of those cocoons over four weeks, dropped by 52.12 percent and 70.7 percent respectively. The authors interpret this reproductive collapse as evidence of physiological stress that forces the animals to reallocate energy away from reproduction and toward basic survival metabolism, a well-documented coping strategy under chronic toxicant exposure. Notably, the reproductive damage from the herbicide-laced particles exceeded even that previously reported for pure, unweathered microplastics, underscoring how aging and chemical loading compound the hazard.

The findings carry implications well beyond the laboratory dish. Planarians sit at a pivotal junction in freshwater food webs, and reductions in their reproductive output or disruptions to their hunting and escape behaviors could cascade through entire communities, generating imbalances that propagate both up and down the food chain. The study also adds nuance to a lively scientific debate: some researchers argue, based on modeling of hydrophobic organic chemicals, that microplastic ingestion is unlikely to meaningfully increase pollutant exposure in marine environments, while others have documented vectoring effects in specific organisms. The Brazilian results suggest that for aged polyethylene and a water-soluble herbicide in a freshwater invertebrate, the combination is genuinely more toxic than either component alone, and the authors explicitly conclude that polyethylene microplastics can serve as a potential vector of organic pollutants. They call for a review and updating of herbicide guidelines and criteria to account for these interaction effects, and recommend future work exploring different concentrations, longer exposure periods, post-exposure recovery to assess reversibility, and histological analyses to uncover the cellular mechanisms behind the observed malformations. As microplastic contamination continues to spread through every corner of the hydrosphere, the message of this study is uncomfortable but clear: the true ecological cost of plastic pollution may only become apparent when it is measured alongside the chemical pollutants it travels with.

Subject of Research: Interaction between aged polyethylene microplastics and the herbicide 2,4-D and its ecotoxicological effects on the freshwater planarian Girardia tigrina

Article Title: Interaction between polyethylene microplastic and 2,4-dichlorophenoxyacetic acid (2,4-D): chemical and ecotoxicological evaluation with Girardia tigrina

Article References: Sousa, R. R., Vasconcelos, R. B., Sarmento, R. A., Pereira, D. H., Souza, N. L. G. D., & Cavallini, G. S. (2025). Interaction between polyethylene microplastic and 2,4-dichlorophenoxyacetic acid (2,4-D): chemical and ecotoxicological evaluation with Girardia tigrina. Discover Toxicology, 2(1), Article 18. https://doi.org/10.1007/s44339-025-00026-w

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00026-w

Keywords: microplastics, polyethylene, 2,4-D herbicide, Girardia tigrina, ecotoxicology, freshwater planarians, adsorption, regeneration, reproductive toxicity, weathering, pollutant vectoring, water pollution