Microplastics modify the toxicity of glyphosate on Daphnia magna

Plastic contamination is an environmental problem spreading to even the most remote aquatic and terrestrial habitats and in particular, microplastics represent an uncertain threat for organisms. Microplastics can have a direct negative effect, but they can also potentially modify the toxicity and bioavailability of pollutants. Here, we tested over 1 week the combined effect of three different glyphosate chemical formulations (glyphosate acid, glyphosate-monoisopropylamine salt, and Roundup Gran) and two types of microplastics (polyethylene (PE) microbeads and polyethylene terephthalate/polyamide (PET/PA) fibers), on Daphnia magna. Glyphosate-monoisopropylamine salt caused the highest mortality after 1 week exposure (23.3%), whereas glyphosate acid the lowest (12.5%), when tested in the absence of microplastics. These results were inverted, however, when the individuals were exposed in combination with the PE microbeads (glyphosate acid: 53.3% and glyphosate-monoisopropylamine salt: 18.3%) or the PET/PA fibers (glyphosate acid: 30.0% and glyphosate-monoisopropylamine salt: 8.3%). The mortality in the Roundup Gran formulation also increased when combined with the two microplastics, though the effect was less pronounced. In all experiments, the effect of the treatments and time was significant, though there was no significant interaction between them. In most treatments, negative effects were not observed after 48 h or later. The change in toxicity of the glyphosate formulations caused by microplastics can be linked to the different sorption properties of the glyphosate-based chemicals formulations. The outcome of this study highlights that beside the potential direct negative effects of microplastics, they can modify the toxicity of pollutants, such as herbicides.