Increased adsorption of diflubenzuron onto polylactic acid microplastics after ultraviolet weathering can increase acute toxicity in the water flea (Daphnia magna).

The ultraviolet (UV) weathering of microplastics (MPs) can lead to higher adsorption of harmful contaminants, thus increasing the potential risks of their combined effects. Because biodegradable MPs are more susceptible to UV weathering than conventional MPs, concerns have arisen about their ecological toxicity and environmental impact. Therefore, this study investigated the mechanisms associated with the adsorption of the pesticide diflubenzuron (DFB) onto polylactic acid (PLA) MP particles after UV weathering and the acute effects (48 h) of their combination on the water flea Daphnia magna. These effects were also compared with those of the conventional MP polyethylene terephthalate (PET). UV weathering led to a greater number of cracks and pores in the PLA particles compared to PET, as well as a higher number of oxygen-based functional groups and a larger surface area. These surface changes in UV-weathered PLA particles promoted higher DFB adsorption, which in turn led to stronger acute toxicity for D. magna compared to UV-weathered PET particles. Combined exposure to 25 ng L DFB and both UV-weathered and non-UV-weathered MPs significantly reduced the chitin content in D. magna, while combined exposure to 12.5 ng L DFB and the MPs increased the chitin content. This effect was more pronounced for UV-weathered PLA exposure than UV-weathered PET exposure. The expression of the genes for chitinase and endocrine glycoprotein, both of which are closely associated with the toxic mechanisms of DFB, showed no significant changes with the combination of 25 ng L DFB and non-UV-weathered MPs but were significantly downregulated after UV weathering. Overall, the UV weathering of PLA promoted the adsorption of DFB, thus increasing its toxic effects. Our findings demonstrate the importance of considering the effects of UV weathering and interactions with environmental pollutants when assessing the ecological risks associated with biodegradable MPs.