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Effect of UV-weathering on chronic toxicity of biodegradable mulch film microplastics to Daphnia magna: Particle versus extract exposure
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Scientists tested how sunlight-worn "biodegradable" mulch film plastic bits affect tiny water creatures called Daphnia, often used to study environmental toxicity. They found that sunlight breaks the plastic into smaller pieces that get absorbed more into bodies, but surprisingly this weathering didn't make things uniformly worse — instead, whole plastic particles mainly caused death and health problems, while chemicals leaching out of the plastic mainly stunted growth and reproduction. This matters because it shows "biodegradable" plastics aren't automatically safer once they break down in the environment, and their real-
Biodegradable mulch films are increasingly applied as sustainable alternatives to conventional plastics. However, their ecological hazards following environmental fragmentation and aging remain insufficiently understood. In particular, how ultraviolet (UV) weathering alters particle-associated and chemically mediated toxicity of biodegradable mulch films microplastics is unclear. Here, we investigated the chronic effects (16 days) of pristine and UV-weathered mulch film microplastics (PMF and UMF, respectively) and their corresponding methanol extracts (EXP and EXU, respectively) on Daphnia magna. By integrating life-history traits, biochemical energy reserves, and transcriptional responses, we disentangled particle and extract exposure. Particle exposure was associated with mortality and broader life-history impairment in D. magna, whereas extract exposure primarily affected somatic growth and reproduction without affecting survival. UV-weathering reduced particle size and increased internal particle burden; however, UMF exposure was associated with comparatively reduced organism-level impairment relative to PMF. Under extract exposure, organism-level responses were broadly comparable between EXP and EXU, whereas EXU exhibited comparatively stronger transcriptional responses related to mitochondrial and carbohydrate metabolism-associated genes. Overall, the effects of UV-weathering were endpoint-dependent rather than reflecting a uniform increase or decrease in toxicity. Instead, UV-weathering modified biological responses differently between particle and extract exposure conditions. These findings highlight the importance of exposure-specific assessment when evaluating the environmental hazards of biodegradable microplastics.
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