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Size-dependent chronic toxicity of fragmented polyethylene microplastics to Daphnia magna
Summary
Researchers tested the long-term effects of irregularly shaped polyethylene microplastic fragments on the freshwater organism Daphnia magna over 21 days. They found that fragmented microplastics were ingested at much higher rates and caused more harm to reproduction and survival than smooth, round plastic beads of similar size, suggesting particle shape matters for toxicity.
Fragmented microplastics (MPs) are emerging contaminants in freshwater environments; however, long-term assessment of their toxicity remains limited. This study aimed to evaluate and compare the chronic toxicity (21 d) of synthesized polyethylene MP fragments and commercial polyethylene MP beads to Daphnia magna. Ingestion of small- and large-sized MP fragments (17.23 ± 3.43 and 34.43 ± 13.09 μm, respectively) by D. magna was significantly (p < 0.05) higher, by 8.3 and 5.2 times, respectively, than that of MP beads (39.54 ± 9.74 μm). The survival of D. magna exposed to small- and large-sized MP fragments (20 and 60%, respectively) was significantly (p < 0.05) lower than that of individuals exposed to MP beads (90%). In particular, small-sized MP fragments significantly (p < 0.05) reduced algal feeding (from 95% to 76%), body length (from 4.20 mm to 3.98 mm), and the number of offspring (from 109 to 74) in D. magna, when compared with MP beads, likely due to their longer retention time and greater interference in the digestive tract. These findings suggest that fragmentation of MPs into μm-scale particles can pose a significant ecological risk to aquatic organisms; moreover, further studies are required to identify the underlying toxicity mechanism.