Fragmentation is a critical process for the formation and weathering of plastics. Sunlight irradiation, physical wear, and chemical oxidation are recognized as important pathways for the fragmentation of microplastics (MPs). In the present work, we identified a new biological fragmentation pathway through the interaction of MPs with rotifers, a widely distributed zooplankton in surface waters. Both marine (Brachionus plicatilis) and freshwater rotifers (Brachionus calyciflorus) could uptake polystyrene (PS) MPs with the size at 5, 10, and 20 μm, while MPs at 30 μm were not observed in mastax or digestive tract of rotifer due to the limit of its trophi. Fragmentation was detected after the ingestion of fluorescent PS MPs (10 μm) in rotifer bodies using confocal microscopy. Fragmentation was also confirmed for non-fluorescent MPs as observed with Raman imaging scanning electron microscopy, and the nanoplastics with the size around 100 nm were generated in rotifer bodies. The fragments were clearly present around trophi, and the Young's modulus of trophi (1.46 GPa) was much higher than PS MPs (0.79 GPa), indicating the fragmentation through grinding. Importantly, photo-aging (12 h) did not change the morphology of PS MPs, but highly increased the fragmentation efficiency by rotifers. The numbers of nanoplastics produced from MPs (5 mg/L) by marine rotifers and freshwater rotifers were 6.41×104 and 8.48×104 particles/individual within 4 h, respectively. This study provides a new source/pathway of the generation of small-size MPs and nanoplastics, and new insight into the fate of MPs in surface waters.

Fragmentation of microplastics by marine and freshwater rotifers

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