Effects of microplastics and natural particles on the aquatic invertebrate Daphnia magna under different dietary quality scenarios

Natural and synthetic particles co-occur in the aquatic environment. However, little information is available about the effects of natural particles on freshwater animals and how these effects differ from those of synthetic particles, especially under the scenarios of decreasing dietary quality and increasing cyanobacteria in the aquatic environment. Therefore, this study evaluated apical and molecular effects of polypropylene (PP) microplastics (MPs) and three natural non-food particles (i.e., kaolin, peat, and sediment) on the freshwater invertebrate Daphnia magna fed either a green alga or a mixture of green alga and cyanobacterium. After the 21-d chronic exposure of 10 mg/L PP when using the green alga Acutodesmus sp. as diet, the size of D. magna was significantly reduced, and the molting time was significantly extended compared with the control. However, the chronic effects of PP were masked when the cyanobacterium Pseudanabaena sp. was added to their diet. The natural particles kaolin, peat, and sediment posed insignificant effects on D. magna regardless of dietary quality. The expression of molting-related genes (e.g., ecr-a) and oxidative stress-related genes (e.g., sod2) was significantly upregulated in D. magna with the exposure of both natural and synthetic particles. The predicted no-effect concentration of PP was derived as 0.025 mg/L, raising concerns relating to their toxicity and risks in the contaminated aquatic environment. This study will improve our understanding of the effects and risks of natural and synthetic particles in freshwater environments, as well as facilitate ecoenvironmental authorities to make informed decisions on the appropriate management of MPs.