Algal density affects the influences of polyethylene microplastics on the freshwater rotifer Brachionus calyciflorus

Most previous researches focused on the toxicity of polystyrene microplastics (MPs) to marine organisms, but less on polyethylene MPs and freshwater zooplanktons. The present study aims to elucidate the toxicity of polyethylene (PE) MPs (diameter = 10-22 μm) to the typical freshwater rotifer Brachionus calyciflorus. Firstly, fluorescent microscope observation showed that rotifers could ingest PE MPs and accumulate them in their digestive tracts. Life-table experiments revealed that exposure to 0.5 × 10, 2.5 × 10, and 1.25 × 10 particles/mL PE MPs significantly reduced net reproductive rate and intrinsic rate of pollution increase of rotifers under algal densities (Scenedesmus obliquus) of 0.1 × 10, and 0.5 × 10 cells/mL, but no significant effects were observed under 2.5 × 10 cells/mL algal density. These results showed that PE MPs suppressed the reproduction of rotifer and this negative effect could be alleviated by increasing food supply. The swimming linear speed of rotifers significantly decreased with increasing MP concentrations. The activities of superoxide dismutase and Na-K-ATPase significantly decreased in treatments with high concentration of PE MPs under 0.1 × 10 cells/mL algal density, but did not change significantly in MP treatments under 0.5 × 10 and 2.5 × 10 cells/mL, compared to the control. Glutathione peroxidase activity significantly increased in treatments with 1.25 × 10 particles/mL and 2.5 × 10 particles/mL under 0.1 × 10 and 0.5 × 10 cells/mL algal density, respectively, but did not change significantly in all MP treatments under 2.5 × 10 cells/mL. Exposure to PE MPs might lower the gathering capacity of algae, induce oxidative stress, trigger cell membrane damages and disturb energy metabolism in rotifers, which can explain the PE MPs toxicity to rotifer reproduction.