Microplastics delay Paramecium multimicronucleatum to eliminate toxic Microcystis aeruginosa population

Assessing the feasibility of elimination of harmful Microcystis by protozoa from aquatic ecosystems requires evaluating the removal ability of protozoa under environmentally relevant pollutant conditions. Microplastics widely present in waters not only affect the individual organisms, but also interfere with the interspecific dynamics between predator and prey, including zooplankton and algae. To evaluate the effects of microplastics on protozoa eliminating toxic Microcystis, we designed an experiment of Paramecium grazing on Microcystis under different initial densities of Microcystis (2.0 × 10⁶ and 1.0 × 10⁷ cells mL^-1) and microplastics concentrations (0, 0.5, 1.0, 2.0, and 4.0 mg L^-1). The results showed that Paramecium was able to eliminate the Microcystis populations, but when microplastics reached 0.5 or 1.0 mg L^-1, it significantly reduced the grazing rate of Paramecium (from 1.3 × 10⁴ to 0.8 × 10⁴ preys predator^-1 d^-1 under 2 × 10⁶ cells mL^-1; from 4.9 × 10⁴ to 2.1 × 10⁴ preys predator^-1 d^-1 under 1 × 10⁷ cells mL^-1) and extended the time to eliminate Microcystis (from 4 to 6 d under 2 × 10⁶ cells mL^-1; from 8.7 to 11 d under 1 × 10⁷ cells mL^-1). Additionally, higher initial densities of Microcystis prolonged elimination time from 4 to 8 to 8.7-11 d. Overall, these findings suggest that microplastics delay the top-down effects through reducing grazing rate of Paramecium, which is an issue that cannot be ignored in practical complex water environments.