Internalization of polystyrene microplastics in Euglena gracilis and its effects on the protozoan photosynthesis and motility

Highlights: • A dose-dependent effect of MPS on E. gracilis was reflected by photosynthetic indices. • MPS reduced the photosynthetic electron transport and promoted energy dissipation. • Internalization of MPS in the cells and chloroplasts was involved in adverse effects. • Both photosynthesis and motility has its own advantage on toxicity evaluation of MPS. In this study, effects of polystyrene microplastics (MPS) on Euglena gracilis were investigated via examination on its photosynthesis and motility, two typical properties of the protozoan. No adverse effects were observed after 4-d exposure except for decrease in motility at two high MPS concentrations (5 and 25 mg/L). After 8-d duration, MPS at 1 mg/L had no obvious effects on E. gracilis, but two higher concentrations (5 and 25 mg/L) of MPS inhibited protozoan growth, motility, and photosynthesis. The reduced protozoan photosynthetic activity was reflected by changes in F_v/F_m (the maximum photochemical yield of PSII), ΔF_IP (difference between F_P and F_I) and PI_ABS (the performance index), indicative of reduced quantum yield of electron transport and enhanced energy dissipation. A dose-dependent effect of MPS on E. gracilis was found in protozoan growth, photosynthesis and motility, especially photosynthetic indices. MPS of small size (75 nm) seemed more toxic to the protozoa than large size (1000 nm). Internalization of MPS in the cells and chloroplasts was observed clearly for the first time, likely responsible for their toxicity. Analysis on photosynthetic process and motility of E. gracilis could provide more comprehensive understanding of MPS toxicity in the aquatic environment, and may potentially serve as a biomonitoring tool.