Effects of polystyrene microplastics on Euglena gracilis: Intracellular distribution and the protozoan transcriptional responses

Microplastics (MPs) introduced into aquatic environments inevitably interact with aquatic organisms such as plankton, potentially yielding adverse effects on the aquatic ecosystem. The extent to which MPs can infiltrate planktonic cells and evoke a molecular response remains largely unknown. In the present study, the internalization of fluorescently labeled polystyrene (PS) MPs on Euglena gracilis cells was investigated, determining the transcriptional responses within protozoa after an 8-day exposure period. The results showed that exposure to 25 mg/L PS-MPs for 8 days, significantly inhibited protozoan growth (P < 0.05) and decreased the chlorophyll a content of E. gracilis. The photosynthetic efficiency of E. gracilis was suppressed by MPs after 4 days, and then recovered to control values by the eighth day. Fluorescence imaging confirmed the presence of MPs in E. gracilis. Transcriptomic analysis revealed the influence of PS-MPs on a diverse range of transcriptional processes, encompassing oxidative phosphorylation, oxidation-reduction process, photosynthesis, and antioxidant enzymes. Notably, a majority of the differentially expressed genes (DEGs) exhibited down-regulation. Furthermore, PS-MPs disturbed the transcriptional regulation of chloroplasts and photosynthesis. These findings indicate a direct interaction between PS-MPs and organelles within E. gracilis cells following internalization, thereby disrupting regular gene expression patterns and posing a substantial environmental risk to the aquatic ecosystem.