Enrichment effects of Paramecium on polystyrene nanoplastics of different sizes and concentrations and the mechanism of reverse toxicity

Polystyrene nanoplastics (PS-NPs) represent a significant environmental threat that has garnered widespread attention. However, the mechanisms underlying the enrichment, degradation, and toxic effects of PS-NPs in single-cell organisms remain poorly understood. In this study, Paramecium was exposed to three sizes of PS-NPs (30 nm, 100 nm, 500 nm) for 24 h. The responses of this sensitive ciliate species were monitored through multiple analytical methods, including comparative transcriptomics, morphological analysis, physiological assessment, and molecular biology techniques. The results demonstrated that Paramecium can efficiently accumulate PS-NPs through adsorption and phagocytosis. Upon internalization, PS-NPs altered antioxidant enzyme activity and content, disrupted mitochondrial membrane potential, and induced apoptosis and necrosis. Furthermore, PS-NPs induced cellular damage in a size- and concentration-dependent manner. Smaller particle sizes and higher concentrations of PS-NPs were associated with increased necrosis. Co-culturing with oxidative stress inhibitors alleviated oxidative damage, shifting cells from necrosis to apoptosis. This study provides insights into the dual mechanisms of cell death induced by PS-NPs in Paramecium and highlights the efficient accumulation of nanoplastics by this organism. These findings underscore the need to understand nanoplastics' toxicological effects across multiple trophic levels and offer valuable guidance for future strategies for their accumulation and degradation.