Impacts of polystyrene nanoplastics at the environmentally relevant and sub-lethal concentrations on the oxidative stress, immune responses, and gut microbiota to grass carp (Ctenopharyngodon idella)

Nanoplastics (NPs) are emerging environmental contaminants with alarming ecological and human health concerns due to their unique physicochemical properties. This study elucidates the impacts of environmentally relevant concentrations (ERCs, 300 and 3000 ng/L) and sublethal concentrations (SLCs, 300 and 3000 mu g/L) of polystyrene nanoplastics (PS-NPs, size 80 nm) on the intestinal immune response and gut microbiota of grass carp (Ctenopharyngodon idella). Exposure to SLCs showed greater accumulation of NPs in gut and gill tissue, and more pronounced gut damage (vacuolization of gut villi) compared to ERCs. SLCs significantly increased ROS levels with increasing exposure duration and exposure concentration (p < 0.05). In addition, SLCs significantly increased the transcription levels of several immune response-related genes such as IL-6, IL-8, IL-10, IL-1 beta, and TNF-alpha (p < 0.05). However, the ERC exposure group at 3000 ng/L significantly increased the expression levels of IL-10 and TNF-alpha, compared to the control group (p < 0.05). Moreover, PS-NPs at both ERCs and SLCs signifi-cantly alter grass carp gut microbiota at the phylum and genus levels and disrupts microbial community composition and homeostasis (p < 0.05). At the phylum level, the abundance of Proteobacteria and Verrucomi-crobiota was increased after PS-NPs exposure, while that of Spirochaetota and Fusobacteriota decreased. At the genus level, the number of beneficial bacteria decreased and the number of pathogenic bacteria increased. Our findings suggest that PS-NPs at the level of ERCs and SLCs can induce inflammation of the intestine and microbiota imbalance in grass carp. Our findings contribute to a deeper understanding of the gut toxicity of PS -NPs under environmentally relevant conditions in freshwater commercial fish. These experimental results sug-gest that PS-NPs may indirectly damage grass carp intestinal epithelium by modulating the gut microbiota, thereby damaging the grass carp gut. Hence, this study provides valuable hints on the intestinal barrier structure and functional damage mediated by NPs exposure.