Toxic impacts of polystyrene nanoplastics and PCB77 in blunt snout bream: Evidence from tissue morphology, oxidative stress and intestinal microbiome

Polystyrene nanoplastics (PS-NPs) and 3,3',4,4'-tetrachlorobiphenyl (PCB77) are common pollutants in freshwater aquatic environments. To investigate the separate and combined toxicity of these two pollutants on the freshwater blunt snout bream (Megalobrama amblycephala), 270 juveniles were randomly assigned to six exposure treatments:the control group, CT (free of PS-NPs and PCB77), three single exposure groups, PS-L (0.2 mg/L PS-NPs), PS-H (2 mg/L PS-NPs), PCB (0.01 mg/L PCB77), and two combined exposure groups, PP-L (0.2 mg/L PS-NPs + 0.01 mg/L PCB77) and PP-H (2 mg/L PS-NPs + 0.01 mg/L PCB77). After a seven-day exposure, the tissue histopathology, antioxidant capacity, inflammatory response, and gut microbiome composition of fish were analyzed. The results showed that tissue fluorescence intensity of PS-NPs increases as the exposure levels of PS-NPs increase, and the combined exposure groups exhibited higher fluorescence intensity compared to their single PS-NPs exposure groups. Histopathological analysis showed that the exposure groups exhibited varying degrees of damage to the intestinal tissue compared to the control group, with more severe damage observed in the combined exposure groups. Additionally, liver damage was evident in the PS-H, PP-L and PP-H groups. Furthermore, the highest catalase (CAT) activities and malondialdehyde (MDA) contents were found in the intestine and liver of fish in the PP-L and PP-H groups. The mRNA levels of inflammatory factors (il, il-1β, il-8, il-6, il-10, and tnf-α) were up-regulated in the PS-H, PP-L and PP-H groups compared to those of the CT group. In addition, remarkable alternations in the intestinal microbiome compositions were observed among the groups:the abundance of Verrucomicrobiome and Planctomycetota increased in all exposed groups compared to that of the control group, while the abundance of Actinobacteriota was significantly reduced in the exposure groups. Functional prediction of microbiota indicated that the amino acid and carbohydrate metabolism, as well as intestinal structure, were impaired in the PS-NPs and PCB77 exposure groups. The results suggested that the toxicity of PS-NPs on M. amblycephala including tissue injury, oxidative stress, and disturbance of intestinal microbiota, depends not only on concentration but also increases when co-exposed to PCB77. This finding raises concerns about the ecological safety in freshwater aquatic environments.