Toxic Effects of Multiwalled Carbon Nanotubes on the Common Carp ( <i>Cyprinus carpio</i> ) Intestines

Carbon nanotubes (CNTs) have been widely used in a variety of industrial fields, and their impacts on the environment have attracted a great deal of attention in recent years. The potential toxic effects of CNTs on fish intestines remain to be elucidated. In this study, common carps were exposed to various concentrations (0, 0.25, and 2.5 mg/L) of multiwalled CNTs (MWCNTs) for 4 weeks. The results showed that 2.5 mg/L MWCNTs induced more obvious histopathological changes and apoptosis as well as more significant expression changes of intestinal barrier tight junction genes than 0.25 mg/L MWCNTs. Further, the intestinal content samples were collected from fish exposed to 2.5 mg/L MWCNTs for microbiome analysis and untargeted metabolomics analysis. Analysis of gut microbiota composition using the 16S rRNA sequencing approach revealed that the abundance of Planctomycetes, Actinobacteria, Verrucomicrobia, as well as Firmicutes was increased after MWCNTs exposure; in contrast, the abundance of Fusobacteria was decreased. Analysis of metabolites using the LC-MS/MS approach revealed 24 upregulated metabolites and 27 downregulated metabolites. A joint analysis of the microbiome and metabolism suggested that significantly altered metabolites among the lipids and lipid-like molecules such as 15-Deoxy-d-12,14-PGJ2, 2-Methoxyestradiol, 5alpha-Cholestanone, 7-Dehydrocholesterol, and Cholesterol sulfate had a significant correlation with microbial genera. This study highlights a microbiome-metabolism axis that revealed the interactions between microbiota imbalance and metabolite disturbance, indicating more therapeutic strategies for MWCNTs-induced injury in fish.