Local Mucosal Toxicity and Inflammatory Responses in the Gallbladder of Cyprinus carpio Exposed to Benzo[a]pyrene: A Transcriptomic and Histological Study

Benzo(a)pyrene (B[a]P) is a pervasive freshwater pollutant, yet its toxicity to the fish gallbladder remains poorly understood. This study investigated the toxicological impacts of 2.5 and 25 μg/L B[a]P on common carp (Cyprinus carpio) using histological, transcriptomic, and single-cell RNA sequencing (scRNA-seq) analyses. Results showed that the gallbladder is a primary site for B[a]P accumulation. High B[a]P concentrations caused vacuolar degeneration of mucosal epithelial cells and nuclear deformities. Transcriptomic analysis revealed that B[a]P stress triggered autoimmune homeostasis imbalance and overinhibited apoptosis. scRNA-seq identified cellular heterogeneity changes, specifically T-cell impairment and epithelial cell (EC) proliferation. Mechanistically, T-cell reduction was linked to the T-cell 2 subset, while EC proliferation involved EC 0 and EC 4 subsets, all participating in the apoptosis pathway. These findings demonstrate that the apoptosis pathway is a key target of B[a]P toxicity in the gallbladder. This work provides a cellular-level framework for assessing environmental polycyclic aromatic hydrocarbon (PAH) risks in aquaculture.