Local Mucosal Toxicity and Inflammatory Responses in the Gallbladder of Cyprinus carpio Exposed to Benzoapyrene: A Transcriptomic and Histological Study

Benzo(a)pyrene (BaP) 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 BaP 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 BaP accumulation. High BaP concentrations caused vacuolar degeneration of mucosal epithelial cells and nuclear deformities. Transcriptomic analysis revealed that BaP 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 BaP toxicity in the gallbladder. This work provides a cellular-level framework for assessing environmental polycyclic aromatic hydrocarbon (PAH) risks in aquaculture.