Physiological and Transcriptome Analyses of Gill and Hepatopancreas of Potamocorbula ustulata Under Ammonia Exposure

Excessive ammonia accumulation poses a significant threat to aquatic species. Potamocorbula ustulata, known for its burrowing behavior and high population density, may experience elevated ammonia levels in its environment. However, its ammonia stress response mechanisms remain unclear. This study investigates the physiological and molecular responses of P. ustulata to acute ammonia exposure. Antioxidant enzyme activity was significantly altered in the gills and hepatopancreas, with GS, GDH, and ARG levels markedly increasing in the hepatopancreas. Transcriptome analysis revealed that after 24 h of exposure, differentially expressed genes (DEGs) were enriched in apoptosis and inflammation-related pathways (MAPK, NF-kB, NOD-like receptor signaling). By 96 h, DEGs in the gills were associated with nitrogen metabolism and transport, while those in the hepatopancreas were linked to oxidative phosphorylation and amino acid metabolism. Key ammonia transport and excretion genes, including V-type H+-ATPase, Ammonium transporter Rh, and Na+/K+-ATPase, were significantly upregulated in the gills, while glutamine synthetase and glutamate dehydrogenase were upregulated in the hepatopancreas (p < 0.05). These findings suggest that ammonia stress disrupts antioxidant defense, triggers inflammation and apoptosis, and enhances ammonia tolerance through excretion, glutamine conversion, and urea synthesis. This study provides insights into the molecular mechanisms underlying ammonia tolerance in bivalves.