Integrated analysis of physiological, transcriptome, and metabolome analyses of the gills in Solenaia oleivora under ammonia exposure

Ammonia is a common toxicant in aquatic systems and one of the key factors affecting aquaculture. However, data on mollusks' toxic response and coping mechanisms to ammonia nitrogen, especially freshwater mollusks, are still lacking. In this study, we evaluated the tolerance of a freshwater mollusk Solenaia oleivora to ammonia and investigated its coping mechanisms by combining physiological, metabolic, and transcriptomic analyses in the gills. The acute toxicity test revealed that the LC50-96 h (temperature-20 celcius, pH-7.4) of ammonia in S. oleivora was 63.29 mg/L. The physiological and TUNEL results showed that although 10 mg/L ammonia exposure increased the activities of antioxidant, immune and ammonia detoxification-related enzymes, it still caused oxidative damage and cell apoptosis of gill tissues. A total of 97 differential metabolites (DMs) and 3431 differential expressed genes (DEGs) were identified after ammonia stress. Among them, most DMs and DEGs were involved in immune response, antioxidant, cell apoptosis, carbohydrate metabolism, amino acid metabolism, and lipid metabolism. The enhancement of glycolysis and lipid metabolisms may provide energy for immune response and ammonia detoxification. In addition, glutamine synthesis, alanine synthesis and urea cycle were involved in ammonia nitrogen detoxification in the gill tissue of S. oleivora. Our results indicate that ammonia leads to individual death in S. oleivora, as wells as oxidative damage, cell apoptosis, immune response, and metabolic changes of gill tissues. The findings will provide valuable information to assess the potential ecological risk of environmental ammonia to freshwater mollusks and theoretical guidance for the healthy aquaculture of S. oleivora.