Transcriptional responses to heat stress and ammonia-nitrite toxicity in largemouth bass

This study is aimed to investigate the combined effect of heat stress and chronic ammonia-nitrite toxicity on metabolism and immunity of largemouth bass. Largemouth bass juveniles were raised at 28C (Celsius) or 34C in aquaria and routinely fed but water exchange was not performed to allow the accumulation of nitrogenous contaminants, ammonia and nitrite. The concentrations of total ammonia and nitrite were almost undetectable at the beginning of the experiment. Ammonia levels increased steadily and peaked on day 7 at 34C and day 9 at 28C. Subsequently, ammonia levels rapidly decreased to near zero on day 10 at 34C and day 13 at 28C. Nitrite levels remained below 0.2 mg/L at both temperatures from day 0 to day 6, slowly increased from day 7 to day 9, followed by a rapid rise from day 9 to day 11 at 34C or day 12 at 28C, and continued to increase at a lower rate until the end of the experiment. The fish were sampled on day 1, day 8 and day 14. Ammonia levels on the sampling days were 0.45 mg/L, 4.93 mg/L and 0.10 mg/L for 28C; and 0.68 mg/L, 5.95 mg/L and 0.15 mg/L for 34C. While nitrite levels on the sampling days were 0 mg/L, 0.56 mg/L and 10.83 mg/L for 28C; and 0 mg/L, 1.50 mg/L and 12.83 mg/L for 34C. Therefore, the fish were consecutively exposed to high levels of ammonia and nitrite with or without heat stress. Liver and spleen tissues were collected and subjected to gene expression analysis by RNA sequencing.