Effect of dissolved oxygen and ammonia nitrogen on Culter alburnus: Physiology, biochemistry, and molecular analyses

Culter alburnus (topmouth culter)is an economically valuable freshwater fish. However, its insufficient tolerance to dissolved oxygen (DO) and ammonia nitrogen (AN) hinders its industrialisation. 360 experimental fish (4.87 ± 1.10 g) were placed in breathing chambers (oxygen level was 0.70-6.50 mg/L) or water tanks (control AN, 0 mg/L; low AN, 8 mg/L; high AN, 16 mg/L). This study analysed the effects of DO and AN on C. alburnus at physiological, biochemical, and molecular levels. (1) Physiology level: the floating point, coma critical point, and coma point at 20 °C group were significantly higher than those at 30 °C. The oxygen consumption rate of C. alburnus at 20 °C, 25 °C, and 30 °C was (256.65 ± 25.87), (470.47 ± 83.84), and (520.87 ± 55.40) mg/kg.h. The LC50 of AN after 96 h was 24.13 mg/L, and the safe concentration was 2.41 mg/L. The survival rate in the high AN group was significantly lower than that in the other two groups. (2) Biochemistry level: The change curves of antioxidant enzyme activity in the liver tissue under hypoxic stress reached a maximum at 12 h and then decreased. In addition, the increase and decrease in enzyme activity (except malondialdehyde) in the high AN group was lower than that in the low AN group. (3) Molecular level: the angiotensin-converting enzyme and carboxypeptidase genes were the major differentially expressed genes (DEGs) in hypoxic stress, and the DEGs were mainly enriched in the ABC transporter signal transduction pathway. In addition, the serum/glucocorticoid-regulated kinase, stearoyl-CoA desaturase, and 3-hydroxy-3-methylglutaryl-coenzyme A reductase genes were among the major DEGs under high AN stress. The DEGs were mainly enriched in steroid biosynthesis or glycine, serine, and threonine metabolism transporter signal transduction pathways. In summary, it is necessary to focus on the DO and AN during C. alburnus breeding.