Effects of ammonia exposure and post-exposure recovery in pacific white shrimp, Litopenaeus vannamei: Histological, physiological and molecular responses.

The toxic effects of ammonia exposure on Litopenaeus vannamei have been widely reported, including tissue damage, oxidative stress, and metabolic disorders, but the ability of L. vannamei to recover from ammonia damage is still unclear. To further understand the adaptation mechanism of L. vannamei to ammonia, this study explored the effects of ammonia exposure and recovery on histopathology, physiological indicators, and transcriptomic responses. In the ammonia exposure (NH-N 25 mg/L) and recovery experiment, shrimp were sampled at 0 h, 24 h, 48 h of exposure, and 24 h, 48 h of recovery. The results showed that histopathological damage to the hepatopancreas and gills caused by short-term ammonia exposure could be alleviated after recovery. Ammonia exposure inhibited superoxide dismutase (SOD) and catalase (CAT) activities, decreased total antioxidant capacity (T-AOC), and increased malondialdehyde (MDA) in shrimp. Restoration of the antioxidant system after exposure mitigated oxidative damage and reduced MDA levels. The inhibition of acid phosphatase (ACP) and alkaline phosphatase (AKP) activities in shrimp caused by ammonia exposure was reversible. Ammonia excretion and metabolism attenuate ammonia toxicity and promote recovery in L. vannamei. Transcriptome analysis identified 1690, 1568, and 1463 differentially expressed genes (DEGs) in the hepatopancreas at 48 h of stress, 24 h, and 48 h of recovery, respectively. KEGG enrichment analysis revealed that ammonia exposure induced oxidative damage, resulting in apoptosis. Furthermore, activation of antioxidant-related pathways, such as glutathione metabolism and peroxisomes, helped reduce oxidative damage during the post-exposure recovery period. The addition of exogenous spermine and spermidine may contribute to post-exposure recovery and enhance ammonia acclimation in L. vannamei. Differential expression of the inflammatory gene STEAP4 in the ammonia stress and recovery phases, as screened by transcriptome analysis, may play a positive role in post-stress recovery. This study demonstrated the reversibility of the toxic effects of ammonia exposure on L. vannamei, complemented the knowledge of the mechanisms of adaptation of shrimp under ammonia exposure, and provided a basis for subsequent ammonia tolerance studies in crustaceans.