Single and combined effects of ammonia and nitrite on Litopenaeus vannamei: Histological, physiological and molecular responses

The Pacific white shrimp (Litopenaeus vannamei) is affected by the accumulation of ammonia and nitrite, in an intensive culture model. However, the mechanisms of combined stress toxicity remain unclear. We analyzed histopathology, antioxidant defense, immune competence, and hepatopancreatic transcriptional responses under ammonia and nitrite stress. The results indicated that combined stress caused more extensive histopathological damage compared to single stresses. Deeper histopathological damage was observed in hepatopancreas and gills than in muscles. Combined stress induces more severe oxidative stress and influences competence. The malondialdehyde (MDA) levels in the combined stress group were higher than those in the single stress group and the control group. The activities of superoxide dismutase (SOD) and acid phosphatase (ACP) activities in the hepatopancreas of the combined stress group were significantly lower than those of the single stress group and the control group. Transcriptome analysis showed that 418, 427, and 1158 differentially expressed genes (DEGs) were identified in the ammonia and nitrite single and combined stress groups, respectively, compared to the control. Lysosomes, drug metabolism - other enzymes, and glycine, serine, and threonine metabolic pathways were found to be significantly enriched by Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis in both single and combined stress groups. The genes CES and BHMT, which are related to detoxifying metabolic and immune, were up-regulated under both single and combined stresses. The response of these upregulated genes provides a better understanding of shrimp resistance to multiple environmental stressors stress. Retinol metabolism, sphingolipid metabolism, apoptosis, and FoxO signaling pathway were enriched only in the combined stress group, suggesting that combined stress had a more significant effect on shrimp xenobiotic metabolism, apoptosis, and oxidative stress than single stress. This study deepens our understanding of the response mechanisms of L.vannamei under ammonia and nitrite combined stresses.