Molecular mechanisms of ammonia nitrogen tolerance in sea cucumbers (Apostichopus japonicus)

Ammonia nitrogen is a major pollutant in aquatic environments, affecting the growth and physiological functions of aquatic animals. This study focused on the sea cucumber Apostichopus japonicus as the research subject, and conducted a series of experiments to evaluate its tolerance to ammonia nitrogen. Enzyme activity in the body wall was measured to assess changes in growth and physiological functions. Additionally, transcriptomic and metabolomic sequencing analyses were performed on intestinal tissue to systematically investigate the effects of ammonia nitrogen on physiological responses, molecular mechanisms, and metabolic regulation in A. japonicus. The results revealed that sea cucumbers exhibited high tolerance to ammonia nitrogen. Under two stress concentrations, their survival rate remained at 100 %, although the weight gain rate (WGR) and specific growth rate (SGR) declined significantly. CAT activity increased significantly with rising ammonia nitrogen concentrations and glutathione metabolism and CYP450 pathways were significantly enriched. Upregulation of MGST3 and CYP1A1 indicated that sea cucumbers mitigated oxidative stress and toxicity through these pathways. Glycolysis and the TCA cycle were suppressed, as indicated by reduced activities of HK, PK, MDH, and SDH, and by downregulation of the PFKM gene. Conversely, metabolic pathways involving amino acids such as alanine and aspartate were significantly enriched, suggesting that accelerated amino acid metabolism help to compensate for energy demands. Notably, apoptosis-related pathways (e.g., MAPK and mTOR signaling pathways) and the oxidative phosphorylation pathway were enriched exclusively under high ammonia nitrogen stress. The upregulation of caspase-3 and caspase-8 suggests that sea cucumbers eliminate the damaged cells via apoptosis. Meanwhile, high expression of COX family genes which are involved in oxidative phosphorylation, likely enhanced energy production. Under low ammonia nitrogen conditions, the NOD receptor signaling pathway was significantly enhanced, which may strengthen the immune response. This study enhances our understanding on the molecular mechanisms underpinning ammonia nitrogen tolerance in sea cucumbers and provides new insights into promoting sustainable aquaculture.