Bacillus velezensis LG37 protects Macrobrachium rosenbergii from ammonia nitrogen stress

This study evaluated the protective effects of Bacillus velezensis LG37 on Macrobrachium rosenbergii under ammonia nitrogen stress, with a focus on survival, antioxidant capacity, and immune response. Juvenile prawns were divided into two groups: a control group without LG37 (Group A) and a treatment group with dietary LG37 supplementation (Group B). The 96-h maximum tolerance and absolute lethal concentrations of ammonia nitrogen were determined as 18.54 ± 1.21 mg/L and 86.52 ± 5.6 mg/L, respectively. Group B exhibited a 28 % higher median lethal dose compared with Group A, indicating enhanced tolerance. After 96 h of exposure, malondialdehyde (MDA) levels increased significantly in both groups (p < 0.05), but remained consistently lower in Group B. Antioxidant enzyme activities (SOD and CAT) were significantly higher in Group B than in Group A. Immune-related enzymes (lysozyme and phenoloxidase) showed tissue-specific responses, with gill activity peaking and then declining, while hepatopancreas and midgut activities increased significantly. Heat shock protein 70 (Hsp70) expression was consistently upregulated in Group B, whereas Caspase-3 expression increased in both groups but was significantly greater in Group B. Overall, dietary supplementation with B. velezensis LG37 improved survival, enhanced antioxidant defenses, and strengthened immune responses in M. rosenbergii under ammonia nitrogen stress, highlighting its potential as a protective probiotic in aquaculture.