Acute effects of salinity and pH stress on <i>Micropterus salmoides</i>

To provide guidance for the cultivation of Micropterus salmoides in saline alkali environment, the effects of salinity and pH stress on enzymatic activity, tissue structure, and gut microbiota were investigated. The experimental groups consisted of group 1 (control group), group 2 (0.75% salinity and 9.0 pH stress group), and group 3 (10.0 pH and 0.5% salinity stress group). Samples were taken at 96 h and 192 h as 1-1, 2-1 and 3-1 groups, and 1-2, 2-2, and 3-2 groups, respectively. The superoxide dismutase (SOD), acid phosphatase (ACP), and alkaline phosphatase (AKP) activities in the experimental groups showed a trend of decreasing. The SOD, ACP, and AKP activities of groups 2-2 and 3-2 were significantly lower than those of group 1-1 and 1-2, whereas the malondialdehyde (MDA) index of groups 2 and 3 was significantly higher than that of group 1-1 and 1-2. Histological observation revealed that the liver cells gradually worsened with increasing duration of stress; the groups 2 and 3 were more severe than those of group 1. The gills were similar to those of the liver. At the phylum level, as stress persisted, harmful bacteria Proteobacteria first decreased and then increased; however, less harmful or beneficial bacteria Fusobacteriota first increased and then decreased. At the genus level, Aeromonas , Mycoplasma , or Candidatus - Bacilloplasma decreased at first, and then increased; Dietzia increased at first and then decreased. Simultaneously, stress with salinity and pH led to a decrease in gut microbiota diversity. In conclusion, 9.0-10.0 pH and 0.5-0.75% salinity caused changes in enzyme activity, damage to the liver and gills, and alterations in the gut microbiota.