Impact of Probiotics on Enzyme Activities, Intestinal Microbial Remodeling, and Metabolic Pathways in American Shad (Alosa sapidissima) at High Temperatures

High temperature restricts the survival and growth of aquatic organisms. Probiotics have significant potential for mitigating the negative effects of temperature stress on fish. In this study, the American shad (Alosa sapidissima), a temperature-sensitive freshwater fish, was selected as the experimental paradigm to dissect the underlying mechanisms governing the interactions between the host and its microbiome, with a particular focus on the impact exerted by the probiotic Lactococcus lactis within a high-temperature setting. We evaluated the effects of probiotics on the growth and biochemistry of A. sapidissima by measuring relevant parameters and enzyme activities and conducted an integrated microbiome–transcriptome analysis to assess the impacts on the gut microbiota and uncover probiotic-regulated metabolic pathways. The findings of our research indicated that probiotics had beneficial effects on growth; the activities of enzymes such as LPS, T-SOD, and GSH-PX; and the gut microbial composition. Furthermore, the configuration of the intestinal microbiota underwent a transformation, as evidenced by the increased relative prevalence of bacteria with potential beneficial properties, including Bacillus, Lactococcus, and Clostridium. Liver transcriptomic analysis revealed 586 differentially expressed genes (DEGs). The expression of immune-related genes (nfil3-2, il17d, and leap2) and lipid metabolism-related genes (pla2g3 and sc5d) was strongly upregulated. KEGG enrichment analysis revealed that the DEGs were predominantly clustered within metabolic pathways such as circadian rhythm and fatty acid degradation. This study revealed that probiotics enhanced intestinal bacterial diversity and eased stress by regulating the circadian rhythm, immunity, and lipid metabolism under high-temperature conditions. This study provides a reference for the use of probiotics in A. sapidissima at high temperatures.