| Summary: | Abstract The aim of this study was to determine the effects of different dietary protein levels on the growth, physiological parameters, and gut microbiome of genetically improved farmed tilapia (GIFT, Oreochromis niloticus). Two pellet feed diets with low (25%, LPD) and normal (35%, NPD) protein levels were fed to GIFT in aquaria at 28°C for 8 weeks. The LPD reduced trypsin activity and inhibited the growth of GIFT. The serum alanine amino transferase and aspartate transaminase activities, hepatic malondialdehyde content, and superoxide dismutase, glutathione peroxidase, and catalase activities were significantly higher in LPD GIFT than in NPD GIFT (p < .05). The LPD led to decreased lysozyme activity and increased levels of C3 (p < .05). A 16S rRNA gene profiling analysis showed that the LPD significantly affected the gut microbial composition. Compared with the NPD, the LPD significantly decreased intestinal microbial diversity (p < .05). The macronutrient distribution affected the taxonomic profile of gut bacteria, mainly the phyla Bacteroidetes, Proteobacteria, and Firmicutes. The LPD favored growth of the genus Bacteroides. The NPD appeared to increase the abundance of the genera Lawsonia, Romboutsia, and Sphingomonas. Our results showed that, compared with NPD GIFT, the LPD GIFT had weakened nonspecific immune function, altered microbial community structure, and decreased gut microbial diversity.
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