Selenium-enriched Lacticaseibacillus paracasei enhances growth performance, antioxidant capacity, and gut health in Micropterus salmoides

This study establishes Lacticaseibacillus paracasei ACCC10639 as a selenium-enriched probiotic through dual-phase optimization for Micropterus salmoides aquaculture. Strain selection under gradient sodium selenite stress (4–64 mg/L) revealed 32 mg/L Na 2 SeO 3 as the optimal production concentration, achieving 13.78 mg/g organic selenium (59.8 % organic speciation) while maintaining 70 % biomass viability. Subsequent dose-response trials identified 2.0 g/kg as the optimal dietary supplementation level through multi-parameter evaluation of growth performance, antioxidant capacity, and microbiota modulation. At this dual-optimized regimen (32 mg/ L -manufactured biomass and 2.0 g/kg feed dose), the selenized probiotic induced systemic improvements versus non-selenized controls: 12.7 % higher specific growth rate (3.89 vs 3.45 %/d), 18.6 % lower feed conversion ratio (1.40 vs 1.72), and 70 % enhanced glutathione peroxidase activity (79.4 vs 46.8 U/mgprot). Hepatic lipid peroxidation was 29 % reduced (MDA 1.06 vs 1.50 nmol/mgprot), accompanied by 15.2 % decreased hepatosomatic index. Ecological analysis using weighted UniFrac distances revealed tight microbiota clustering (NMDS stress=0.02) and distinct community separation via Principal Coordinate Analysis (72.9 % variation along PC1) in the SeLp group, with Firmicutes dominance at 89.8 % (+9.4 points) and butyrate production peaking at 182 % control levels. Pathogen suppression reached 97.6 % ( Aeromonas ) and 99.7 % ( Vibrio ). Spearman correlations revealed Lactobacillus abundance strongly associated with growth (ρ=0.92) and oxidative stress mitigation (ρ=-0.94), while butyrate emerged as a key antioxidant mediator (ρ=-0.94 with MDA). These findings validate selenium probiotic optimization through strain-specific production (32 mg/L) and dose-dependent application (2.0 g/kg), providing a sustainable strategy to replace inorganic selenium in carnivorous aquaculture. • Lactobacillus converts selenite to selenocystine with high efficiency. • Synergistic selenium-probiotic enhances fish growth and liver function. • Selenium-probiotics boost Firmicutes and butyrate-producers, suppress pathogens. • Microbiota and metabolites mediate selenium-probiotic driven host benefits.