Biosynthetic Nano-Selenium by <i>Bacillus licheniformis</i> Enhances Growth and Health of Largemouth Bass ( <i>Micropterus salmoides</i> )

Selenium is an essential micronutrient in aquaculture, with its efficacy and safety strongly dependent on the chemical form used for supplementation. In this study, we systematically compared the effects of three selenium sources-sodium selenite (SS), selenium-enriched <i>Bacillus licheniformis</i> fermentation broth (BlSe), and selenium-enriched <i>Saccharomyces cerevisiae</i> fermentation broth (ScSe)-on the growth performance and physiological parameters of largemouth bass (<i>Micropterus salmoides</i>). The SS group exhibited a survival rate of 90%, whereas both the BlSe and ScSe groups achieved 100% survival. Compared with the control, the ScSe group showed a significant increase in weight gain rate. In contrast, the BlSe group displayed markedly reduced serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phospholipase (ALP). All selenium-supplemented groups demonstrated substantially enhanced muscle selenium content-by 326% (SS), 455% (BlSe), and 88% (ScSe)-together with elevated activities of hepatic glutathione peroxidase (GSH-Px) and total superoxide dismutase (T-SOD), and reduced malondialdehyde (MDA) content. Intestinal microbiota analysis revealed a shift from Firmicutes dominance (50%) in the control to Proteobacteria dominance (60%) across all treatment groups at the phylum level. At the genus level, <i>Mycoplasma</i> was predominant in the control and BlSe groups (~20% abundance), while <i>Sphingomonas</i> and <i>Chloroplast</i> were enriched in the SS and ScSe groups, respectively. Moreover, KEGG pathway analysis indicated upregulation of taurine and hypotaurine metabolism, biosynthesis of unsaturated fatty acids, and fatty acid biosynthesis in all selenium-treated groups. These findings underscore the potential of <i>B. licheniformis</i>-derived nano-selenium as a highly effective selenium source for sustainable aquaculture.