Se-biofortified Pediococcus acidilactici SKR-2 enhances fish thermotolerance over inorganic Se

Selenium (Se) is a crucial trace element that is essential for the maintenance of physiological and metabolic functions. Probiotics have the ability to bioconvert inorganic Se into less toxic, more bioavailable forms, such as seleno-substituted amino acids and nano-selenium (nano-Se), offering dual benefits of Se supplementation and probiotic activity. In this study, Pediococcus acidilactici SKR-2, which is highly tolerant to sodium selenite, was identified from Qualified Presumption of Safety recommended probiotics. Optimal conditions for Se enrichment were established, resulting in a total Se content of 300-350 mg/g, primarily in the form of nano-Se. Characterization confirmed the presence of spherical Se nanoparticles on the surface of Se-enriched P. acidilactici SKR-2 (P-Se), while transcriptomic analysis revealed that the strain synthesizes nano-Se through multiple pathways. In Zebrafish trials, P-Se at 0.056 mg/kg did not affect growth performance compared to inorganic Se supplemented at 0.112 mg/kg and significantly improved survival under heat stress, highlighting P. acidilactici SKR-2 as a highly efficient Se-enriching strain with potential to replace commercial Se. An enriched strain with potential to replace commercialized Se-enriched lactic acid bacteria and Se-enriched yeast would offer significant economic value and promising applications in enhancing aquatic animal health and stress resistance. • P. acidilactici SKR-2 accumulates 300-350 mg/g Se, nearly 100% of which is in organic or nano-Se form. • P . acidilactici SKR-2 synthesizes SeNPs via selenium compound and proline metabolic pathways. • P. acidilactici -Se (0.056 mg/kg) enhanced zebrafish heat survival significantly over inorganic Se (0.112 mg/kg).