Host-associated probiotic Bacillus altitudinis HCY710 enhances growth performance and biosafety in white scar oyster (Crassostrea belcheri) seed production: Functional and whole-genome insights

Probiotics are increasingly recognized as sustainable tools to enhance productivity and health in bivalve hatcheries. This study evaluated the probiotic potential, functional performance, and biosafety of a host-associated bacterium, Bacillus altitudinis HCY710, for application in laboratory-scale white scar oyster ( Crassostrea belcheri ) seed production. Oysters received dietary and waterborne supplementation of B. altitudinis HCY710 (10 6 CFU mL⁻ 1 ), and growth, survival, immune responses, digestive gland histology, pathogen resistance, and water quality were assessed. Probiotic-treated oysters showed significantly improved growth performance, survival, and visceral mass compared with controls. Innate immune parameters, including lysozyme activity and antibacterial capacity, were enhanced and associated with increased survival following experimental challenge with Vibrio alginolyticus strain VA04. Histological analysis demonstrated improved digestive gland structure, characterized by increased digestive gland cellular height and enhanced cellular vacuolization, indicating greater digestive and absorptive capacity. Ammonia and nitrite concentrations were significantly reduced in probiotic-treated systems. Phenotypic screening confirmed a non-hemolytic profile and strong sporulation ability. Whole-genome sequencing supported biosafety, revealing no virulence-associated genes, a low prevalence of mobile genetic elements, and only intrinsic, non-transferable antimicrobial resistance determinants. Functional annotation identified genes linked to metabolism, stress tolerance, adhesion, and host interaction, providing mechanistic support for probiotic efficacy. Overall, B. altitudinis HCY710 improves oyster seed performance through integrated physiological, immunological, and environmental mechanisms, supporting its application in sustainable hatchery production. • Bacillus altitudinis HCY710 significantly improved growth and survival of juvenile Crassostrea belcheri . • Probiotic supplementation enhanced lysozyme activity and antibacterial capacity, improving resistance to Vibrio alginolyticus . • Digestive gland morphology was improved, indicating enhanced nutrient absorption. • Water quality was improved through reduced ammonia and nitrite concentrations. • Whole-genome analysis confirmed biosafety and revealed functional traits supporting probiotic efficacy.