Effects of dietary Phaffia rhodozyma on the growth, immunity, antioxidant activity, and digestive tract microbiota of juvenile Strongylocentrotus intermedius

Strongylocentrotus intermedius is a commercially valuable aquaculture species. Phaffia rhodozyma , a yeast rich in astaxanthin and β-glucans, has immune-enhancing and antioxidant properties in aquatic animals; however, its effects on S. intermedius remain underexplored. Here, we examined how P. rhodozyma affects growth performance, digestive enzyme activity, antioxidant capacity, immune response, and digestive tract microbiota in juvenile S. intermedius . Juveniles (480) were assigned to four dietary groups: control (C, 0 %), low (L, 0.8 %), medium (M, 1.6 %), and high (H, 3.2 %) supplementation, for 60 days. Group M exhibited significant improvements in specific growth rates of shell diameter and body weight (SGRSD and SGRBW, P < 0.05), digestive tract index (DTI, P < 0.05), and digestive enzyme activities (PEP, LPS, AMS, and CL, P < 0.05). Group L showed enhanced immune enzyme activities (ACP, AKP, and LZM, P < 0.05) and reduced malondialdehyde (MDA) ( P < 0.05). In contrast, Group H demonstrated suppressed growth, digestive function, and immune response, and elevated MDA ( P < 0.05). Microbiome analysis revealed that Groups L and M had higher microbial diversity and beneficial taxa (e.g., Bacteroidota, Desulfobacterota); Group H had elevated potential pathogens (e.g., Pseudomonadota). Functional prediction indicated that moderate supplementation enriched pathways related to cell growth and immunity, whereas high supplementation activated stress-related pathways. Overall, P. rhodozyma exerted dose-dependent effects on S. intermedius : 1.6 % optimally promoted growth, digestive health, and microbial homeostasis; 0.8 % enhanced immunity and antioxidant status; and 3.2 % produced adverse effects, including inhibited growth. These findings support the application of P. rhodozyma in sustainable aquaculture. • Dose-dependent effects of P. rhodozyma on juvenile S. intermedius were investigated. • Moderate dose (1.6 %) improved growth and enzyme activity in S. intermedius. • Moderate dose (1.6 %) enhanced antioxidant and immune responses in S. intermedius . • Moderate dose increased beneficial gut microbes and decreased pathogenic taxa. • This study promotes the application of P. rhodozyma in sustainable aquaculture.