The Effects of Probiotics on the Recovery of Growth, Digestive, Antioxidant, Immune Functions, and Gut Microbiota of Chinese Hooksnout Carp (Opsariichthys bidens) Under Microplastic Stress

Microplastics threaten freshwater fish health, with probiotics potentially alleviating this stress. This study subjected Opsariichthys bidens fry to 10 μg/L polystyrene microplastics (diameter 100 μm) for seven days, then established control (CK, 0 CFU/g), low (LC, 1 × 107 CFU/g), medium (MC, 1 × 108 CFU/g), and high (HC, 1 × 109 CFU/g) Bacillus coagulans supplementation groups for a 56−day rearing experiment. Results indicated that microplastic stress significantly reduced intestinal digestive enzyme activity in O. bidens and induced oxidative stress. Following stress removal, probiotic-treated groups exhibited markedly superior growth performance compared to the control (CK). Notably, the high-concentration probiotic group (HC) demonstrated intestinal trypsin levels approaching pre-stress levels, with no significant differences in liver antioxidant capacity (T−AOC, CAT, SOD) compared to pre-stress levels (p > 0.05). Furthermore, compared to the control group (CK), probiotic-treated fish exhibited upregulated growth- and immune-related genes (igf, ghr, tnf−α, il−1β), alongside optimized gut microbiota composition, characterized by increased abundance of Bacillus and decreased abundance of Pseudomonas and Aeromonas. This study demonstrates that B. coagulans alleviates microplastic-induced stress in O. bidens, offering insights for aquatic ecosystem conservation and biological remediation of microplastic pollution.