Effect of replacing soybean meal with Clostridium autoethanogenum protein on the growth performance, digestive capacity, antioxidant capacity, and protein metabolism of Hefang bream

The supply of soybean raw materials in China has always relied on imports, and recent fluctuations in international trade have seriously disrupted the stability of China's soybean raw material supply. Establishing suitable alternative protein sources is one of the effective ways to address this issue. Clostridium autoethanogenum protein (CAP) is a single-cell protein produced by microbial fermentation using industrial waste gas. Due to its high nutritional value, stable supply, and sustainability, it has attracted widespread attention in the field of aquatic feed. To investigate the effects of replacing SBM with CAP in diets on Hefang bream, a 56-day feeding trial was conducted on Hefang bream with an initial body weight of 8.01 ± 0.02g. Five iso-nitrogenous and iso-lipidic diets were formulated with graded replacement levels of SBM by CAP at 0% (control), 30%, 50%, 70%, and 100%. Compared to the control group, the complete replacement of SBM with CAP in the diet had no significant effect on SR, FBW, WGR, and SGR of juvenile Hefang bream. The length of midgut villi in fish fed with diets that replaced more than 50% of SBM with CAP was significantly higher than that in the control group. Intestinal α-amylase activity in fish fed the diet with CAP replacing 100% of SBM was significantly higher than that in the control group. The hepatic T-AOC and GSH content in fish fed the diet with CAP replacing 100% of SBM was significantly lower than those in the control group. The hepatic MDA content in fish fed with diets that replaced more than 50% of SBM with CAP was significantly higher than that in the control group. Muscle protein content in fish fed the diet with a complete substitution of SBM by CAP were significantly lower than those in the control group. In fish fed the diet with CAP replacing 100% of SBM, the expression of protein synthesis related genes tor, s6k1, and igf-1 (mTOR-related) was significantly suppressed in both liver and muscle. The gene expression of 4ebp1, a protein synthesis suppressor, showed significant upregulation in hepatic tissue of fish fed diets with CAP replacing 70% and 100% of SBM. In summary, CAP completely replacing SBM in the diet has no adverse effects on the survival and growth performance of juvenile Hefang bream. CAP substitution of 30% SBM in the diet significantly increased the essential amino acid content in muscles of juvenile Hefang bream. However, replacing more than 50% of SBM in the diet can cause liver oxidative stress and inhibit muscle protein synthesis.