A 10-week growth study was conducted to evaluate the effects of replacement of soybean meal (SBM) with different levels of Methylococcus capsulatus (0%, 2%, 4%, 6%, 8%, 10%) meal on the growth, digestibility, intestinal development and serum biochemical indices of juvenile Jian carp. The present results showed that compared to the control group (0% M. capsulatus meal), 6%, 8%, and 10% M. capsulatus meal significantly improved the final average weight (FW), weight gain rate (WGR), specific growth rate (SGR), and decreased feed conversion rate (FCR). No significant difference was found in the survival rate (SR) in the treatment groups, and SR of each group was more than 95%. Jian carp had relatively high apparent digestibility of diets containing M. capsulatus meal, the apparent digestibility coefficient of Jian carp for dry matter, crude protein, crude lipid, energy and amino acid of M. capsulatus meal was 70.85%, 90.68%, 67.68%, 80.82%, and 88.00–91.17%, respectively. The addition of M. capsulatus meal had no adverse effect in the intestinal tissue of Jian carp juveniles, and 8% M. capsulatus meal significantly increased the villus width compared with the control group. Improved serum biochemical indices and increased antioxidant capacity of fish was noted in fish fed diets containing M. capsulatus meal. Replacement of M. capsulatus meal at the 6% level significantly decreased alanine aminotransferase (ALT) activity compared to the control group. The blood glucose (GLU) of all treatment groups had a decreasing trend, and the 8% and 10% M. capsulatus meal diets had significant effects on GLU content. Compared to the control group, 6%, 8%, and 10% M. capsulatus meal significantly decreased blood urea nitrogen (BUN) content. Compared to the control group, 6% and 8% M. capsulatus meal diets significantly increased serum total antioxidant capacity (T-AOC) of Jian carp, and 8% M. capsulatus meal diet significantly decreased malondialdehyde (MDA) activity. The study has demonstrated that the replacement of SBM with Methylococcus capsulatus shows promise as an alternative protein source in the aquaculture industry.

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