Effects of methanotroph (Methylococcus capsulatus) bacteria meal replacement of fish meal on growth performance, serum biochemical indices and intestinal health of hybrid grouper (Epinephelus fuscoguttatus ♀×E. lanceolatus ♂)

Methanotroph (Methylococcus capsulatus) bacteria meal (MBM), which has the advantages of high protein content and balanced amino acid composition, is a kind of the single-cell protein produced by M. capsulatus, which using methane as the sole carbon and energy source. In order to investigate the effects of substituting MBM replacement for fish meal on growth performance and intestinal health of hybrid grouper (Epinephelus fuscoguttatus ♀×E. lanceolatus ♂), five groups of isonitrogenous and isolipid diets were prepared by adding 2%, 4%, 8% and 12% MBM to replace 5% (MBM5), 10% (MBM10), 20% (MBM20) and 30% (MBM30) fish meal based on control diet, respectively, and hybrid grouper initial weight (28.85±0.04) g was fed for 8 weeks. The results showed that the survival rate of grouper in each group was 100%. There were no significant differences in weight gain rate, specific growth rate, feed intake rate, feed conversion ratio and protein efficiency ratio among replacement groups compared with control group (P>0.05). In whole fish composition, the moisture in all replacement groups and the ether extract in MBM20 and MBM30 were significantly higher than those in control group (P<0.05), but the crude protein of whole fish and the moisture, ether extract, crude protein, total essential amino acids and flavor amino acids of muscle showed no significant difference in all groups (P>0.05). Serum triglyceride, total cholesterol, low density lipoprotein and glutamic-oxalacetic transaminase in replacement groups were significantly lower than those in control group (P<0.05), but there were no significant differences in serum albumin and high density lipoprotein among all groups (P>0.05). Intestinal total antioxidant capacity and superoxide dismutase activity in replacement groups were higher than those in control group, while malondialdehyde content was lower than that in control group, but there were no significant differences (P>0.05). There was no significant difference in the thickness of intestinal muscle layer among all groups (P>0.05), and no significant difference in the height and width of intestinal villi in MBM10, MBM20 and MBM30 replacement groups compared with control group (P>0.05). The height of intestinal villi in MBM10, MBM20 and MBM30 groups and control group were significantly higher than that in MBM5 group (P<0.05). The width of intestinal villi in MBM30 group was significantly higher than that in MBM5 group (P<0.05). The vacuolated area of liver increased with the increasing replacement of MBM without significant difference (P>0.05). In conclusion, when the fish meal content in the basal diet is 40%, MBM can replace at least 30% of fish meal without negatively affecting the survival, growth performance and feed utilization rate of hydrid grouper, and can promote the reduction of blood lipid level, promote the accumulation while reducing the catabolism of lipids in liver and improves of intestinal antioxidant capacity of grouper. This study confirmed the feasibility of using methanotroph (Methylococcus capsulatus) bacteria meal in the diet of E. fuscoguttatus ♀×E. lanceolatus ♂, which has a facilitating effect on saving the use of fish meal in the diet of the groupers and reducing the cost of breeding.