Effects Of Fermented Soybean Meal With Phytate Degraded Bacteria On Juvenile Coho Salmon(Oncorhynchus Kisutch)

The aquaculture industry is growing rapidly and is an important contributor to global food production.However,unstable feed supply and high feed costs are major problems in aquaculture.Feed cost accounts for about 60%of aquaculture cost,and protein source is the key factor determining feed cost.At present,fish meal is still used as the main proteogen of aquatic feed due to its rich protein content,balanced amino acid structure,high nutrient digestibility,no anti-nutrient factors and other advantages.However,due to its large market demand and high price,its supply has decreased significantly.Therefore,to reduce the proportion of fish meal in feed and to find alternative high quality protein source has always been a hot topic in aquatic feed research.Soybean meal is a potential protein substitute.However,phytic acid in soybean mealis often chelated with cations such as iron,zinc,magnesium and calcium ions,thus affecting the absorption of mineral ions by the body.Therefore,it is considered to be the most harmful anti-nutrient factor in food,and also the cause of mineral ion deficiency in animal and human nutrition.Microbial fermentation is the most effective method to degrade polymeric proteins and phytic acid to enhance soybean meal nutrition.The phytic acid content in soybean meal can be reduced by fermentation,and the utilization rate of nutrients in feed can be increased.The purpose of this study was to study the effects of fermented soybean meal on growth performance,serum biochemical indices,antioxidant capacity and antioxidant gene expression of juvenile coho salmon,which were fed with a partial fish meal instead of juvenile coho salmon,in order to provide a basis for the development of low fish meal feed and new protein sources.The results were as follows:（1）Mangrove root soil was collected,phytic acid screening medium was selected,several phytic acid degrading strains were isolated and screened,and numbered.Two strains with better performance were selected after preliminary screening of transparent ring and rescreening of phytase activity.Strain a A8 was determined to be dextral Sporolactobacillus laevolacticus and strain a B13 to be Bacillus cereus by 16S r DNA sequencing.The phytic acid hydrolysis ability of the strain was investigated.The strain was inoculated in the medium with soybean meal as the only nitrogen source,and the phytic acid degradation rate was optimized by L₁₆（4⁵）orthogonal test.The results showed that the optimal fermentation conditions of a A8 were sucrose as fermentation carbon source,fermentation at 37℃for 24 h,inoculation amount of 10%（v/m）,ratio of material to water 1:1.2,phytic acid degradation rate of soybean meal was 89.1%.Under the optimal fermentation conditions of 24h fermentation at 45℃with brown sugar as fermentation carbon source,inoculation rate of 10%（v/m）and ratio of material to water of 1:0.8,the phytic acid degradation rate of soybean meal was 95.37%.The experimental results showed that the strain could efficiently degrade phytic acid in soybean meal,which provided a theoretical basis for feeding fermented soybean meal in live culture.（2）The phytolytic strain was fermented by soybean meal,and some fish meal protein was replaced by fermented soybean meal to make experimental feed.The results showed that the replacement of some fish meal protein by fermented soybean meal had no significant effect on the body composition and survival rate of juvenile coho salmon（P>0.05）.After 10 weeks of feeding the study diet,the growth indexes of juvenile coho salmon fed fermented soybean meal were significantly higher than those fed the fish meal diet,such as specific growth rate,weight gain rate,liver body index and fatness（P<0.05）.（3）The phytolytic strain was fermented by soybean meal,and some fish meal protein was replaced by fermented soybean meal to make experimental feed for feeding.The results showed that replacing some fish meal protein with fermented soybean meal could significantly improve the serum indexes（glucose level,alkaline phosphatase activity,total protein level,albumin level and total cholesterol level）（P<0.05）of juvenile coho salmon.In terms of liver antioxidant,replacing fish meal protein with fermented soybean meal can significantly improve the activity of liver antioxidant enzymes（P<0.05）,promote the expression of Nrf2antioxidant pathway in the liver of juvenile coho salmon,and thus promote the gene expressions of glutathione system,superoxide dismutase and catalase in the downstream,so as to improve the antioxidant capacity of the body.In conclusion,fermented soybean meal can replace part of fish meal protein,but has no significant difference in growth antioxidant of juvenile coho salmon,and can increase the active expression level of antioxidant enzymes in liver.This study provides an important theoretical basis for the substitution of fish meal protein with fermented soybean meal.