Effects of substitution of lard oil and soybean oil with Rhodotorula toruloides biomass on growth, muscle fatty acids, digestive enzyme activities, lipid metabolism, liver and intestinal health, and gut microbiota of zebrafish (Danio rerio)

Microbial oil, as an alternative to fish oil, animal and vegetable oils in aquafeeds, have a great potential to fulfil the high demand for lipid sources in the fast-growing aquaculture. However, their use as a feed ingredient in aquaculture is not common. For that, Rhodotorula toruloides (51.90 % lipid) was used to replace 0.00 % (control), 9.38 % (RT3), 18.75 % (RT6), 37.50 % (RT9), and 56.25 % (RT18) of the lard and soybean oils commonly used in aquaculture in the diet of zebrafish. After 6 weeks of feeding, compared with the control group, (1) the FBW and WGR were significantly increased ( p <0.05) in RT9 group, (2) the content of C18:1 and C18:3 fatty acids were increased, while that of C18:0, C18:2, and C20:4 were decreased in the muscle of zebrafish, indicating that the fatty acid content of the feed affect the muscle content of fatty acids, (3) the intestinal lipase activity was markedly elevated in RT9 group ( p <0.05), (4) in RT9 and RT18 groups the content of AST was significantly reduced ( p <0.05), (5) the content of TAG in the liver was significantly reduced in RT6, RT9 and RT18 groups ( p <0.05), probably due to the down-regulation of the lipid synthesis-related gene dgat2 and up-regulation of the lipolysis-related gene ucp2 , (6) the expression of il-8 in the intestine was notably downregulated in RT9 and RT18 groups ( p <0.05), whereas the expression of inhibitory gene il-10 was significantly upregulated in RT9 group ( p <0.05), (7) feeding with R. toruloides reduced the relative abundance of Proteobacteria and Vibrio spp , while increasing the abundance of Firmicutes. In sum, the replacement of R. toruloides at a level of 37.50 % can improve the growth, digestive enzyme activity, liver health and gut microbiota profile of zebrafish fed with high-fat diet and can be used to replace the oil sources from other unsustainable ingredients in aquafeed. • The 37.50 % R. toruloides improved zebrafish growth performance. • The R. toruloides altered zebrafish muscle fatty acid composition. • The 18.75 %-56.25 % R. toruloides decreased zebrafish liver TAG accumulation. • The R. toruloides improved the gut microbiota of zebrafish.