Dietary fish oil supplementation provides n-3 long-chained polyunsaturated fatty acids for supporting fish growth and metabolism and enriching fillet with eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; c22:6n-3). Two experiments were performed as a 3 × 2 factorial arrangement of dietary treatments for 16 wk to determine effects and mechanisms of replacing 0%, 50%, and 100% fish oil with DHA-rich microalgae in combination with synthetic vs. microalgal source of astaxanthin in plant protein meal (PM)- or fishmeal (FM)- based diets for juvenile rainbow trout (Oncorhynchus mykiss). Fish (22 ± 0.26 g) were stocked at 17/tank and 3 tanks/diet. The 100% fish oil replacement impaired (P < 0.0001) growth performance, dietary protein and energy utilization, body indices, and tissue accumulation of DHA and EPA in both diet series. The impairments were associated (P < 0.05) with upregulation of hepatic gene expression related to growth (ghr1and igf1) and biosynthesis of DHA and EPA (fads6 and evol5) that was more dramatic in the FM than PM diet-fed fish, and more pronounced on tissue EPA than DHA concentrations. The source of astaxanthin exerted interaction effects with the fish oil replacement on several measures including muscle total cholesterol concentrations. In conclusion, replacing fish oil by the DHA-rich microalgae produced more negative metabolic responses than the substitution of synthetic astaxanthin by the microalgal source in juvenile rainbow trout fed 2 types of practical diets.
Microalgal docosahexaenoic acid (DHA) and astaxanthin (AST) may substitute for fish oil and synthetic AST in aquafeeds. This study explored the effects and mechanisms of those substitutions on AST metabolism and redox status of rainbow trout fed plant protein meal (PM)- or fishmeal (FM)-based diets. Two parallel experiments (PM vs. FM) were performed with 612 juvenile rainbow trout for 16 weeks as a 2 × 3 factorial arrangement of treatments with two AST sources (synthetic (SA) vs. microalgal (AA), at 80 mg/kg) and three levels (0, 50, and 100%) of fish oil substitutions with DHA-rich microalgae. The fish oil substitutions exhibit main effects (p < 0.05) and/or interactive effects (p < 0.05) with the source of AST on AST deposition, malondialdehyde and glutathione concentrations, and mRNA levels and activities of major redox enzymes (glutathione reductase (GR), glutathione peroxidase (GPX), glutathione S-transferase (GST), and superoxide dismutase (SOD)) in the muscle and liver of trout fed both diet series. The AST source produced only differences in tissue AST deposition (p < 0.05) and number of metabolites. In conclusion, the substitutions of fish oil by the DHA-rich microalgae exerted more impacts than those of SA by AA on redox status and functional expression of antioxidant enzymes in the tissues of rainbow trout.
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