Exploring the potential of seaweed Gracilaria gracilis and microalga Nannochloropsis oceanica, single or blended, as natural dietary ingredients for European seabass Dicentrarchus labrax

Gracilaria gracilis and Nannochloropsis oceanica, single or blended, were tested in European seabass (Dicentrarchus labrax) diets. A control (CTRL) diet was compared with experimental diets including either 8% G. gracilis (GRA8), 8% N. oceanica (NAN8), or a blend of 4% of each alga (NAN4GRA4). After 106 days of feeding, growth, nutrient utilization, antioxidant defense, immunological status, and end-product quality were evaluated. All fish exhibited similar feed intake (1.4–1.5%), body weight, growth, and feed conversion ratio (1.6). Dietary inclusion N. oceanica did not affect digestible N intake and gain. Fish fed GRA8 had the lowest digestible N and energy intake (P < 0.05), and simultaneously the highest nitrogen retention efficiency and energy retention efficiency, resulting in a N and energy gain similar to all other treatments. All fish had well-preserved intestinal morphology; feeding NAN8 resulted in a significant increase in neutral goblet cells compared with GRA8. Fish fed the algal diets had significantly lower (P < 0.05) hepatosomatic index (1.7–1.8 vs 2.1) and plasma triglyceride levels than CTRL, but whole body composition remained similar among treatments. The liver total antioxidant capacity of fish fed NAN8 was significantly higher than that of fish fed GRA8 but did not differ significantly from the CTRL group. NAN4GRA4 resulted in lower values of total glutathione, glutathione peroxidase, and alternative complement. N. oceanica decreased fillet springiness; however, with NAN4GRA4, the muscle fillet became less resilient. G. gracilis and N. oceanica biomass, either used single (8%) or blended (4% each), can be valuable natural ingredients for partial replacement of fish meal in European seabass diets.