Exploring the potential of seaweed Gracilaria gracilis and microalga Nannochloropsis oceanica, single or blended, as natural dietary ingredients for European seabass Dicentrarchus labrax Sónia Batista 1,2 & Ricardo Pereira 1,2 & Beatriz Oliveira 1,2 & Luis F. Baião 1,2 & Flemming Jessen 3 & Francesca Tulli 4 & Maria Messina 4 & Joana L. Silva 5 & Helena Abreu 6 & Luisa M. P. Valente 1,2 Received: 13 January 2020 /Revised and accepted: 25 March 2020 # Springer Nature B.V. 2020 Abstract 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. Keywords Antioxidant capacity . Gracilaria gracilis . Gut integrity . Fish nutrition . Nannochloropsis oceanica . Novel ingredients Introduction In recent years, the use of both seaweeds and microalgae has attracted increasing interest from the feed, food, and pharma- ceutical industries for the development of novel products, as well as sources of high-value-added compounds. Global trends point towards increasing demand for natural sustain- able sources of nutrients and biologically active substances able to simultaneously promote fish health and function as a vehicle of valuable compounds for consumers. Seaweeds can be natural sources not only of nutrients for aquafeeds, but also of bioactive compounds like pigments and water-soluble and liposoluble vitamins, as well as minerals (Valente et al. 2006; Holdt and Kraan 2011; Bellou et al. 2014; Silva et al. 2015; Sarker et al. 2016). Several studies have demonstrated the benefits of including seaweeds in fish diets in terms of growth, nutrient utilization, stress resistance, immune system, and fillet quality (Mohamed et al. 2012; * Luisa M. P. Valente lvalente@icbas.up.pt 1 CIIMAR, Centro Interdisciplinar de Investigação Marinha e Ambiental, Terminal de Cruzeiros do Porto de Leixões, Av. General Norton de Matos, S/N, 4450-208 Matosinhos, Portugal 2 ICBAS, Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal 3 National Food Institute, Technical University of Denmark, Søltofts Plads, Building 221, DK-2800 Kgs. Lyngby, Denmark 4 Department of Agriculture, Food, Environment and Animal Science, University of Udine, 33100 Udine, Italy 5 Allmicroalgae, Natural Products S.A., Apartado 9 EC Pataias, 2449-909 Pataias, Portugal 6 ALGAplus, Production and Trading of Seaweed and Derived Products Ltd, PCI-Via do Conhecimento, 3830-352 Ílhavo, Portugal https://doi.org/10.1007/s10811-020-02118-z / Published online: 9 May 2020 Journal of Applied Phycology (2020) 32:2041–2059