In vitro evaluation of algae and their effect as dietary ingredient on growth, chemical composition and intestinal functionality in juvenile turbot (Scophthalmus maximus)

The objectives of this study were to evaluate the in vitro protein hydrolysis of five algae (Arthrospira sp., Chlorella sp., Tisochrysis sp., Microchloropsis sp. and Ulva rigida) by the digestive proteases of Scophthalmus maximus, and to assess the effect of feeding turbot juveniles on diets supplemented with these algae in a 60-day feeding trial. The in vitro results revealed that algal protein was hydrolysed by turbot digestive proteases. Besides, the quantification of total amino acids released during the in vitro hydrolysis evidenced noticeable amino acid bioavailability, mainly in the case of Arthrospira sp. with up to 42.7% of the amino acids released after 120 min of in vitro hydrolysis. However, the structural characteristics of cell wall of each species influenced the protein accessibility as evidenced by the lower values obtained in Microchloropsis sp. For the in vivo feeding trial, 255 specimens of turbot, with an average initial weight of 26.4 ± 7.6 g, were used, and were fed with diets including 5% and 10% of a blend of the same microalgae tested in vitro (M5 and M10) or U. rigida (U5 and U10), compared to a control feed without algal biomass (CT). At the end of the feeding trial, growth performance and nutrient utilization did not show differences among the experimental groups. In addition, the fish fed with the diets supplemented with algae showed lower lipid oxidation in the muscle, and an improvement in digestive functionality, as the activity of pancreatic and intestinal enzymes increased. The ultrastructural study of the intestinal mucosa revealed an increase in the length of the microvilli and the absorptive surface in fish fed with the algae-supplemented diets, as well as decreased thickness of the lamina propria and the number of goblet cells. These results confirmed that the supplementation of turbot juvenile diets with low percentage of microalgae or U. rigida is appropriate for improving digestive functionality, as well as reducing the oxidation of lipids in the muscle. •Turbot intestinal proteases are able to hydrolyse algal protein in vitro.•Turbot were fed diets supplemented with a blend of microalgae or U. rigida for 60 days.•Feeding turbot with algae-supplemented diets modified protein and lipid content in muscle.•Inclusion of microalgae or U. rigida improved the digestive functionality in turbot.•Muscle lipid oxidation was reduced by dietary use of microalgae or Ulva rigida.