Dietary supplementation of a plant-based diet with hydrolyzed microalgae–grape marc extracts: effect on oxidative status, intestinal functionality and microbiota in Sparus aurata juveniles

The expansion of aquaculture and the rising cost of fishmeal have driven research into fish gut health and the replacement of marine-derived ingredients with alternative protein sources, within sustainable strategies aligned with the Blue Transformation. In this sense, the objective of this study was to evaluate the effect of including a functional ingredient based on hydrolyzed microalgae and grape marc extracts in feeds with high proportion of plant protein sources on growth, muscle composition, oxidative status, intestinal microbiota, and digestive functionality in gilthead seabream juveniles ( Sparus aurata ). Three experimental diets were formulated, all isoproteic (47%) and isolipidic (18%), namely: CT (control diet - a diet mimicking the commercial aquafeeds used for this species based on 20% fishmeal); PP (a plant-based diet replacing 60% of fishmeal with hydrolyzed vegetable protein); and PP-GG (the same PP diet supplemented with 2% of the microalgal-grape marc based nutraceutical, LB-Green Grape ). Results indicated a trend for improved growth performance and FCR in fish fed the PP-GG diet compared to PP group, although differences were not statistically significant. The inclusion of plant protein ingredients reduced muscle protein content, however, supplementation with microalgae and grape marc extract in the PP-GG diet partially mitigates this loss. Fishmeal replacement with vegetable proteins also affected lipid composition, with higher mono-unsaturated fatty acid content and increased poly-unsaturated fatty acid retention in PP and PP-GG groups. Additionally, PP diet posed an oxidative challenge, evidenced by higher lipid peroxidation, while this was significantly reduced in the PP-GG group, confirming the antioxidant effect of the nutraceutical ingredient. Digestive enzyme activities significantly increased in pancreatic enzymes of PP-GG group. Finally, although no significant differences in intestinal microbiota composition were observed, PP group showed increased bacterial diversity. In conclusion, dietary fortification with the nutraceutical based on microalgae and grape marc extracts improved growth, reduced oxidative damage, and enhanced digestive capacity in gilthead seabream juveniles, highlighting its potential as a nutritional strategy for sustainable aquaculture.