From by-product to benefit: the effect of white grape marc extracts on European seabass growth, gut microbiota, immune status, and resistance to Vibrio harveyi

Aquaculture intensification has increased the susceptibility of farmed fish to infectious diseases, which are traditionally managed with antibiotics. Growing environmental concerns and the emergence of antimicrobial resistance, however, have driven the search for more sustainable approaches to disease management. White grape marc, a polyphenol-rich by-product of the wine-making industry with known antimicrobial and antioxidant properties, represents a promising functional extract for aquafeeds. This study evaluated the effects of three white grape marc extracts obtained within the NeoGiANT project, using alcohol (ELN), glycol (PLN), and ketone (ACA) solvents, on growth performance, oxidative status, immune response, disease resistance, and gut microbiota of European seabass (Dicentrarchus labrax) juveniles. Fish were fed with four isoproteic and isolipidic diets: a control diet (CTRL) and three diets supplemented with white grape marc extracts at 450 ppm of total polyphenolic content per kg of feed. Fish were distributed into triplicate groups and fed for 48 days. Dietary supplementation did not affect survival, growth performance, or feed utilization. However, fish fed the PLN450 diet showed improved protein efficiency and antioxidant status, with higher catalase activity and reduced lipid peroxidation. Immune and stress-related gene expression also tended to be lower in fish fed the PLN450 diet than in those fed the other diets, while gut microbiota composition remained largely unchanged. Following a Vibrio harveyi challenge, mortality did not differ significantly among groups, although fish receiving the supplemented diets showed higher absolute survival than those receiving the CTRL diet. Overall, NeoGiANT grape marc extracts were safe for inclusion in the diets of European seabass juveniles and provided moderate antioxidant benefits, highlighting the potential of winery by-products as functional ingredients to support more sustainable aquaculture.