Effect of algal extracts on the metabolism, intestinal functionality and oxidative status of gilthead seabream (Sparus aurata)

Intensive aquaculture systems are associated with decrease in water quality and increased environmental stress and disease prevalence. This promotes the exploration of bioactive compounds that can be included in aquafeeds to enhance fish resilience under such negative conditions. The present study investigated the effects of dietary supplementation with two different extracts obtained from the green seaweed Ulva spp. on the growth performance, intestinal functionality, metabolic profile and oxidative status of gilthead sea bream ( Sparus aurata ). These extracts, either characterized to be enriched in protein (P) or carbohydrate (F) fractions, were included at 12 % in respective diets. After a 42-day feeding trial, a 10-day starvation challenge was conducted as a putative proof-of-concept on the effects of bioactive compounds under food-deprivation. No significant differences in growth performance were observed between the experimental groups during the feeding period. However, after starvation challenge, fish fed the Ulva -based diets exhibited reduced weight loss (∼5 % lower than the control group), indicating enhanced physiological resilience. No significant effect of the inclusion of algal extracts was observed on intestinal morphology or functionality. Biochemical analyses revealed that plasma lactate levels were significantly lower (∼15 %) in the P group, while plasma protein concentrations were reduced (∼9 %) in the F group compared to the control. Hepatic glycogen depletion was more pronounced in fish fed both supplemented-experimental diets after the starving period. Conversely, oxidative stress markers indicated enhanced antioxidant capacity in both experimental groups. Superoxide dismutase (SOD) activity was significantly higher (∼13 %), while malondialdehyde (MDA) levels, a marker of lipid peroxidation, were ∼ 15 % lower in Ulva -fed fish compared to controls. These findings suggest that Ulva spp. extracts may confer metabolic and antioxidant benefits, particularly under starvation-induced stress, without compromising growth performance. Further research is warranted to optimize inclusion rates and elucidate their long-term impacts. • Ulva spp. at 12 % inclusion level did not negatively affect growth performance and intestinal health in Sparus aurata. • Fish fed Ulva -enriched diets showed improved resilience to starvation with ∼5 % less weight loss. • Ulva supplementation enhanced antioxidant status by increasing SOD and decreasing MDA activities.