Low-inclusion raw and hydrolysed algal biomass in aquafeeds: effects on muscle composition, intestinal integrity, and gut microbiota in juvenile gilthead seabream (Sparus aurata)

Introduction: Early-life nutritional interventions can have lasting effects on fish performance and health. Macro- and microalgae are promising aquafeed ingredients, although their use may be limited by indigestible cell wall polysaccharides. This study evaluated whether raw or enzymatically hydrolyzed algal blends affect muscle composition, intestinal integrity, and gut microbiota in juvenile gilthead seabream.

Methods: Juvenile gilthead seabream (initial weight: 25.30 ± 2.97 g) were fed diets containing a mixed algal blend + Alaria esculenta biomass in raw (R) or hydrolyzed (H) form at 5% or 10% inclusion and were evaluated in triplicate (3 tanks per dietary treatment; n = 60 fish/tank; initial density 4.00 ± 0.02 kg m⁻³). Fish were sampled after 45 and 90 days. Muscle proximate composition and fatty acid profiles were analyzed to assess nutritional quality, while intestinal health was evaluated histologically. Gut microbiota composition and diversity were characterized at the end of the trial. Statistical analyses were performed using SPSS version 25 (IBM Corporation Inc.) and the significant differences between fish fed different treatments were detected by ANOVA and calculated using Fisher’s least significant difference (LSD) post hoc test (p < 0.05).

Results: After 45 days, no significant differences in muscle proximate composition were observed. After 90 days, all algae-fed groups showed higher muscle protein and lower lipid content than the control group. DHA increased significantly in all algae-supplemented groups, while EPA was the highest in R10. No intestinal damage was detected in any group. Although overall microbial diversity was unaffected, diet-driven shifts in community composition were evident. Pseudomonadota and Bacillota dominated across treatments, with hydrolyzed diets increasing Acinetobacter and raw diets, especially R5, being associated with higher Pseudoalteromonas and Thalassotalea.

Conclusions: Algal supplementation improved muscle nutritional quality after prolonged feeding without compromising intestinal integrity. These findings support raw and hydrolyzed algal blends as functional feed ingredients for juvenile Sparus aurata.