Microalgae-derived feed additives improve physiological health, intestinal integrity, and welfare in juvenile gilthead seabream (Sparus aurata) fed plant-based diets

Healthy aquaculture diets, which contain beneficial ingredients for the organism, are an emerging option. Within this framework, dietary additives play a crucial role, providing numerous advantages to animal health and physiological status. The present study aimed to assess how the growth and physiological health of gilthead seabream (Sparus aurata), a carnivorous fish species, are affected by the incorporation of two microalgae-based additives (LB-IMMUNOboost and LB-LIVERprotect from LifeBioencapsulation, Almería, Spain) into functional feeds formulated with only 5 % of fishmeal (FM) and low fish oil (FO, 5 %). Four experimental diets were tested for 90 days: i) a formulation mimicking the commercial diets for this species (C+: 20 % FM, 9 % FO); ii) a diet with 5 % of FM based mainly on plant-based ingredients and 5 % FO (C–); and the C– diet supplemented with iii) 1 % LB-IMMUNOboost (IB) or iv) 1 % LB-LIVERprotect (LP). The results showed reduced growth performance, metabolic rates, and overall welfare in fish fed the C– diet. However, IB supplementation in the diet partially reversed the adverse effects produced by the plant-based formulations, improving growth, metabolic parameters, and plasma cortisol levels. These results were consistent with the expression of various genes involved in the stress response, indicating adaptive negative feedback mechanisms within the hypothalamus-pituitary-interrenal (HPI) axis during the secondary stress response after three months of feeding on plant-based diets. The histological findings revealed a significant accumulation of adipocytes around the exocrine intrahepatic pancreas in animals fed plant-based diets (C–, IB, LP). Additionally, intestinal modifications in the subepithelial spaces were observed in these groups, which could be attributed to an increase in intestinal length (ILI, %), leading to the separation of the mucosa and submucosa layers. Nonetheless, fish fed the IB diet and, to a lesser extent, the LP diet, exhibited improvements in epithelial resistance (Rt) and apparent tissue permeability (Papp), which helped counteract the detrimental effects observed in the C– group. Overall, our results suggest that dietary additives like IB can partially mitigate some adverse effects of plant-based raw materials. Yet, given the specific effects of IB and LP on the immune system and hepatic integrity, further studies are needed to fully evaluate their potential benefits.