Introduction
European seabass (Dicentrarchus labrax) is a cornerstone species of Mediterranean aquaculture, with growing production driven by strong market demand. However, its early-life stages remain a critical vulnerability for the sector. Larvae display limited physiological capacity to cope with environmental fluctuations, handling procedures, and opportunistic pathogens, resulting in high sensitivity and variable survival. Ensuring that these fragile larvae develop into robust juveniles is therefore essential to improve production efficiency and sustainability. Functional diets have emerged as a promising strategy to enhance robustness during these early developmental stages. By incorporating bioactive compounds able to modulate immune, antioxidant, and stress���related pathways, functional diets can support the maturation of key physiological systems and improve resilience during common production stressors. Among functional ingredients, algae stand out due to their rich biochemical profile. Sulfated polysaccharides stimulate innate immunity, carotenoids and phenolic compounds enhance antioxidant defenses, and polyunsaturated fatty acids contribute to membrane stability and inflammatory regulation. These mechanisms can collectively reduce oxidative damage, modulate endocrine stress responses, and strengthen mucosal barriers. The objective of this work is to evaluate the potential of algae-based functional diets to enhance robustness European seabass during the early life-stages, particularly regarding stress modulation and resistance to a pathogen challenge.
Materials and Methods
Two sequencial experimental trials were conducted at IPMA facilities to assess the effects of algae���based functional diets on the robustness of European seabass during early development. In Trial 1, dietary treatments consisted of a commercial���like control diet and three functional diets containing different combinations of Euglena sp., Ulva sp. and Tetraselmis sp. biomasses. In Trial 2, dietary treatments included a commercial diet, a commercial���like control diet and three functional diets containing combinations of Ulva sp., Tetraselmis sp. and Chlorella sp. biomasses. Fish from both trials (Trial 1: 83 days after hatching (DAH); Trial 2: 108 DAH) were reared in triplicate 300���L cylindroconical tanks under controlled zootechnical conditions and fed ad libitum with experimental microdiets for one week. Water quality parameters were kept within optimal ranges for the species, and routine husbandry ensured stable rearing conditions throughout the trial. Fish were then subjected to an acute air���exposure challenge to evaluate physiological stress responsiveness prior to bacterial infection. Fish were subsequently infected with Pasteurella damselae subsp. piscicida by bath exposure to evaluate the effect of dietary treatments on seabass disease resistance over a 14���day period. Fish sampling occurred at the end of the feeding period (before stress), 3 hours after stress and 24 hours after pathogen infection to analyse stress, immune and antioxidant���related enzymes and gene expression. Survival during the bacterial challenge was analysed using Kaplan–Meier curves.
Results and Discussion
No significant differences were observed on fish growth performance or survival during the experimental period of both trials. However, algae-based functional diets demonstrated measurable effects on stress resilience and pathogen resistance, although the magnitude of responses varied among trials and dietary treatments. In trial 1, no significant differences were observed between dietary treatments for stress (e.g. post-stress cortisol) and antioxidant-related biomarkers (CAT, SOD and GPX), but a blend composed of Euglena, Ulva and Tetraselmis biomasses led to an increase in the expression of immune-related genes (e.g. CCR3, CXCR4 and IL1β). Survival analysis following stress and pathogen infection revealed no significant differences among treatments. Results obtained in he second trial so far showed no significant differences for fish mortality following the air exposure stress test (overall survival of 89 %). However, results from the pathogen challenge revealed that a combination of Chlorella, Ulva and Tetrasselmis significantly reduced mortality risk (final value:56 %) in comparison to the commercial (final value:76 %) and the commercial���like control diet (p=0.022; final value: 69%). In addition, a blend of Chlorella and Ulva showed a strong trend toward improved survival (p=0.067) in comparison to the commercial-like control. Overall, results from both trials demonstrate that algae blends can contribute to improved stress and pathogen resilience during the early life stages of European seabass. Determining species combinations and optimal inclusion levels is critical for modulating physiological responses, as each algal biomass provides a distinct profile of nutrients and bioactive compounds. Therefore, this work supports the integration of algae biomasses into commercial microdiets as a sustainable strategy to reduce mortality during critical production phases in aquaculture.
Acknowledgments
This work was financially supported by "Pacto da Bioeconomia Azul" (Project No. C644915664-00000026) within the WP5 Algae Vertical, funded by Next Generation EU European Fund and the Portuguese Recovery and Resilience Plan (PRR), under the scope of the incentive line "Agendas for Business Innovation" through the funding scheme C5 – Capitalization and Business Innovation.