Introduction
Algae, either micro- or macro- algae, are sustainable natural ingredients that have been used in human nutrition due to their rich composition in high valued nutrients such as proteins and long-chain polyunsaturated fatty acids (LC-PUFAs) . More recently, they have been acknowledged for their potential as a nutraceutical resource due to their enriched composition in pigments, minerals, vitamins and other bioactive compounds. In early life stages, fish are immature in their responses to the environment and their health and development can easily be compromised. Microalgae have been used to promote larvae health in larviculture , however the study of their effects in feeds as a health promoter is recent . Peixoto et al (2021) showed the potential of microalgae as larvae robustness enhancer in Senegalese sole (Solea senegalensis ), however, considering the contents of bioactive compounds in macroalgae, their synergetic application may be a cost-efficient strategy to potentiate algae as functional ingredient in aquaculture (Batista et al 2020). The aim of this study was to evaluate the immune response of S. senegalensis postlarvae when fed microdiets w ith algae blends (Nannochloropsis sp. and Gracilaria gracilis) included as functional ingredient to boost immunocompetence and overall performance.
Material and methods
Senegalese sole post-larvae were fed the CTRL diet, or the blend at 3% or 6% inclusion (BLEND3 and BLEND6 in equal proportions of micro- and macroalgae). The trial was conducted from 34 to 63 days after hatching (DAH) and molecular and physiological immune response was evaluated at 49 and 63 DAH (12 and 29 days of feeding) . Growth performance and whole-body immune response were assessed , as well as the expression of a panel of genes related with immune response. Here, the modulation of the expression of hepcidine (HAMP) , complement C3 , g type lysozyme (gLys ) the cytokines IL1b and IL10 and the toll like receptors TLR1 and TRL5 was evaluated and integrated with the observed physiological output. Data a nalysis was performed at univariate level and data were further integrated in a canonical discriminant analysis to infer different performances. A follow-up trial was performed to evaluate the capacity of post-larvae to cope with a Tenacibacullum maritimum infection inflicted by bath exposure.
Results and Discussion
Overall growth performance was enhanced in fish fed diets with algae blends, mainly BLEND3. Lysozyme and peroxidase activity was higher in both blend groups but was more evident in BLEND6 group . Interestingly, molecular analysis revealed lower expression of gLys in fish fed the blends at 49DAH. Expression of HAMP , IL10 and TLR5 was modulated in BLEND6 at 63DAH , and were slightly higher than in BLEND3. Multivariate analysis showed that at 49DAH groups are not separated based on the evaluated parameters, however at 63DAH both groups with blend inclusion, BLEND3 and BLEND6 were significantly discriminated from CTRL group. When fish were challenged with T. maritimum fish responded with a 20% increase in survival when compared with CTRL group, validating that algae blends inclusion in post-larvae microdiets improve fish robustness and resistance to diseases.
Conclusions
Micro- and macro- algae blends inclusion in microdiets have a great potential to promote health and resistance in Senegalese sole post-larvae. Algae blend is a sustainable ingredient and supports a cost-efficient strategy for larviculture since the inclusion of macroalgae mitigate the high production costs of microalgae and provides a more diverse bioactive compounds composition to the mixture. Here we observed that overall, the BLEND3 produced similar results than BLEND 6 therefore the former is a more cost-effective inclusion rate.
References
Peixoto, D.; Pinto, W.; Gonçalves, A.T.; Machado, M.; Reis, B.; Silva, J.Navalho , J.; Dias, J.; Conceição, L.; Costas, B. (2021) Microalgal biomasses have potential as ingredientes in microdiets for Senegalese sole (Solea senegalensis) post-larvae . Journal of Applied Physology . DOI: 10.1007/s10811-021-02431-1
Batista, S.; Pereira, R.; Oliveira, B.; Baião , L.F.; Jessen, F.; Tulli, F.; Messina, M.; Silva, J.L.; Abreu, H.; Valente, L.M.P (2020) Exploring the potential of seaweed Gracilaria gracilis and microalga Nannochloropsis oceanica , single or blended, as natural dietary ingredients for European seabass Dicentrarchus labrax . Journal of Applied Phycology 32, 2041–2059.
Acknowledgements
This work was funded by project 24517_VALORMAR supported by Compete 2020, Lisboa 2020, CRESC Algarve 2020, Portugal 2020 and the European Union through FEDER/ERDF.