Assessment of the immunomodulatory effect of using raw or hydrolysed Nannochloropsis gaditana in diets for juvenile gilthead seabream specimens

Microalgae are known to have bioactive compounds able to modulate fish immunity. So, they are good candidates to be incorporated into aquafeeds in order to improve fish health and welfare against opportunistic outbreaks. Enzymatic hydrolysis of microalgae biomass prior to its inclusion into aquafeeds represents a suitable strategy for increasing the bioavailability of bioactive compounds. In this framework, our objective was to assess the possible immunomodulatory effect of the microalga Nannochloropsis gaditana on the innate immune response of gilthead seabream (Sparus aurata) juveniles (15.5 ± 0.5 g of body weight) fed with isoproteic and isolipidic diets containing the microalgae at two inclusion level (2.5 % or 5 %), and two formats: raw or enzymatically hydrolysed. We analyzed the host immune status during 45 and 90 days of feeding and after a bacterial challenge performed at day 91 and mimicking an opportunistic outbreak. The middle-term effect were up-regulation of variable amounts of immune activities and genes depending on the diet. However, the long-time effects depended on the microalgae format. Thus, hydrolysed microalgae decreased or have no effect on immune parameters at long term, while raw microalgae induced up or down regulation depending on the parameter studied. Interestingly, the highest mortality rates were observed in the fish fed with raw microalgae diets. Thus, we have identified two molecules (il6 and Hamp) which exacerbated up-regulation might trigger the mortalities observed. Further studies will be recommended for controlling il6 and Hamp expression in order to protect fish from opportunistic bacterial infections.