Antioxidant and innate immunity of Danio rerio against Edwardsiella tarda in response to diets including three kinds of marine microalgae

Disease is one of the most critical factors restricting the green and sustainable development of aquaculture. Microalgae, serving as excellent aquatic bait with high nutritional value, can strengthen the immune protection of aquatic organisms. The purpose of this study is to select superior marine microalgae to as feed additives that can defend against pathogens to a certain extent. In this study, the nutrient compositions of three marine microalga species were analyzed, and their antioxidant and innate-immunity protection effects as feed additions on Danio rerio were further investigated. Results demonstrated that Dunaliella salina contained 36.72% carbohydrates, 0.51% total chlorophyll, and 1.20% total carotenoids. Isochrysis galbana contained 1.33% total chlorophyll and 0.91% total carotenoids. The total fatty acid content of Phaeodactylum tricornutum and I. galbana was 60.77 μg mg−1 dry cell weight (DCW) and 76.40 μg mg−1 DCW, respectively. The docosahexaenoic acid content of I. galbana was 8.57 μg mg−1 DCW. After 30 days of feeding with different diets on D. rerio, the relative percent survival of the D. salina and I. galbana group challenged with Edwardsiella tarda EIB 202 were 17.39% and 26.09%, respectively. In the I. galbana group, the relative expression levels of the antioxidant factor catalase and glutathione peroxidase increased 1.5–2.5 times. Furthermore, the Toll-like receptors 2 signaling pathway was activated, and the signaling transcription-factor nuclear factor kappa-B was activated depending on the myeloid differentiation factor 88 pathway to promote cytokine expression. The relative expression of defensins increased 1.26 times, which helped enhance the innate-immunity defense system. Therefore, I. galbana can be developed as feed additives in the freshwater-fish field and be an excellent carrier for the development of recombinant microalga vaccine in aquaculture.