From Brewery Waste to Sustainable Aquafeed: Harnessing Nannochloropsis Microalgae for Fishmeal‐Free Gilthead Sea Bream Diets

The microalgae Nannochloropsis oceanica cultivated from brewery waste was considered a sustainable feed ingredient for gilthead sea bream ( Sparus aurata ), evaluating physiological, molecular and microbial responses. Over 96 days, fish were fed four isoproteic and isolipidic diets: a commercial‐based diet (D1), and three fishmeal‐free diets with increasing Nannochloropsis inclusion levels (0%, D2; 2%, D3; 14%, D4). The D4 diet was additionally supplemented with docosahexaenoic acid (DHA)‐rich algae oil instead of fish oil (FO). Growth performance remained almost equal in fish fed all the fishmeal‐free diets with only a slight and final improvement (<4%) in the control D1 fish, although an enhanced expression with a discriminant capacity was reported for hepatic growth‐related markers ( igf1 and ghr1 ) in D4 fish. Gene expression profiling also revealed a down‐regulation of fatty acid synthesis in combination with the up‐regulation of tissue lipid‐uptake genes, mostly linked to the dietary DHA enrichment. Antioxidant capacity was increased in both D3–D4 fish, suggesting a progressive activation of enzymatic and non‐enzymatic defences with the dietary microalgae diets (D3–D4). Moreover, intestinal transcriptomic profiling revealed a tightly regulated immune milieu, with coordinated modulation of pro‐ and anti‐inflammatory pathways indicative of an immune surveillance state induced by dietary microalgae. Water microbiota also shifted with diet composition, indicating a link between feeding strategy and environmental microbial communities. Taking all together, Nannochloropsis inclusion, particularly when combined with DHA‐rich microalgae oil, enhances fish health, immune and antioxidant status and beneficial gut microbiota markers. Altogether makes the circularly‐produced Nannochloropsis oceanica a valuable resource for the sustainable development of aquaculture feeds.