From Brewery Waste to Sustainable Aquafeed: Harnessing Nannochloropsis Microalgae for Fishmeal-Free Gilthead Sea...

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.