This study aimed to valorize Codium sp. and Osmundea sp. as functional ingredients for European seabass diets. For this purpose, triplicate groups of 25 fish (20.6 g) were fed, during 8 weeks, one of seven diets: the control (CTR), 5% of Codium and Osmundea ground (diets CO and OS, respectively), 5% of Codium and Osmundea ground and autoclaved (diets COA and OSA, respectively), and 0.5% of Codium and Osmundea polysaccharide extracts (diets COP and OSP, respectively). The same diets were used for a digestibility trial. Fish fed the CO diet presented lower growth and an apparent digestibility coefficient (ADC) for dry matter and protein compared to fish fed the CTR and OS diets. Diet COA counteracted these negative effects. No differences were observed in feed intake, feed efficiency, and lipid ADC. Antioxidant enzyme activities and distal intestine histomorphology, an indicator of gut health, were not affected. The expression of interleukin-1β and interleukin-6 increased in fish fed the COP diet. In conclusion, the processing methods counteracted the negative effects of raw Codium, enhancing its value as a dietary ingredient, while its polysaccharides showed immunomodulatory potential that could be valuable during stress or disease periods. These findings support the valorization of these algae for aquafeeds, with Osmundea being safely included at 5% without processing.

Fish oil has traditionally been the primary source of n-3 long-chain polyunsaturated FA (n-3 LC-PUFA) in aquafeeds but is increasingly being replaced by alternative lipid sources. However, marine carnivorous fish require dietary n-3 LC-PUFA, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which these alternative sources lack. This study assessed the long-term impact of graded levels of dietary EPA+DHA from microalgal oil on European seabass growth, feed utilization, immunological and oxidative status, and muscle FA profile. Six isoproteic (47% protein) and isolipidic (17% lipid) diets containing 0.56%, 0.66%, 0.71%, 0.90%, 1.11%, and 1.27% EPA+DHA were fed to fish (107 g initial weight) for 180 days at 23 °C, followed by a 7-day thermal challenge at 30 °C. At the end of the feeding trial, growth performance, feed utilization, organosomatic indices, blood and innate immune parameters, and oxidative stress were unaffected. Muscle FA composition mirrored that of the diets, and linear correlations were observed between muscle and dietary n-3 LC-PUFA levels. In the thermal challenge, no mortalities occurred, but hematological parameters, immune responses, and oxidative stress were intensified. Muscle FA composition remained unchanged, indicating that a dietary EPA+DHA level of 0.56% appears sufficient to meet physiological needs during an acute temperature challenge.