International Journal of Aquaculture, 2025, Vol.15, No.5, 221-228 http://www.aquapublisher.com/index.php/ija 225 These findings are consistent with those observed in salmonids, in which the combination of Schizochytriumspp. and Nannochloropsis gaditana in diets formulated for Salmo salar promoted activation of innate immunity, evidenced by increased expression of the genes C3 and NK-lysin, as well as elevation in the number of monocytes and immature erythrocytes (Sánchez et al., 2023). Similarly, the inclusion of Haematococcus sp. in diets for trout (Oncorhynchus mykiss) resulted in higher myeloperoxidase activity and increased survival rate after challenge with Vibrio anguillarum, reinforcing the immunomodulatory potential of microalgae (Aulia et al., 2024). Furthermore, a study with Oncorhynchus mykiss evaluated six diets containing progressive FO replacements by Schizochytrium sp. The best zootechnical performance was observed in the T20 diet (20% FO replacement), which resulted in weight gain, specific growth rate, and feed efficiency significantly higher (p < 0.05). This same diet showed the highest lysozyme activity level among treatments and provided greater survival rate of fish after challenge with Lactococcus garvieae, compared to the CON (control, 100% FO), T80 (80% replacement), and T100 (100% replacement) groups, indicating a relevant immunoprotective effect of the microalga even with partial FO replacement (Lee et al., 2022). Immune system modulation is considered one of the main benefits associated with the use of microalgae, which are rich in polyunsaturated fatty acids (PUFAs), β-glucans, and natural antioxidants (Bahi et al., 2023). Regarding the antimicrobial aspect, both extracts from Microchloropsis gaditana and Tetraselmis suecica demonstrate effective activity (Parra-Riofrio et al., 2023; Díaz et al., 2025). Specifically, M. gaditana can increase the antibacterial activity of Salmo salar serum against Piscirickettsia salmonis by more than 85% (Díaz et al., 2025). Additionally, extracellular polysaccharides fromT. suecica and Porphyridium cruentumshowed antiviral action against VHSV (Viral Hemorrhagic Septicemia Virus) in cell cultures, interfering at different stages of the viral cycle (Parra-Riofrio et al., 2023). In general, microalgae are rich sources of compounds with antimicrobial potential, including peptides, long-chain fatty acids, pigments (such as carotenoids and astaxanthin), phenols, and sulfated polysaccharides (Ahmed et al., 2022; Ilieva et al., 2024). These compounds may act directly or indirectly in reducing bacterial load and inhibiting common aquaculture pathogens. Antimicrobial activity related to the content of sulfated polysaccharides, for example, may be associated with alteration of bacterial cell wall integrity and inhibition of pathogen adhesion to the host (Rajasekar et al., 2019). Regarding antioxidant effects, several microalgae have demonstrated the ability to reduce oxidative stress and improve antioxidant enzyme activity. In Scophthalmus maximus, inclusion of Nannochloropsis sp. in the diet significantly increased the activities of the enzymes SOD (superoxide dismutase), GSH-Px (glutathione peroxidase), and total antioxidant capacity, while reducing hepatic levels of MDA (malondialdehyde) (Qiao et al., 2019). Similarly, in tilapia fed with different microalgae strains, an increase in T-AOC (total antioxidant capacity) in muscle tissue was observed, reduction in reactive oxygen species production, and positive regulation of the expression of the genes GSH-Px, CAT(catalase), and SOD, as well as higher resistance to infection by Aeromonas hydrophila (Ibrahim et al., 2022). Similar results were observed in mullet (Mugil liza) subjected to partial replacement of oil and FM by flaxseed oil and Spirulina sp. In this study, the 50% replacement treatment resulted in increased antioxidant capacity and improved zootechnical performance, without compromising the fatty acid profile of the fillet. These findings suggest that the combination of plant-based and microalgal ingredients can positively modulate the antioxidant response and contribute to maintaining product quality (Rosas et al., 2019). Modulation of intestinal microbiota is also a recurring effect associated with microalgae use. In Sparus aurata, for example, diets containing microalgae altered the bacterial profile of the intestine, promoting growth of the genera Pseudomonas and Bacillus (Katsoulis-Dimitriou et al., 2024). These effects may be related to the presence of complex polysaccharides, such as fucose, which act as selective substrates for probiotic bacteria.
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