Composite single-cell protein as a sustainable fishmeal substitute: Enhancing growth and health in largemouth bass (Micropterus salmoides)

This study evaluated the potential of a composite single-cell protein (CSCP) containing Chlorella vulgaris and Clostridium autoethanogenum at a 1:4 ratio as a substitute for fishmeal (FM) in largemouth bass ( Micropterus salmoides ) feed. A 56-day trial assessed CSCP's effects on disease resistance, antioxidant capacity, immune response, digestive function, and intestinal microbiota. Three diets were formulated: the control diet (FM52) with a high proportion of FM, the FM26 diet with 50 % of the FM replaced by CSCP, and the FM0 diet with complete FM replacement by CSCP. Three groups of 360 largemouth bass (initial average weight: 4.00 ± 0.03 g) were assigned to diets with three replicates of 40 fish each. Exposure to Aeromonas hydrophila resulted in higher survival rates (SR) in CSCP-supplemented groups, with the FM26 group showing the highest SR. Serum biochemical analysis indicated that CSCP improved lipid metabolism and liver health, evidenced by changes in triglycerides, cholesterol, alanine aminotransferase and aspartate aminotransferase. Antioxidant defense was strengthened with increased superoxide dismutase, peroxidase, and glutathione activities, and reduced malondialdehyde, indicating reduced oxidative stress. Gene expression analysis showed that the anti-apoptotic bcl-2 and anti-inflammatory il-10 genes were up-regulated, while the pro-apoptotic casp3 and casp9 as well as the pro-inflammatory tnf-α and il-8 genes were down-regulated. Intestinal digestive enzyme activities (trypsin, α-amylase, lipase) were enhanced, and intestinal microbiota composition was altered, with an increase in potential beneficial bacteria ( Cetobacterium , Clostridium_sensu_stricto_1 ) and a reduction in potential pathogens ( Mycoplasma , Aeromonas ) in the FM26 group. Functional predictions indicated enhanced pathways in “Cell motility” and “Immune system,” with enrichment of the NOD-like receptor signaling pathway. In conclusion, CSCP is a promising FM substitute, improving disease resistance, antioxidant capacity, immune response, digestive function, and intestinal microbiota in largemouth bass, with the FM26 diet yielding the most optimal results. This study provides valuable insights for sustainable aquaculture feed optimization. • CSCP ( Chlorella vulgaris and Clostridium autoethanogenum ) boost largemouth bass disease resistance against A. hydrophila . • CSCP diets improve antioxidant defense and reduce oxidative stress. • CSCP modulates immune genes, boosting anti-inflammatory responses. • 50 % replacement of FM with CSCP boosts immunity and intestinal health.