Chitosan-flocculated Picochlorum maculatum MACC3 as a functional feed for improved growth and health in guppies (Poecilia reticulata)

Microalgae offer a sustainable alternative to traditional aquaculture feeds, promoting enhanced growth performance and disease resistance. This study evaluated the effects of dietary supplementation with chitosan-flocculated biomass of the marine picoalga Picochlorum maculatum MACC3 on growth, digestive enzyme activity, immunity, ornamentation, and gut microbiome composition in juvenile Silverado guppies (Poecilia reticulata). Two groups were studied for 120 days: a control group fed freeze-dried bloodworm meal and a treatment group fed the same diet supplemented with 10 % chitosan-flocculated, freeze-dried P. maculatum MACC3. The supplemented group showed significant (p < 0.05) improvements in growth, survival, and digestive enzyme activities (proteases, amylases, and lipases), with a non-significant (p > 0.05) increase in alkaline phosphatase activity. Upon challenge with Aeromonas salmonicida, the treatment group exhibited a 46.67 % higher survival rate (p < 0.05) than the control group. Immunological markers, including total protein, immunoglobulin, alkaline phosphatase, protease, and bactericidal activities, were significantly (p < 0.05) enhanced in algal biomass-fed fish. Skin pigmentation and ornamentation were also significantly (p < 0.05) enhanced, with increased carotenoid, pteridine, and melanin levels and a 10.23-fold higher expression of the pigment pattern-related csf1ra gene. Colordistance image analysis confirmed two distinct clusters, showing clear pigmentation differences between the experimental groups. Gut microbiome analysis revealed a significantly (p < 0.05) higher abundance of probiotic genera (Weissella, Lactobacillus, and Streptococcus) in the treatment group, while the control group was dominated by pathogenic genera, with PICRUSt predictions indicating more disease-related pathways. The treatment group also had a lower Bacteroidetes to Firmicutes ratio than the control, which was correlated with the observed growth benefits. In conclusion, P. maculatum MACC3 biomass is an effective functional feed for ornamental fish, enhancing growth, digestion, immunity, and pigmentation through gut microbiome modulation. The findings underscore the potential of P. maculatum MACC3 biomass as a sustainable feed ingredient in aquaculture.