Dietary Inclusion of Micro-Algal Astaxanthin on Gut Health of Rainbow Trout Oncorhynchus mykiss: Insights from Gut Morphology, Physiological Indices and Microbiota Diversity

The green alga Haematococcus pluvialis, rich in natural astaxanthin, is a key feed additive for salmonid pigmentation. This study evaluated dietary micro-algal astaxanthin effects on structure, antioxidative and immune response, as well as microbiota in different gut segments of rainbow trout Oncorhynchus mykiss (initial average weight: 0.67 ± 0.02 kg). Three diets contained 0 (Diet 1, control), 18.57 (Diet 2) and 31.25 mg/kg (Diet 3) micro-algal astaxanthin. After a 4-month feeding trial, dietary astaxanthin promoted the goblet cell proliferation of pyloric caeca and increased hindgut tunica muscularis thickness (p < 0.05). It also improved antioxidant capacity, characterized by the upregulation of gpx and cat expression in the midgut, accompanied by a significant decrease in MDA content (p < 0.05). Furthermore, dietary astaxanthin could upregulate tgf-β, tor1 and pcna levels in midgut and igm in hindgut, while il1β, il6, il8 and tnfα in hindgut were significantly downregulated in Diet 2 (p < 0.05). Additionally, dietary astaxanthin also enhanced the α-diversity of hindgut and altered the core microbiota (reduced Proteobacteria, increased Actinobacteria). Diet 2 increased microbic abundance associated with reducing gut inflammation and promoting nutrient absorption while decreasing that of pathogenic bacteria. Overall, dietary 18.57 mg/kg astaxanthin supplementation could promote gut structure, antioxidant and immune capacity, reduce inflammation and modulate microbiota. These findings indicate that natural astaxanthin from H. pluvialis has potential as an immunostimulant to promote gut health in salmonids.