Engineered Saccharomyces cerevisiae-fermented soybean meal biofortified with astaxanthin enhances nutritional value and functionality in largemouth bass feed

This study applied synthetic biology principles to engineer Saccharomyces cerevisiae with a heterologous astaxanthin pathway, creating a high-yield strain. Fermenting soybean meal (SBM) with this strain produced astaxanthin-enriched fermented SBM (AFSBM; 43.8 mg/kg astaxanthin) while significantly reducing antinutritional factors (ANFs, e.g., oligosaccharides, urease). AFSBM’s potential in largemouth bass ( Micropterus salmoides ) feed was evaluated using five isonitrogenous (48% crude protein) and isolipidic (10% crude fat) diets where AFSBM replaced 0%, 15%, 30%, 45%, or 60% of fish meal (FM) protein (basal diet: 40% FM). After an 8-week feeding trial (initial weight: 8.70 ± 0.03 g), AFSBM replacement ≤30% did not inhibit growth performance (weight gain rate (WGR), specific growth rate (SGR)), but levels ≥45% caused significant inhibition. Higher replacement upregulated intestinal amino acid transporter ( slc7a5 , slc7a8 ) mRNA expression. Replacing 30% FM protein with AFSBM significantly enhanced tissue antioxidant capacity, increased intestinal villus height, and improved muscle texture as assessed by texture profile analysis (TPA). AFSBM substitution also reshaped gut microbiota: it reduced the abundance of the potential pathogen Chryseobacterium at all replacement levels, decreased microbial richness at higher replacement levels, and shifted the predicted functional potential of the microbiota toward carbohydrate, nucleotide, and energy metabolism pathways. In a 40% FM basal diet, replacing 30% FM protein with AFSBM enhances antioxidant status, improves intestinal morphology and muscle quality in largemouth bass without compromising growth. This demonstrates the potential of synthetic biology for the development of high-value, upgraded plant-based feed ingredients.