The effects of microalgal feed supplementation on growth and digestive function of Acipenser baerii and Sparus aurata

This study aims to assess the potential use of microalgae, produced through a sustainable process, as protein and lipid source in practical diets for two commercially valuable fish species: Siberian sturgeon ( Acipenser baerii ) and gilthead seabream ( Sparus aurata ). To achieve this goal, the study evaluated different biomasses of Microchloropsis gaditana, cultivated on either conventional synthetic medium (MSM) or diluted pig manure (MPM), incorporated crude or hydrolysed into aquafeeds. Specifically, the impact on growth performance, digestive enzyme activity, and intestinal morphology was evaluated in both species. Regarding growth performance, no significant improvement was observed in sturgeon fed M. gaditana -supplemented diets. However, gilthead seabream significantly increased body weight except for the group fed diets elaborated with hydrolysed microalgae. Digestive enzyme activities were also modified by the use of algal biomass, with negative effects in Siberian sturgeon, whereas gilthead seabream proteolytic activities were enhanced, which evidenced a species-specific digestive response. Finally, morphological analyses of intestinal mucosa revealed that dietary inclusion of M. gaditana induced significant changes in both species. In Siberian sturgeon, the inclusion of microalgae reduced the total absorption surface, regardless of the growth medium used for producing the microalgae. Conversely, in gilthead seabream, dietary inclusion positively influenced intestinal morphology, increasing the intestinal fold diameter, enterocyte height, and microvilli length, indicating enhanced nutrient uptake capacity. In conclusion, these results evidenced the potential application of microalgae obtained from biorefinery as a protein and lipid source in practical diets for both fish species. However, the contrasting effects observed in both species bring to light the need for future works to assess the species-specific responses to dietary changes. • M. gaditana- supplemented diets boosted growth in seabream but not in sturgeon. • Dietary M. gaditana modulated digestive enzyme activities in seabream and sturgeon. • Seabream intestinal function improved with M. gaditana dietary use, unlike sturgeon.