Suitability of disrupted biomass of microalgae Tetradesmus obliquus and Raphidonema monicae cultivated in drainwater from soilless farming as alternative ingredients in Atlantic salmon feeds

Microalgae-based ingredients can improve aquafeed sustainability by reducing reliance on fish- and plant-based ingredients. This study evaluated partial fishmeal replacement in Atlantic salmon diets with disrupted biomass of Tetradesmus obliquus and Raphidonema monicae, produced in nutrient-rich drainwater from soilless farming and processed using a biorefinery approach. Two experimental diets (AT, containing T. obliquus, and AR, containing R. monicae) were formulated with 10% inclusion of each microalga, and their performance was compared with that of a control diet (CT). After the 9-week feeding period at 8 °C, Atlantic salmon (initial weight 150 g) grew 2.4-fold, with no significant differences in growth or feed performance among dietary treatments, as assessed by specific growth rate, feed conversion ratio, and protein efficiency ratio. Whole-body or fillet nutrient composition were unaffected by the diets; however, protein and lipid digestibility were significantly higher in the AR group. Regarding health status indicators, AR-fed fish had a significantly higher goblet cell density in the distal intestine (DI), increased neutrophil counts, and upregulation of the pro-interleukin 1 beta (il1b). Conversely, fish fed the AT diet exhibited lower phagocytic activity, lower circularity of head kidney adherent cells, shorter villi, and higher expression of annexin A1 (anxa1) in the DI, in comparison to CT and AR. While diets had no impact on alpha- and beta-diversity of DI bacterial communities, AR-fed fish exhibited a higher relative abundance of Staphylococcus, Enterococcus, Paenibacillus, and Nocardioides. Despite the mild signs of inflammation in the AR-fed fish, no negative effects on fish performance were observed. Nonetheless, long-term exposure studies are required to evaluate any adverse effects in Atlantic salmon. Overall, both microalgae species, sourced through sustainable operations, exhibited significant potential for incorporation as a fishmeal replacement into Atlantic salmon diets, highlighting the value of a circular-economy approach in developing new aquafeed ingredients.