Defatted microalgae (Nannochloropsis sp.) from biorefinery as a potential feed protein source to replace fishmeal in European sea bass diets.

The present work focuses on the use of defatted biomass of the microalga Nannochloropsis sp. from the biodiesel industry, as a partial substitute of fish meal (FM) in diets for European sea bass. The effects of increasing inclusion levels of microalgal meal on growth performance, body composition, nutrient utilization, gut morphology, and innate immunity were evaluated after 93 days. A reference alga-free diet was the control (CTRL) diet, and the three experimental diets contained 5 (MA5), 10 (MA10), and 15% (MA15) of the microalgal meal. The microalga-rich diets were supplemented with DL-methionine to assure sea bass dietary requirement. Overall, nutrient apparent digestibilities (ADCs) of the diets were not altered by the microalgal inclusion, but energy ADC was highest in fish fed the CTRL diet (90% vs 88%). At the end of the trial, fish growth performance was similar among dietary treatments (DGI of 1.0), but fish fed MA10 had a significantly higher feed conversion ratio than those fed CTRL and MA5. Final whole body composition and nutrient gain of fish fed the different diets were similar. No significant differences were detected in gut morphology among treatments. Innate immune parameters (lysozyme, alternative complement pathway-ACH50, and peroxidase) were examined, and ACH50 of the fish fed MA15 was significantly lower than those fed MA10, suggesting a dose-dependent stimulation of the innate immune response. The present results indicate that defatted microalgal meal can replace fishmeal in European sea bass diets-at inclusion levels of up to 15%-contributing to a circular economy approach.