| Summary: | This work aimed to assess the suitability of a microalgal blend as a dietary ingredient for feeding juveniles of marine carnivorous and herbivorous teleost, as is the case of <i>Sparus aurata</i> and <i>Mugil cephalus</i>, respectively, and to isolate microorganisms from different media and characterize them on the base of their enzymatic activities and their antagonism against important fish pathogens. Thirty juveniles of each species (70 ± 3.2 g <i>S. aurata</i> mean weight and 47 ± 2.8 g <i>M. cephalus</i> mean weight) were distributed in four tanks (15 individuals each) corresponding to four independent dietary treatments (control and microalgae diets designed for each species). Fish were fed their corresponding diets ad libitum for 108 days. At the end of the trial, fish were weighed, and plasma, liver, perivisceral fat, and the entire intestines were obtained for the evaluation of growth performance and metabolic assessment. Furthermore, 117 bacterial strains were isolated in different culture media from the gastrointestinal tract of <i>S. aurata</i> fed the microalgae blend and further characterized for their potential use as probiotics in aquaculture. <i>S. aurata</i> fed the microalgae-supplemented diet (25% dietary inclusion) showed a significant increase in weight gain, specific growth rate, feed efficiency, hepatosomatic, and intestine length indices. However, growth performance and somatic indices in <i>M. cephalus</i> were not affected by the experimental diets. Plasma samples from <i>S. aurata</i> fed the microalgal diet revealed higher levels of glucose and triglycerides and a decrease in cortisol levels. No significant differences were found in any biochemical parameters among the experimental diets in <i>M. cephalus</i>. In conclusion, both species demonstrated a favorable adaptation to the nutritional formulation employed in this study, and bacterial strains UMA-169 and UMA-216 (both identified as <i>Bacillus pumilus</i>) could be considered for use in aquaculture as they might benefit host health by improving digestion and absorption of different energy sources and by minimizing the colonization of pathogenic species.
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