The inclusion of microalgae in the diet of farmed fish such as sea bream (Sparus aurata) is proposed as a sustainable alternative to the traditional diet with fishmeal, since numerous benefits have been reported to the fish. The gills act as the first defence barrier against pathogenic organisms and have an autochthonous microbiota that contributes to this function, in the fact that it is important to know if diets supplemented with microalgae can cause any alteration. In this work, three diets (control, raw Nannochloropsis gaditana
seaweed and hydrolyzed seaweed) were fed to juvenile sea bream for 90 days, later they were sacrificed and their organs frozen. 16S
rRNA was sequenced on an Illumina® platform and bioinformatic analysis with MOTHUR was performed. A significant increase in
the richness index (Chao1) was shown in the microbiota of the gills of the fish fed N. gaditana. The most significant changes occurred
in the fish fed the raw algae, with a notable increase in the genus Shewanella in this group.