Influence of the biofloc bacterial community on the digestive activity of Nile tilapia (Oreochromis niloticus)

High-throughput sequencing was performed to study the diversity of bacterial communities in the water of an aquaculture production system using Biofloc technology (BFT) and its effect on the activity of digestive enzymes in Nile tilapia, Oreochromis niloticus. Sex-reversed juveniles were reared in six circular indoor tanks (1 m³) for 12 weeks. The following treatments were performed: biofloc with 0% protein restriction in the diet (BFTH = high protein diet); biofloc with 11% dietary protein restriction (BFTL = low protein diet), and clear water control with 0% dietary protein restriction (CW). Biofloc groups showed a higher bacterial diversity and a completely different composition compared to CW, with significant increase of Actinobacteria and Planctomycetes and a decrease in Verrumicrobia and Fusobacteria. At the genus level, Flavobacterium, Thermomonas, Comamonas, Microbacterium, and Intestinibacter present in BFT significantly correlated with the activity of trypsin, aminopeptidase, and alkaline proteases. Twenty percent of the difference between the biofloc groups was due to the higher activity of acid proteases and aminopeptidases. The metagenome prediction revealed metabolic pathways differentially expressed in BFTH and BFTL. Most were similarly expressed in both treatments; except for the CoA degradation pathway I, which was expressed in BFTL, and the biosynthesis of isoprene II and mevalonate II in BFTH. The biofloc microbiota provided enzymes that, in synergy with the endogenous digestive enzymes of the fish, improved digestion and nutrient assimilation. The study provides knowledge on the exogenous enzymatic contribution and diversity of the bacterial groups present in the biofloc; the contribution of these microorganisms in digestive metabolism and fish performance is also highlighted.