The effect of plant origin bioflavonoid-containing diet and season on gut microbiota composition in rainbow trout (Oncorhynchus mykiss).

This study aimed to investigate the effect of seasonal (sampling date) changes (from May to November) with a dietary supplement of dihydroquercetin (DHQ), a bioflavonoid of plant origin, on the composition of the gastrointestinal tract (GIT) microbiota of rainbow trout (Oncorhynchus mykiss). At the beginning of the study, fish had an average weight of 546 ± 25.8 g and later at the end of the study the weight had changed significantly in the control group (2071.6 ± 84.7 g) and in the experimental group (1909.9 ± 101.1 g) that received a diet supplemented with DHQ for 6-months. Amplicon sequencing targeted to the V3-V4 region of 16S rRNA gene fragments was used for the taxonomic identification of the bacterial composition. Firmicutes (54.6 ± 3.3%), Proteobacteria (32.6 ± 3.4%), Fusobacteriota (6.0 ± 1.6%), and Bacteroidota (5.0 ± 0.9%) were the major phyla in the fish GIT of both control and experimental groups. According to the ADONIS test (weighted and unweighted UniFrac distances), significant effects of sampling date and section of digestive tract have been shown, indicating that both of these factors are more relevant than DHQ supplementation. The functional composition associated with Mycoplasma in the gut remains less variable across sampling dates, suggesting that this dominant group acts as a stable core, resistant to environmental fluctuations in the water body. Dietary DHQ selectively affected total composition of the gastrointestinal microbiota in rainbow trout. This should be taken into account in the long-term assessment of the predictive role of bacterial communities and the effective search for new feed additives in order to obtain high-quality aquaculture products.