Effect of glycerol feed-supplementation on seabass metabolism and gut microbiota

Dietary glycerol supplementation in aquaculture feed is seen as an alternative and inexpensive way to  fuel fish metabolism, attenuate metabolic utilization of dietary proteins and, subsequently reduce nitrogen  excretion. In this study, we evaluated the impact of dietary glycerol supplementation has on nitrogen excretion of European seabass (Dicentrarchus labrax) and its effects on metabolite profile and bacterial community composition of gut digesta. These effects were evaluated in a 60-day trial with fish fed diets supplemented with 0, 2.5 or 5% (w/w) refined glycerol. Nuclear magnetic resonance spectroscopy and high-throughput 16S rRNA sequencing characterized the effects of glycerol supplementation of digesta metabolite and bacterial community composition of 6h postprandial fish. Our results showed ammonia excretion was not altered by dietary glycerol supplementation and the highest glycerol dosage was associated with significant increases in amino acids and a significant decrease of ergogenic creatine in digesta metabolome. Concomitantly, significant decreases in putative amino acid degradation pathways were detected in the predicted metagenome analysis, suggesting a metabolic shift. Taxon-specific analysis  revealed significant increases in abundance of some specific genera (e.g. Burkholderia and Vibrio) and bacterial diversity. Overall, our results indicate glycerol supplementation may decrease amino acid catabolism without adversely affecting fish gut bacterial communities.