Metatranscriptomic discovery of plant biomass-degrading capacity from grass carp intestinal microbiomes

Despite the economic importance of fish, the ecology and metabolic capacity of fish microbiomes are largely unknown. Here, we sequenced the metatranscriptome of the intestinal microbiota of grass carp, Ctenopharyngodon idellus, a freshwater herbivorous fish species. Our results confirmed previous work describing the bacterial composition of the microbiota at the phylum level as being dominated by Firmicutes, Fusobacteria, Proteobacteria and Bacteriodetes. Comparative transcriptomes of the microbiomes of fish fed with different experimental diets indicated that the bacterial transcriptomes are influenced by host diet. Although hydrolases and cellulosome-based systems predicted to be involved in degradation of the main chain of cellulose, xylan, mannan and pectin were identified, transcripts with glycoside hydrolase modules targeting the side chains of noncellulosic polysaccharides were more abundant. Predominant 'COG' (Clusters of Orthologous Group) categories in the intestinal microbiome included those for energy production and conversion, as well as carbohydrate and amino acid transport and metabolism. These results suggest that the grass carp intestinal microbiome functions in carbohydrate turnover and fermentation, which likely provides energy for both host and microbiota. Grass carp intestinal microbiome thus reflects its evolutionary adaption for harvesting nutrients for an herbivore with a high-throughput nutritional strategy that is not dominated by cellulose digestion but rather the degradation of intracellular polysaccharides.