Dietary polysaccharides: fermentation potentials of a primitive gut ecosystem.

The alimentary canal of the earthworm is representative of primitive gut ecosystems, and gut fermenters capable of degrading ingested biomass-derived polysaccharides might contribute to the environmental impact and survival of this terrestrial invertebrate. Thus, this study evaluated the postulation that gut microbiota of the model earthworm Lumbricus terrestris ferment diverse biomass-derived polysaccharides. Structural polysaccharides (e.g. cellulose, chitin) had marginal impact on fermentation in anoxic gut content treatments. In contrast, nonstructural polysaccharides (e.g. starch, glycogen) greatly stimulated (a) the formation of diverse fermentation products (e.g. H2 , ethanol, fatty acids) and (b) the facultatively fermentative families Aeromonadaceae and Enterobacteriaceae. Despite these contrasting results with different polysaccharides, most saccharides derived from these biopolymers (e.g. glucose, N-acetylglucosamine) greatly stimulated fermentation, yielding 16S rRNA gene-based signatures of Aeromonadaceae-, Enterobacteriaceae- and Fusobacteriaceae-affiliated phylotypes. Roots and litter are dietary substrates of the earthworm, and as proof-of-principle, gut-associated fermenters responded rapidly to root- and litter-derived nutrients including saccharides. These findings suggest that (a) hydrolysis of certain ingested structural polysaccharides may be a limiting factor in the ability of gut fermenters to utilize them and (b) nonstructural polysaccharides of disrupted biomass are subject to rapid fermentation by gut microbes and yield fatty acids that can be utilized by the earthworm.