Dietary polysaccharides: fermentation potentials of a primitive gut ecosystem

Abstract

Summary 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.

Amino Acids and Ribose: Drivers of Protein and RNA Fermentation by Ingested Bacteria of a Primitive Gut Ecosystem

“…In the present study, numerous responsive phylotypes, including five group phylotypes (GPT), were also affiliated to these families (Fig. 6; see also Table S10 in the supplemental material) (note, a group phylotype consists of identical or nearly identical phylotypes based on sequence similarity [8]). The relatively short read lengths generated by Illumina sequencing can compromise the taxonomic assignment of sequences at the species level (72, 73), and Illumina phylotype assessments should be qualified accordingly.…”

“…Phylotypes are based on a sequence similarity cutoff of 97% and were considered stimulated when a phylotype in at least one of the supplemented treatments displayed a ≥2% net increase in relative abundance. Net increases of relative abundances were calculated as follows (8): (i) the calculation is based either on mean relative abundances when samples from the three replicates were analyzed separately (i.e., all RNA and DNA samples of control treatments and RNA samples at 30 h of supplemented treatments) or on single relative abundances when samples of the three replicates were pooled for sequence analyses (i.e., DNA samples at 0 h and 30 h and RNA samples at 0 h of supplemented treatments); (ii) mean or single relative abundances at the beginning of incubation were subtracted from those at the end of incubation for control and supplemented treatments; (iii) the resulting time-corrected relative abundances of control treatments were subtracted from those of supplemented treatments (negative time-corrected relative abundances of control treatments were ignored). CAA, Casamino Acids; Glu, glutamate; Asp, aspartate; Thr, threonine; Ala, alanine; Gly, glycine; Val, valine; S, succinate; F, formate; G, glucose.…”

“…Because responsive gut fermenters are dominated by ingested transients (24), the potential for a fermentative response is dependent on the occurrence of a given fermentative phylotype in preingested soil. For example, the Fusobacteriaceae in gut content is responsive in some cases but in other cases is essentially nondetectable, thus reflecting the variable detectability of this family in the soil on which an earthworm is maintained (8, 24, 40). In the current study, the Fusobacteriaceae were represented by GPT-5 which was responsive to amino acids (Fig.…”

“…In this regard, microbe- and plant-derived organic carbon rapidly stimulate fermentation by gut bacteria (8, 40), and dietary polymers that are more easily hydrolyzed are likely primary sources of fermentable organic carbon. For example, structural polysaccharides that are difficult to hydrolyze (e.g., cellulose and chitin) are not readily utilized for gut content fermentation, whereas protein and nonstructural polysaccharides (e.g., starch and glycogen) are easily hydrolyzed and rapidly fermented by the gut community (8, 40). Thus, the fermentative capacity of the gut community to convert ingested biomass to products that can be utilized by the earthworm can contribute to the sustenance of the animal host.…”

“…Fossil evidence suggests that ancestors of earthworms (annelids) and other worm-like animals existed 0.5-to-1.1 billion years ago (1, 2), making these invertebrates evolutionarily among the oldest known animal lineages (e.g., winged insects, termites, and ruminants date back approximately 300, 150, and 50 million years ago, respectively [3–5]). Earthworms are mostly unseen in the environment, but these primitive subsurface animals can reach enormous densities and have profound impact on the cycling of matter in the terrestrial biosphere, an impact linked to the ingestion of soil, plant material, and associated microorganisms (6–8). Although earthworms have positive effects on soil fertility and are of value for vermicomposting environmental wastes (9–14), their invasiveness may have negative consequences (15, 16).…”