Dietary Biopolymers : Fermentation Potentials of a Primitive Gut Ecosystem

Abstract

Gut microbial communities are of interest because of their importance to animal development and health. The alimentary canal of the earthworm is representative of primitive gut ecosystems, and gut fermenters capable of degrading ingested biomass-derived biopolymers might contribute to the environmental impact and survival of this important terrestrial invertebrate. However, relatively little is known about the capacity of fermentative microbes in the earthworm gut to utilize such biopolymers. Thus, the work described in this dissertation evaluated the hypothesis that the gut microbiota of the model earthworm L. terrestris hydrolyze and ferment diverse dietary plant- and microbial-derived biopolymers. Structural polysaccharides had a marginal impact on the fermentation in anoxic gut content treatments. In marked contrast, nonstructural polysaccharides greatly enhanced the formation of diverse fermentation products and stimulated Aeromonadaceae and Enterobacteriaceae. Although the experimental design required a 1:10 dilution of the gut contents, comparative analysis of the fermentative community in diluted und undiluted gut contents indicated that the dilution did not cause a major disturbance of the system. The disruption of ingested plant and microbial cells by the earthworm gizzard and other lytic events introduces protein and RNA in the anoxic alimentary canal of earthworms. Yeast cell lysate, as proof-of principle, augmented the production of H2, CO2, and diverse fatty acids in anoxic gut content microcosms, indicating that the cell lysate triggered diverse fermentations. Likewise, protein and RNA enhanced fermentations in gut contents and yielded contrasting product profile. The combined product profile of protein and RNA treatments was similar to that of cell lysate treatments, and 16S rRNA gene- and 16S rRNA-based analyses indicated that many taxa that responded to cell lysate were similar to taxa that responded to protein or RNA. To further resolve protein- and RNA-derived fermentations, amino acids and ribose were evaluated as potential drivers of fermentation in gut content of the model earthworm L. terrestris. Of eight amino acids tested, glutamate, aspartate, and threonine were most stimulatory and yielded dissimilar fermentations facilitated by contrasting taxa. Ribose yielded a complex fermentation profile primarily produced by the Aeromonadaceae. Although theoretical considerations suggest that most microbes in the earthworm gut are likely ingested and transient, the non-responsiveness of soil microbes to a specific ‘high quality’ gut substrate and anoxia has made it difficult to demonstrate that responsive gut fermenters are derived from soil. Therefore, soil and gut content of L. terrestris were further examined for their fermentative capabilities. In unsupplemented anoxic treatments, fermentation was negligible with soil but rapid with gut content. However, both soil and gut content facilitated similar fermentations when challenged with complex nutrients, and the responsive fermentative taxa this…