Amino Acids and Ribose: Drivers of Protein and RNA Fermentation by Earthworm Gut Bacteria.

Earthworms are important to plant growth and the turnover of organic matter in the terrestrial biosphere. These invertebrates ingest microbial cells that are subject to disruption by the gizzard. Protein and RNA are the dominate polymers of disrupted microbes, and these biopolymers facilitate robust fermentations by gut bacteria of the model earthworm Lumbricus terrestris. To further resolve these fermentations, various amino acids and ribose (as fermentable hydrolysis-derived substrates from protein and RNA, respectively) were evaluated as potential drivers of gut content fermentation. Of eight amino acids tested, glutamate, aspartate, and threonine were most stimulatory and yielded dissimilar fermentations facilitated by contrasting taxa (e.g., glutamate yielded H2 and formate, and a stimulation of Fusobacteriaceae, whereas aspartate yielded succinate and propionate, and a stimulation of Aeromonadaceae). Only a marginal potential for Strickland fermentation was observed in co-amino acid treatments (e.g., glycine plus alanine). Ribose yielded a complex fermentation profile facilitated primarily by the Aeromonadaceae. The consumption of transiently produced succinate was linked to its decarboxylation to propionate and the stimulation of Fusobacteriaceae, and the consumption of transiently produced formate was linked to its conversion to H2 and CO2 and the stimulation of Peptostreptococcaceae. These findings (i) demonstrate that specific amino acids, ribose, and transient intermediates stimulate diverse fermenters in gut content and (ii) reinforce the likelihood that the earthworm and fermentative bacteria compete for the products of protein and RNA hydrolysis in the alimentary canal.