Revisiting the Caldicellulosiruptor pan-genome: Biodiversity of extremely thermophilic cellulolytic communities from Obsidian Pool YNP

Conversion of plant biomass to usable biofuels or chemicals will require microorganisms that are capable of deconstructing the complex network of polysaccharides that comprise plant cell walls. Interest in the genus Caldicellulosiruptor is due to its ability to deconstruct plant biomass at temperatures above 70°C, via large, modular, multi-functional glycoside hydrolases. Previously, eight members from the genus were sequenced to assess the diversity of the Caldicellulosiruptor pan-genome. Because of the dynamic nature of their habitat, the pan-genome was predicted to be open, indicating yet undiscovered plant biomass degrading enzymes within the genus. Comparative analyses identified genetic determinants for cellulolytic capacity, namely the presence of glycoside hydrolase family 48 domains, carbohydrate binding module family 3 domains, and novel cellulose-binding ‘adhesins’. These markers are now useful tools to screen novel isolates and communities from high temperature biotopes for ligncellulose degraders. Previously, a strongly cellulolytic Caldicellulosiruptor species was isolated from Obsidian Pool (Yellowstone National Park). Given that diverse Caldicellulosiruptor species have been isolated at geothermal pools in New Zealand and Russia, Obsidian Pool was examined with respect to the Caldicellulosiruptor pan-genome. Sixteen sites around the pool were sampled, with enrichment communities from four of those sites exhibiting a cellulolytic phenotype. Ribosomal RNA (16S) phylogeny and metagenomic sequencing of Avicel-enrichments showed that Caldicellulosiruptor were present in these cellulolytic communities, although other genera are present, including novel clostridia. Discussed here will the updated status of the Caldicellulosiruptor pan-genome, based on metagenomics analysis, as well as the structure and features of the enrichment communities identified.