Spatial Distribution and Diverse Metabolic Functions of Lignocellulose-Degrading Uncultured Bacteria as Revealed by Genome-Centric Metagenomics

The mechanisms by which specific anaerobic microorganisms remain firmly attached to lignocellulosic material, allowing them to efficiently decompose organic matter, have yet to be elucidated. To circumvent this issue, microbiomes collected from anaerobic digesters treating pig manure and meadow grass were fractionated to separate the planktonic microbes from those adhered to lignocellulosic substrate. Assembly of shotgun reads, followed by a binning process, recovered 151 population genomes, 80 out of which were completely new and were not previously deposited in any database. Genome coverage allowed the identification of microbial spatial distribution in the engineered ecosystem. Moreover, a composite bioinformatic analysis using multiple databases for functional annotation revealed that uncultured members of the Bacteroidetes and Firmicutes follow diverse metabolic strategies for polysaccharide degradation. The structure of cellulosome in Firmicutes species can differ depending on the number and functional roles of carbohydrate-binding modules. In contrast, members of the Bacteroidetes are able to adhere to and degrade lignocellulose due to the presence of multiple carbohydrate-binding family 6 modules in beta-xylosidase and endoglucanase proteins or S-layer homology modules in unknown proteins. This study combines the concept of variability in spatial distribution with genome-centric metagenomics, allowing a functional and taxonomical exploration of the biogas microbiome.IMPORTANCE This work contributes new knowledge about lignocellulose degradation in engineered ecosystems. Specifically, the combination of the spatial distribution of uncultured microbes with genome-centric metagenomics provides novel insights into the metabolic properties of planktonic and firmly attached to plant biomass bacteria. Moreover, the knowledge obtained in this study enabled us to understand the diverse metabolic strategies for polysaccharide degradation in different species of Bacteroidetes and Clostridiales. Even though structural elements of cellulosome were restricted to Clostridiales species, our study identified a putative mechanism in Bacteroidetes species for biomass decomposition, which is based on a gene cluster responsible for cellulose degradation, disaccharide cleavage to glucose, and transport to cytoplasm.