| Summary: | Bacterial hydrolysis of polysaccharides is an important step for the production of sustainable energy, for example during the conversion of plant biomass to methane-rich biogas. Previously, <i>Hungateiclostridium thermocellum</i> was identified as cellulolytic key player in thermophilic biogas microbiomes with a great frequency as an accompanying organism. The aim of this study was to physiologically characterize a recently isolated co-culture of <i>H. thermocellum</i> and the saccharolytic bacterium <i>Defluviitalea raffinosedens</i> from a laboratory-scale biogas fermenter. The characterization focused on cellulose breakdown by applying the measurement of cellulose hydrolysis, production of metabolites, and the activity of secreted enzymes. Substrate degradation and the production of volatile metabolites was considerably enhanced when both organisms acted synergistically. The metabolic properties of <i>H. thermocellum</i> have been studied well in the past. To predict the role of <i>D. raffinosedens</i> in this bacterial duet, the genome of <i>D. raffinosedens</i> was sequenced for the first time. Concomitantly, to deduce the prevalence of <i>D. raffinosedens</i> in anaerobic digestion, taxonomic composition and transcriptional activity of different biogas microbiomes were analyzed in detail. <i>Defluviitalea</i> was abundant and metabolically active in reactor operating at highly efficient process conditions, supporting the importance of this organism for the hydrolysis of the raw substrate.
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