Involvement of a novel fermentative bacterium in acidification in a thermophilic anaerobic digester

Acidification results from the excessive accumulation of volatile fatty acids and the breakthrough of buffering capacity in anaerobic digesters. However, little is known about the identity of the acidogenic bacteria involved. Here, we identified an active fermentative bacterium during acidification in a thermophilic anaerobic digester by sequencing and phylogenetic analysis of isotopically labeled rRNA. The digestion sludge retrieved from the beginning of pH drop in the laboratory-scale anaerobic digester was incubated anaerobically at 55 °C for 4 h during which ¹³C-labeled glucose was supplemented repeatedly. ¹³CH₄ and ¹³CO₂ were produced after substrate addition. RNA extracts from the incubated sludge was density-separated by ultracentrifugation, and then bacterial communities in the density fractions were screened by terminal restriction fragment length polymorphism and clone library analyses based on 16S rRNA transcripts. Remarkably, a novel lineage within the genus Thermoanaerobacterium became abundant with increasing the buoyant density and predominated in the heaviest fraction of RNA. The results in this study indicate that a thermoacidophilic bacterium exclusively fermented the simple carbohydrate glucose, thereby playing key roles in acidification in the thermophilic anaerobic digester.