Unveiling the ecology, taxonomy and metabolic capabilities of MBA03, a potential key player in anaerobic digestion

Abstract

AbstractBiogas, a mix of CO2, CH4and small proportions of other gases, is a biofuel obtained by anaerobic digestion (AD). Biogas production is often considered a black box process, as the role and dynamics of some of the microorganisms involved remain undisclosed. Previous metataxonomic studies in the frame of the MICRO4BIOGAS project (www.micro4biogas.eu) revealed that MBA03, an uncharacterised and uncultured bacterial taxon, was very prevalent and abundant in industrial full-scale AD plants. Surprisingly, no culturable specimen or genome of this taxon has ever been reported, so its role in AD has remained unclear. In the present work, thirty samples derived from anaerobic digesters were sequenced, allowing the reconstruction of 108 metagenome-assembled genomes (MAGs) potentially belonging to MBA03. According to phylogenetic analyses and genomic similarity indices, MBA03 constitutes a new bacterial order, proposed asDarwinibacterialesord. nov., which includesDarwinibacter acetoxidansgen. nov., sp. nov. of the familyDarwinibacteriaceaefam. nov., along withWallacebacter crypticagen. nov., sp. nov. of theWallacebacteriaceaefam. nov. Ecotaxonomic studies determined that AD processes are the main ecological niche ofDarwinibacteriales. Moreover, metabolic predictions identifiedDarwinibacteraceaemembers as putative syntrophic acetate oxidising bacteria (SAOB), as they encode for the reversed Wood-Ljungdahl (W-L) pathway coupled to the glycine cleavage system. This suggests thatDarwinibacteraceaemembers work in collaboration with hydrogenotrophic archaea to produce methane in industrial biogas plants. Overall, our findings presentDarwinibacterialesas a potential key player in anaerobic digestion and pave the way towards the complete characterisation of this newly described bacterial taxa.