バイオガス生産に関わる新しい微生物

Researching sustainable production of biogas with optimised microbial communities.

Biogas, a mix of CO2, CH4 and 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 as Darwinibacteriales ord. nov., which includes Darwinibacter acetoxidans gen. nov., sp. nov. of the family Darwinibacteriaceae fam. nov., along with Wallacebacter cryptica gen. nov., sp. nov. of the Wallacebacteriaceae fam. nov. Ecotaxonomic studies determined that AD processes are the main ecological niche of Darwinibacteriales. Moreover, metabolic predictions identified Darwinibacteraceae members as putative syntrophic acetate oxidising bacteria (SAOB), as they encode for the reversed Wood-Ljungdahl pathway coupled to the glycine cleavage system. This suggests that Darwinibacteraceae members work in collaboration with hydrogenotrophic archaea to produce methane in industrial biogas plants. Overall, our findings present Darwinibacteriales as a potential key player in anaerobic digestion and pave the way towards the complete characterisation of this newly described bacterial taxa. ### Competing Interest Statement The authors have declared no competing interest.

Abstract: This study represents one of the most comprehensive characterisations of the anaerobic digestion (AD) microbiome with 80 samples from 45 different large-scale reactors in three countries. Technical, chemical and taxonomic data was thoroughly collected, analyzed and correlated to identify the main drivers of AD processes. Our results showed that MBA03, Proteiniphilum, a member of Dethiobacteraceae, and Caldicoprobacter were present in all the samples, while Methanosarcina was the most abundant and prevalent archaea. Two distinct bacterial clusters were identified by correlating microbial abundances. One was correlated with hydrogenotrophic and the other with acetoclastic methanogenesis. Organic acids, ammonia, nitrogen, COD and the trace elements Fe, Mo, and the macronutrient P had the greatest impact on AD microbiomes. Temperature, reactor type and substrate also influenced the formation of a specialized microbial community. Overall, this work sheds light on the microbial key players involved in AD and evaluates how they are affected by technical and chemical parameters. ### Competing Interest Statement The authors have declared no competing interest.

Center of a biogas plant, aerial view (top-down) – Download this photo by Julia Koblitz on Unsplash