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Anaerobic digestion (AD) is an important technology for the effective conversion of waste and wastewater to methane. Here, syntrophic bacteria transfer molecular hydrogen (H2), formate, or directly supply electrons (direct interspecies electron transfer, DIET) to the methanogens. Evidence is accumulating that the methanation of short-chain fatty acids can be enhanced by the addition of conductive material to the anaerobic digester, which has often been attributed to the stimulation of DIET. Since little is known about the transcriptional response of a complex AD microbial community to the addition of conductive material, we added magnetite to propionate-fed laboratory-scale reactors that were inoculated with wastewater sludge. Compared to the control reactors, the magnetite-amended reactors showed improved methanation of propionate. A genome-centric metatranscriptomics approach identified the active SCFA-oxidizing bacteria that affiliated with Firmicutes, Desulfobacterota and Cloacimonadota. The transcriptional profiles revealed that the syntrophic bacteria transferred acetate, H2 and formate to acetoclastic and hydrogenotrophic methanogens, whereas transcription of potential determinants for DIET such as conductive pili and outer-membrane cytochromes did not significantly change with magnetite addition. Overall, changes in the transcriptional profiles of syntrophic Bacteria and Archaea in propionate-fed lab-scale reactors amended with magnetite refute a major role of DIET in the studied system.
Die mikrobielle Umsetzung von organischem Material zu dem erneuerbaren Energieträger Methan ist eine bewährte und verbreitete Strategie der effektiven Abfallwirtschaft. Dennoch können in Biogasanlagen Prozessstörungen und im Extremfall Prozessausfälle auftreten und erhebliche Kosten verursachen. Forschende der Hochschule Emden/Leer untersuchen die Mikroorganismen und ihr Zusammenleben, um die Prozessstabilität und Produktivität zu steigern.
Background: Anaerobic digestion (AD) is a globally important technology for effective waste and wastewater management. In AD, microorganisms interact in a complex food web for the production of biogas. Here, acetoclastic methanogens and syntrophic acetate-oxidizing bacteria (SAOB) compete for acetate, a major intermediate in the mineralization of organic matter. Although evidence is emerging that syntrophic acetate oxidation is an important pathway for methane production, knowledge about the SAOB is still very limited.
Results: A metabolic reconstruction of metagenome-assembled genomes (MAGs) from a thermophilic solid state biowaste digester covered the basic functions of the biogas microbial community. Firmicutes was the most abundant phylum in the metagenome (53%) harboring species that take place in various functions ranging from the hydrolysis of polymers to syntrophic acetate oxidation. The Wood-Ljungdahl pathway for syntrophic acetate oxidation and corresponding genes for energy conservation were identified in a Dethiobacteraceae MAG that is phylogenetically related to known SAOB. 16S rRNA gene amplicon sequencing and enrichment cultivation consistently identified the uncultured Dethiobacteraceae together with Syntrophaceticus, Tepidanaerobacter, and unclassified Clostridia as members of a potential acetate-oxidizing core community in nine full-scare digesters, whereas acetoclastic methanogens were barely detected.
Conclusions: Results presented here provide new insights into a remarkable anaerobic digestion ecosystem where acetate catabolism is mainly realized by Bacteria. Metagenomics and enrichment cultivation revealed a core community of diverse and novel uncultured acetate oxidizing bacteria and point to a particular niche for them in dry fermentation of biowaste. Their genomic repertoire suggests metabolic plasticity besides the potential for syntrophic acetate oxidation.
Propionate is an important intermediate in the anaerobic mineralization of organic matter. In methanogenic environments, its degradation relies on syntrophic associations between syntrophic propionate-oxidizing bacteria (SPOB) and Archaea. However, only 10 isolated species have been identified as SPOB so far. We report syntrophic propionate oxidation in thermophilic enrichments of Candidatus Syntrophosphaera thermopropionivorans, a novel representative of the candidate phylum Cloacimonetes. In enrichment culture, methane was produced from propionate, while Ca. S. thermopropionivorans contributed 63% to total bacterial cells. The draft genome of Ca. S. thermopropionivorans encodes genes for propionate oxidation via methymalonyl-CoA. Phylogenetically, Ca. S. thermopropionivorans affiliates with the uncultured Cloacimonadaceae W5 and is more distantly related (86.4% 16S rRNA gene identity) to Ca. Cloacimonas acidaminovorans. Although Ca. S. thermopropionivorans was enriched from a thermophilic biogas reactor, Ca. Syntrophosphaera was in particular associated with mesophilic anaerobic digestion systems. 16S rRNA gene amplicon sequencng and a novel genus-specific quantitative PCR assay consistently identified Ca. Syntrophosphaera/Cloacimonadaceae W5 in 9 of 12 tested full-scale biogas reactors thereby outnumbering other SPOB such as Pelotomaculum, Smithella and Syntrophobacter. Taken together the ubiquity and abundance of Ca. Syntrophosphaera, those SPOB might be key players for syntrophic propionate metabolism that have been overlooked before.