Vertical distribution of microbial community and metabolic pathway in a methanogenic propionate degradation bioreactor

•Syntrophobacter was the main propionate oxidizer in the digester.•The dominant methanogens shifted from Methanoculleus to Methanothrix with depth.•Propionic acid was used for catabolism and anabolism.

The methanogenic propionate degradation consortia were enriched in a propionate-fed semi-continuous bioreactor. The microbial community shift with depth, the microbial network and its correlation with metabolic pathway were also investigated. The results demonstrated that the maximum organic loading rate (OLR) of the reactor was 2.5 g propionic acid (HPr) L−1 d−1 with approximately 1.20 LL−1 d−1 of volumetric methane production (VMP). The organisms in the enrichment were spanning 36 bacterial phyla and 7 archaeal orders. Syntrophobacter, the main Hpr oxidizer in the digester, dominated bacteria with relative abundance changing from 63% to 37% with depth. The predominant methanogens shift from hydrogenotrophic Methanoculleus (∼60%) at the upper liquid layer to acetoclastic Methanothrix (∼51%) at the lower sediment layer in the bioreactor. These methanogens syntrophically support Syntrophobacter by degrading HPr catabolism by-products (H2 and acetate). Other bacteria could scavenge anabolic products (carbohydrate and protein) presumably derived from detrital biomass produced by the HPr-degrading community.