Identification and quantification of key microbial trophic groups of methanogenic glucose degradation in an anaerobic digester sludge

► Key trophic groups in anaerobic glucose degradation were identified and quantified. ► Population size of propionate degraders was the smallest among the trophic groups. ► Population size of acetate degraders was comparable to the glucose degraders. ► The specific degradation rate of acetate was low (1/28 of that of glucose). ► We explained why the degradation of propionate and acetate was rate-limiting. We investigated the major phylogenetic groups and population size of glucose-, propionate-, and acetate-degrading bacteria in the glucose-degrading anaerobic digester sludge by stable-isotope probing analysis of 16S rRNA (RNA-SIP) with [13C6]glucose followed by time course analysis of microautoradiography combined with fluorescent in situ hybridization (MAR-FISH) with [U-14C]glucose. The results indicated that glucose was predominately degraded to CH4 and CO2 by glucose-degrading Propionibacterium and Olsenella that are belonging to the phylum Actinobacteria, propionate-degrading Smithella and Syntrophobacter, and acetate-degrading Methanosaeta and Synergistes group 4 in this anaerobic sludge. The population size of propionate degraders was the smallest among three trophic groups and the specific degradation rate of propionate was also low. The specific degradation rate of acetate was low even though their population size was comparable to the glucose degraders. These results could explain why the degradation of propionate and acetate was the rate-limiting step in methanogenic glucose degradation.