Characterization of a 4-methylbenzoate-degrading methanogenic consortium as determined by small-subunit rDNA sequence analysis

A methanogenic consortium that degrades 4-methylbenzoate (MBA) as the sole carbon and energy source was successfully enriched in an upflow anaerobic s ludge bed bioreactor and studied. Electron microscopic observation showed t hat long rods with a distinct collar feature resembling Desulfomonile tiedj ei rods were the predominant population, and that these rods formed a close spatial orientation with Methanobrevibacter-like bacteria. In addition, th in filaments and bamboo-shaped filaments that highly resembled the acetocla stic Methanosaeta were also frequently observed. A 16S rDNA clone library w as constructed for the domain Bacteria, and 20 sequence types or operationa l taxonomic units (OTUs) were found out of 139 clones screened. Phylogeneti c analysis classified these 20 nearly full-length OTUs into the delta (50.3 % of total clones) and gamma (4.3%) subdivisions of the division Proteobact eria, the green non-sulfur bacteria subdivision I (7.2 %), Cytophagales (7. 2%), Planctomycetes (5.7%), gram-positive low G + C group (8.6%), candidate divisions OP8, OP10 and OP11 (9.3%), and a novel candidate division MBA1 ( 7.2%) that had an interdivisional sequence similarity less than 75%. Howeve r, only 3 OTUs had a sequence similarity higher than 90% to known isolates or environmental 16S rDNA clones, suggesting that the microbial community w as diversified and largely unidentified. In particular, those 8 OTUs found in the delta-Proteobacteria were either clustered into novel groups or show ed a low sequence similarity to closely related bacteria. It is highly poss ible that the delta-Proteobacteria were the long rods with a distinct colla r feature observed microscopically, and together with the methanogens were mainly responsible for the syntrophic degradation of MBA. The unique and no vel microbial populations identified explained the requirement of a long st art-up period of up to 426 d for the MBA-degrading consortium.