Functional bacterial and archaeal dynamics at slightly acidic conditions revealed by pyrosequencing in a propionate-fed UASB reactor

Abstract

To reveal the response mechanism of syntrophic propionate degrading communities to slightly acidic environments, the performance and microbial community structure at pH 7.0, 6.0 and 5.0 in a propionate-fed upflow anaerobic sludge blanket (UASB) reactor were investigated. The propionate removal and the specific methane production rate were reduced by 33.9%–69.5% and 10.9%–18.9% independently as pH decreased to 5.0 from 7.0. Pyrosequencing based on 16S rRNA gene showed that the microbial diversity in the UASB reactor was increased as pH decreased. Syntrophobacter was the dominant propionate-oxidizing bacteria (POB) at pH 7.0 and its relative abundance reached 23.0%. When the pH decreased from 7.0 to 5.0 in stages, the dominant POB were replaced by Smithella and Pelotomaculum. The hydrogenotrophic and acetotrophic methanogens were dominated by Methanospirillum and Methanosaeta at pH 7.0. pH decrease resulted in the relative abundance of acetotrophic methanogens decreased by 88.6% and hydrogenotrophic methanogens became the major methanogens present. The relative abundance of Methanobacterium was up to 5.0% at pH 5.0. pH decrease broke the normal metabolic network, and then further exacerbated the instability of process. These results reveal the microbial mechanism of process instability caused by pH reduction and provide guidance for the microbial management of anaerobic digesters.