Effect of ammonium and acetate on methanogenic pathway and methanogenic community composition

Methanogenesis from acetate (aceticlastic methanogenesis or syntrophic acetate oxidation (SAO) coupled with hydrogenotrophic methanogenesis) is the most important step for the biogas process. The major environmental factors influencing methanogenesis are volatile fatty acids, ammonia, pH, and temperature. In our study, the effect of acetate and ammonia concentration on the methanogenic pathway from acetate and on the methanogenic communities was elucidated in two experiments: one where inocula were gradually exposed to increasing concentrations of acetate or ammonia, and another with direct exposure to different ammonia concentrations. The methanogenic pathway was determined by following the production of (14) CH(4) and (14) CO(2) from acetate labeled in the methyl group (C-2). Microbial communities' composition was determined by fluorescence in situ hybridization. Upon acclimatization to acetate and ammonia, thermophilic cultures clearly shifted their acetate bioconversion pathway from SAO with subsequent hydrogenotrophic methanogenesis (mediated by Methanobacteriales spp. and/or Methanomicrobiales spp.) to aceticlastic methanogenesis (mediated by Methanosarcinaceae spp.). On the contrary, acclimatization process resulted in no pathway shift with the mesophilic acclimatized culture. When nonacclimatized thermophilic culture was exposed to high ammonia levels (7 g NH4 +-N L(-1)), aceticlastic Methanosarcinaceae spp. was found to be the dominant methanogen.