The importance of syntrophic acetate oxidation for process stability in methanogenic systems operating at high ammonia concentrations
has previously been emphasized. In this study we investigated bioaugmentation of syntrophic acetate-oxidizing
(SAO) cultures as a possible method for decreasing the adaptation period of biogas reactors operating at gradually increased ammonia
concentrations (1.5 to 11 g NH4
-N/liter). Whole stillage and cattle manure were codigested semicontinuously for about
460 days in four mesophilic anaerobic laboratory-scale reactors, and a fixed volume of SAO culture was added daily to two of the
reactors. Reactor performance was evaluated in terms of biogas productivity, methane content, pH, alkalinity, and volatile fatty
acid (VFA) content. The decomposition pathway of acetate was analyzed by isotopic tracer experiments, and population dynamics
were monitored by quantitative PCR analyses. A shift in dominance from aceticlastic methanogenesis to SAO occurred simultaneously
in all reactors, indicating no influence by bioaugmentation on the prevailing pathway. Higher abundances of Clostridium
ultunense and Tepidanaerobacter acetatoxydans were associated with bioaugmentation, but no influence on
Syntrophaceticus schinkii or the methanogenic population was distinguished. Overloading or accumulation of VFA did not
cause notable dynamic effects on the population. Instead, the ammonia concentration had a substantial impact on the abundance
level of the microorganisms surveyed. The addition of SAO culture did not affect process performance or stability against
ammonia inhibition, and all four reactors deteriorated at high ammonia concentrations. Consequently, these findings further
demonstrate the strong influence of ammonia on the methane-producing consortia and on the representative methanization
pathway in mesophilic biogas reactors.