Ammonia inhibition of anaerobic volatile fatty acid degrading microbial communities

Ammonia inhibition is an important reason for reactor failures and economic losses in anaerobic digestion. Its impact on propionic and butyric acid degradation has received little attention compared to acetic acid degradation and is even neglected in the Anaerobic Digestion Model No. 1 (ADM1). To compare ammonia inhibition of the degradation of these three volatile fatty acids (VFAs), we fed a mixture of them as sole carbon source to three continuous stirred tank reactors and increased ammonium bicarbonate concentrations in the influent from 52 mM to 277 mM. While butyric acid degradation was hardly affected by the increase in ammonia concentration, propionic acid degradation turned out to be even more inhibited than acetic acid degradation as opposed to the roughly sustained degradation in a control reactor without ammonia inhibition. The inhibited reactors acclimatized and approximated pre-disturbance degradation efficiencies towards the end of the experiment, which was accompanied by strong microbial community shifts, as analyzed by amplicon sequencing of 16S rRNA genes and terminal restriction fragment length polymorphism of mcrA genes. The acetoclastic methanogen Methanosaeta was completely replaced by Methanosarcina. The propionic acid degrading genus Syntrophobacter was replaced by yet unknown propionic acid degraders. The butyric acid degrading genus Syntrophomonas and hydrogenotrophic Methanomicrobiaceae were hardly affected. We hypothesized that the initial dominance of the ammonia sensitive taxa Methanosaeta and Syntrophobacter led to a stronger inhibition of the acetic and propionic acid degrading populations compared to butyric acid degrading and hydrogenotrophic populations, which were dominated by ammonia tolerant taxa Syntrophomonas and Methanomicrobiaceae.