Insights into Ammonia Adaptation and Methanogenic Precursor Oxidation by Genome-Centric Analysis

Ammonia
released from the degradation of protein and/or urea usually leads
to suboptimal anaerobic digestion (AD) when N-rich organic waste is
used. However, the insights behind the differential ammonia tolerance
of anaerobic microbiomes remain an enigma. In this study, the cultivation
in synthetic medium with different carbon sources (acetate, methanol,
formate, and H2/CO2) shaped a common initial
inoculum into four unique ammonia-tolerant syntrophic populations.
Specifically, various levels of ammonia tolerance were observed: consortia
fed with methanol and H2/CO2 could grow at ammonia
levels up to 7.25 g NH+-N/L, whereas the other two groups
(formate and acetate) only thrived at 5.25 and 4.25 g NH+-N/L, respectively. Metabolic reconstruction highlighted that this
divergent microbiome might be achieved by complementary metabolisms
to maximize biomethane recovery from carbon sources, thus indicating
the importance of the syntrophic community in the AD of N-rich substrates.
Besides, sodium/proton antiporter operon, osmoprotectant/K+ regulator, and osmoprotectant synthesis operon may function as the
main drivers of adaptation to the ammonia stress. Moreover, energy
from the substrate-level phosphorylation and multiple energy-converting
hydrogenases (e.g., Ech and Eha) could aid methanogens
to balance the energy request for anabolic activities and contribute
to thriving when exposed to high ammonia levels.