Ammonia Stress on Anaerobic Digestion: The More Sensitive Propionic/Butyric Acid-Degrading Bacteria over Methanogens

Nitrogenous organic matter could be converted into ammonia nitrogen during anaerobic digestion (AD) which could lead to the inhibition of methane production. Therefore, it is crucial to investigate the effect of ammonia stress on AD. In this study, the addition of ammonia nitrogen concentration of 1.5 g N/L led to an obvious extended lag-period time of the AD with propionic acid or n-butyric acid as the sole substrate, while it showed no effect on the AD of acetic acid, indicating that the biotransformation of propionic acid and butyric acid might be more susceptible under low inhibitory ammonia stress. Analysis of the microbial consortia revealed that the community composition of bacteria and archaea was not obviously affected by a low inhibitory ammonia stress. Metatranscriptomic analysis showed that the enzymes related to the biotransformation of propionic and butyric acids were more susceptible to ammonia toxicity than those corresponding to acetoclastic methanogenesis. During protein synthesis, the expression of RNA polymerase and the subunits of ribosomes, which affected the transcription process, were significantly down-regulated. Additionally, ammonia toxicity caused DNA damage in microorganisms and triggered the SOS response of microorganisms. This study provided new insights for further understanding of the mechanism of ammonia stress at a low inhibitory level.