Efficient ammonia oxidation by Pseudomonas citronellolis strain YN-21 under strongly acidic conditions: Performance and mechanism.

Ammonia oxidation microorganisms generally tend to have low rates of ammonia oxidation under acidic conditions, as the protonated ammonia is not a substrate for ammonia monooxygenase. In this work, heterotrophic ammonia oxidation bacteria (HAOB) Pseudomonas citronellolis strain YN-21 showed high efficiency in removing NH (12.7 mg/L/h) even at initial pH 4.5. The potential acid resistance mechanisms (H efflux, H consumption, and production of alkaline substances) maintained intracellular pH neutrality. Transcriptome analysis showed that genes involved in amino acid metabolism, carbohydrate metabolism, ABC transporter and nitrogen metabolism were significantly up-regulated, which facilitated the rapid removal of NH in an acidic environment. Moreover, urea could be used as an alternative nitrogen source for YN-21 in a strongly acidic environment, and the production of NH from urea hydrolysis provided a substrate for ammonia oxidation. These results provide new insights into efficient ammonia oxidation in acidic environments.