Insights into the physiology of ammonia-oxidizing microorganisms

Nitrification is the aerobic process of the nitrogen cycle that converts ammonia to nitrate and is facilitated by ammonia-oxidizing and nitrite-oxidizing microorganisms. Ammonia-oxidizers are unique chemolithotrophs that evolved specialized networks of electron carriers to generate proton motive force using ammonia as a sole energy source as well as mechanisms to tolerate cytotoxic intermediates of their metabolism. Cultivation and genome sequencing of ammonia-oxidizing bacteria (AOB), archaea (AOA), and comammox bacteria (i.e. COMplete AMMonia OXidizers) have revealed new enzymology, mechanisms to tolerate low pH and hypoxia, and mechanisms for production of the potent greenhouse gas, nitrous oxide. The role of ammonia-oxidizers in natural and engineered environments is of keen interest as they are essential to the nitrogen cycle, wastewater treatment, and flux of greenhouse gases to the atmosphere.