Nitric oxide-dependent anaerobic ammonium oxidation

Nitric oxide (NO) has important functions in biology and atmospheric chemistry as a toxin, signaling molecule, ozone depleting agent and the precursor of the greenhouse gas nitrous oxide (N₂O). Although NO is a potent oxidant, and was available on Earth earlier than oxygen, it is unclear whether NO can be used by microorganisms for growth. Anaerobic ammonium-oxidizing (anammox) bacteria couple nitrite reduction to ammonium oxidation with NO and hydrazine as intermediates, and produce N₂ and nitrate. Here, we show that the anammox bacterium Kuenenia stuttgartiensis is able to grow in the absence of nitrite by coupling ammonium oxidation to NO reduction, and produce only N₂. Under these growth conditions, the transcription of proteins necessary for NO generation is downregulated. Our work has potential implications in the control of N₂O and NO emissions from natural and manmade ecosystems, where anammox bacteria contribute significantly to N₂ release to the atmosphere. We hypothesize that microbial NO-dependent ammonium oxidation may have existed on early Earth.