Nitric oxide is an obligate bacterial nitrification intermediate produced by hydroxylamine oxidoreductase

The enzymatic reactions that occur during nitrification are nature's means to use ammonia as cellular fuel. Complete understanding of nitrification and related processes are vital to sustainable agriculture and renewable energy technologies. The prevailing view of the first phase of nitrification is that ammonia oxidizing bacteria use two enzymes, ammonia monooxygenase and hydroxylamine oxidoreductase, to oxidize ammonia to nitrite via hydroxylamine as an obligate intermediate. Our work reveals nitric oxide as an additional obligate intermediate. The presented findings necessitate revision of a key biogeochemical process, identify a new bioenergetic role for nitric oxide, predict participation of a third enzyme in the biological oxidation of ammonia to nitrite, and will inform models toward sustainable agriculture.