text: The two-step conversion of ammonia to nitrate observed in ammonia-oxidizing bacteria, ammonia-oxidizing archaea and nitrite-oxidizing bacteria (such as Nitrobacter) is puzzling to researchers.  Complete nitrification, the conversion of ammonia to nitrate in a single step known as comammox, has an energy yield (∆G°′) of −349 kJ mol NH, while the energy yields for the ammonia-oxidation and nitrite-oxidation steps of the observed two-step reaction are −275 kJ mol NH, and −74 kJ mol NO, respectively. These values indicate that it would be energetically favourable for an organism to carry out complete nitrification from ammonia to nitrate (comammox), rather than conduct only one of the two steps. The evolutionary motivation for a decoupled, two-step nitrification reaction is an area of ongoing research. In 2015, it was discovered that the species Nitrospira inopinata possesses all the enzymes required for carrying out complete nitrification in one step, suggesting that this reaction does occur.