In nitrogen cycle the process 2NH₃ + 3O₂ $\to$2NO^-₂ + 2H⁺ + 2H₂O is carried out by

In nitrogen cycle the process 2NH₃ + 3O₂ $\to$2NO^-₂ + 2H⁺ + 2H₂O is the first step in nitrification and it is carried out by Nitrosomonas and Nitrococcus type bacteria. Thiobacillus does denitrification, Rhodospirillum carries out nitrogen fixation and Nitrobacter is involved in the formation of nitrates from nitrites during nitrification.