Question

Explain the role of diazotrophs in nitrification.

Answer 1

1. Diazotrophs are microorganisms that have the ability to fix atmospheric nitrogen gas into a usable form, such as ammonia.
2. Diazotrophs are either symbiotic or free living. The cyanobacteria fix significant amount of nitrogen in specialized cells called heterocysts.
3. The best known nitrogen fixing symbiotic bacterium is Rhizobium.
4. Diazotrophs, while essential for nitrogen fixation, do not directly participate in nitrification.
5.  Diazotrophs convert atmospheric nitrogen (N₂) into ammonia (NH₃).
6. This process, called nitrogen fixation, requires 16 ATP molecules per nitrogen molecule.
7. It is an endothermic reaction.
   N₂ + 8H⁺ + 8e^- + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
8. Then nitrifying bacteria which are responsible for nitrification, converts ammonia into nitrate, the form of nitrogen which can be used by plants and animals. 

9. Nitrification involves two steps:

   1. Nitrosation: Ammonia is converted into nitrite (NO₂) by bacteria like Nitrosomonas and Nitrosococcus.
   2.  Nitrate oxidation: Nitrite is converted into nitrate by bacteria like Nitrobacter.