Biological Nitrogen Fixation
The Nitrogen Fixation is the procedure by which atmospheric Nitrogen is converted into ammonia. The Nitrogen fixation is one of the important components of the Nitrogen cycle.
Nitrogen fixation can be biotic or abiotic. The examples of abiotic processes are lightening, Industrial processes such as Haber-Bosch Process, and combustion. The biotic nitrogen fixation was discovered by Martinus Beijerinck.
How does it work?
Two molecules of ammonia are produced from one molecule of nitrogen gas, at the expense of 16 units of ATP and a supply of electrons and protons (hydrogen ions):
N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
Please note that exclusively the prokaryotes do this reaction. The enzyme used is called nitrogenase. The nitrogenase enzyme has two kinds of proteins viz. Iron Protein, and Iron-Molybdenum protein. The N2 is is bound to the nitrogenase enzyme complex. The Fe protein is first reduced by electrons donated by ferredoxin. Then the reduced Fe protein binds ATP and reduces the molybdenum-iron protein, which donates electrons to N2, producing HN=NH. There are two more cycles and each requires electrons donated by ferredoxin) HN=NH is reduced to H2N-NH2, and this in turn is reduced to 2NH3 .
Thus in summary
- 16 ATP are used in BNF (Biological Nitrogen Fixation)
- Two minerals viz. Iron and Molybdenum play important role in BNF.
- End product is ammonia + Hydrogen
- Enzyme used is Nitrogenase
Role of Bacteria
Both anaerobic bacteria as well as the aerobic bacteria do biological nitrogen fixation however, the process occurs in absence of Oxygen and thus is anaerobic process. Further, biological nitrogen fixation is done by both free living and symbiotic bacteria. The notable examples are given below:
- Free living aerobic bacteria: Azotobacter
- Free living anaerobic bacteria: Clostridium, purple sulphur bacteria
- Symbiotic in legumes and pulses: Rhizobium (found in root nodules)
- Symbiotic in sugarcane: Glucoacetobacter diazotrophicus (found in stem knots)
- Symbiotic in other plants: Frankia, Azospirillum
Why BNF occurs only in anaerobic conditions?
The enzyme nitrogenase is susceptible to destruction by oxygen. Many bacteria cease production of the enzyme in the presence of oxygen that is why many nitrogen-fixing organisms exist only in anaerobic conditions. Some aerobic bacteria which carry out the Nitrogen Fixation use another protein called Leghemoglobin to bind the oxygen and bring its level down.
BNF in Legume Plants
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with plants such as pulses, groundnut, clover, soybeans, alfalfa, lupines, peanuts etc. They contain symbiotic bacteria called Rhizobium within nodules in their root systems. Further, it is not necessary that ONLY symbiotic bacteria are able to fix nitrogen by BNF. It is also NOT necessary that only leguminous plants do this. BNF is also found in sugarcane in which such bacteria live in stem nodules. Moreover, fixed nitrogen is released only when the plant dies. This helps to fertilize the soil.
Bacteria in Nitrification
Nitrification is the process in which the ammonia is converted into Nitrate. Nitrification is a two step process and based upon these two steps, the bacteria are divided into Nitrosifying and Nitrite-Oxidizing bacteria. Example of Nitrosifying bacteria is Nitrosomonas, which converts the Ammonia (NH3) into Nitrite (NO2-). Example of Nitrite-Oxidizing bacteria is Nitrobacter which are able to oxidize the Nitrite and crate Nitrate (NO3-).
