Question

Name any one symbiotic nitrogen fixing bacteria. How does it help in nitrogen fixation ?

Answer 1

Nitrogen fixation is a process by which nitrogen in the air is converted into ammonia ($N{H}_3$) or related nitrogenous compounds. Atmospheric nitrogen, is molecular dinitrogen ($N_2$), a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts $N_2$ into ammonia, which is metabolized by most organisms.

Biological nitrogen fixation can be represented by the following equation, in which two moles of ammonia are produced from one mole of nitrogen gas, at the expense of 16 moles of ATP and a supply of electrons and protons (hydrogen ions):
$N_2$ + $8H$+ + $8e-$ + $16ATP$ = $2N{H}_3$ + $H_2$ + $16ADP$ + $16Pi$

This reaction is performed exclusively by prokaryotes (the bacteria and related organisms), using an enzyme complex termed nitrogenase. This enzyme consists of two proteins - an iron protein and a molybdenum-iron protein. The reactions occur while $N_2$ 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 $N_2$, producing $HN=NH$. In two further cycles of this process (each requiring electrons donated by ferredoxin) $HN=NH$ is reduced to $H_{2}N-N{H}_2$, and this, in turn, is reduced to $2N{H}_3$. Depending on the type of microorganism, the reduced ferredoxin which supplies electrons for this process is generated by photosynthesis, respiration or fermentation. Nitrogen fixation is carried out naturally in the soil by a wide range of microorganisms termed diazotrophs that include bacteria such as Azotobacter, and archaea.