Nitrogen Fixation

Nitrogen Fixation

Nitrogen fixation involves the cleavage of the  triple bond of N2 by the enzyme nitrogenase.

The ability to fix nitrogen depends on presence of nitrogenase. The only confirmed nitrogen fixers are bacteria (Eubacteria and Archaebacteria)

Nitrogenase activity decreases sharply upon exposure to oxygen and methods have been devised for the protection of nitrogenase. These are physical, behavioral or biochemical such as high respiration rate, scavenging of oxygen, production of leghaemoglobin.



Clostridium Avoidance
Klebsiella pneumoniae avoidance, respiratory consumption of O2
Azospirillum Migration away from O2
Azobacter Respiration, conformational protection
Photosynthetic bacteria avoidance,some respiratory protection
Legume root nodules respiration, leghaemoglobin
actinorhizal nodules respiration, barrioer (vesicle) formation
Cyanobacteria- heterocystous strains spatial separation (heterocysts), respiration
Cyanobacteria- non-herterocystous temporal separation - clumping, respiration, nitrogenase


Most well known is the Rhizobium-Legume association. Cyanobacteria will also form Symbiotic relationships:

N-Fixing Cyanobacterial -plant associations

Plant division Cyanobacteria forming symbiosis
Chrysophyta (Diatom) Chaetoceros, Rhizosolenia
Anthocerophyta (mosses) Anthoceros, Phaeoceros, Notothylas, Dendroceros
Hepaticophyta (liverworts) Blasia, Cavicularia
Pteridophyta (ferns) Azolla
Cycarophyta (cycads) Bowenia, Ceratozamia, Cycas, Dioon, Encephalartos, Lepidozamia, Macrozamia, Microcycas, Strangeria, Zammia
Angiospermae Gunnera


To the Life Sciences Interactive Educational Center
To the Hebrew University of Jerusalem