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Chapter: Essential Microbiology: Procaryote Diversity

Nitrogen-fixing Proteobacteria

The α-Proteobacteria includes certain genera of nitrogen-fixing bacteria.

Nitrogen-fixing Proteobacteria

The α-Proteobacteria includes certain genera of nitrogen-fixing bacteria. These are able to fix (reduce)atmospheric N2 as NH4+ for subsequent incorporation into cellular materials, a process that requires a consid-erable input of energy in the form of ATP:

N2 + 8e + 8H+ + 16ATP −−−−−−−→, 2NH4+ + 16ADP + 16Pi

Nitrogen-fixing bacteria may be free-living in the soil (e.g. Azotobacter), or form a symbiotic relationship with cells on the root hairs of leguminous plants such as peas, beans and clover (e.g. Rhizobium). The nitrogenase responsible for the reaction (actually a complex of two enzymes) is highly sensitive to oxygen; many nitrogen fixers are anaerobes, while others have devised ways of keeping the cell interior oxygen-free.


Closely related to Rhizobium, but unable to fix nitrogen, are members of the genus Agrobacterium. Like Rhizobium, these enter the tissues of plants, but instead of forminga mutually beneficial association, cause cell proliferation and tumour formation. A.tumefaciens has proved to be a valuable tool in the genetic engineering of plants.

Representative genera: Rhizobium, Azotobacter

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