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A Handbook of Biology
BÏØLØGÏÇÅL ÑÏTRØGËÑ FÏXÅTÏØÑ
It is the reduction of
N2 by living organisms
in presence of
nitrogenase enzyme.
Very few
organisms can
utilise the nitrogen
in the form of N2 in
the air.
Only certain
prokaryotic species
have nitrogenase
enzyme and
capability to fix N2.
They are called N2–
fixers.
1
2
3
Nitrogen-fixing microbes are of 2 types:
Free-living: E.g.
Azotobacter
& Beijerinckia
(aerobic microbes),
Rhodospirillum &
Bacillus (anaerobic),
cyanobacteria such as
Anabaena & Nostoc.
(AIPMT 2010)
Symbiotic:
E.g. Rhizobium.
SÝMBÏØTÏÇ BÏØLØGÏÇÅL ÑÏTRØGËÑ FÏXÅTÏØÑ
Legume-bacteria relationship: Most prominent symbiotic biological
nitrogen fixation relationship. Rhizobium species (rod-shaped) seen in
the roots of legumes such as alfalfa, sweet clover, sweet pea, lentils,
garden pea, broad bean, clover beans, etc.
The most common association on roots is as nodules.
The microbe, Frankia also produces N2 fixing nodules on the roots of
non-leguminous plants (Alnus).
Rhizobium and Frankia are free-living in soil, but as symbionts, can fix
atmospheric nitrogen.
Central part of a nodule is red or pink coloured due to the presence of
leguminous haemoglobin (leg- hamoglobin).
During biological nitrogen fixation, inactivation of nitrogenase by oxygen
poisoning is prevented by leg-hamoglobin.
(AIPMT 2015)