Biological Nitrogen fixation - An Overview

• Some natural phenomena like lightning and ultraviolet rays stimulate the process of fixation of Nitrogen in the form of  nitric oxide.

• But   Major amounts of nitrogen are fixed as ammonia, nitrites, and nitrates by soil microorganisms.

• More than Ninety percent of all nitrogen fixation in nature is done by the microbes.

• In the Biosphere there are  few microbes that  can use  the nitrogen  available abundantly in the air.

• Only certain prokaryotic species like Bacteria Fungi and cyanobacteria  are capable of fixing atmospheric  nitrogen.

• These microbes reduce the atmospheric nitrogen and convert it into ammonia.

• The reduction of nitrogen to ammonia by living organisms is termed biological nitrogen fixation.

• An  enzyme, nitrogenase which is capable of nitrogen reduction is present exclusively in prokaryotes. 

• The nitrogen-fixing microbes may be  free-living or symbiotic.

• Examples of free-living nitrogen-fixing aerobic microbes are Azotobacter Azospirullum and Beijernickia whereas Rhodospirillum  and Bacillus are free-living.

• Some members of  cyanobacteria such as Anabaena Nostoc  oscillatoria are also free-living nitrogen-fixers. 

• In addition to free living, some examples of symbiotic biological nitrogen fixation  are also present.

• Fungi Bacteria and cyanobacteria are also present as symbiotic relationships in higher plants.

• A bacterial Species  Rhizobium is present as a symbiotic relationship with the roots of several legumes plants like Alfalfa, sweet pea, lentils, garden pea, broad bean, clover beans and Azospirillum species is present in  cereal and grasses.

• The symbiotic nitrogen-fixing bacteria reside in  the root hairs of plants . In the root hairs,they multiply and their multiplication appears as outgrowth   called root nodule.

• Within the nodules, the bacteria convert free nitrogen to ammonia that is used by  the  plant  for development. 

• Fungi like the genus Glomus are also present in the root of higher plants as a symbiotic relationship.This relationship is called Mycorrhiza.

• Cyanobacteria or blue green algae also fix atmospheric nitrogen  by the special called heterocyst.

• Bacteria ,Fungi and cyanobacteria are therefore also called Biofertilizers because they increase the nitrogen content  in soil by process of Biological nitrogen fixation.