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.

Next Article - Root Nodule formation in Plants - Fate of Ammonia