Describe the process of Symbiotic Biological Nitrogen fixation.

Explanation

1. Loose Association of Nitrogen Fixing Bacteria. Certain nitrogen fixing bacteria live around the roots of higher plants without developing any intimate relationship. It is often called rhizosphere association. The bacteria obtain some plant exudate and use the same as part of their food requirement. The bacteria fix nitrogen and exude a part of the fixed nitrogen for use by the plant. The phenomenon is Urmed as associative mutualism (= associative symbiosis).
2. Symbiotic Nitrogen Fixing Bacteria. They form a mutually beneficial association with the plants. The bacteria obtain food and shelter from plants. In return, they give a part of their fixed nitrogen to the plants. The most important of the symbiotic nitrogen fixing bacteria is Rhizobiurn (pi Rhizobia). It forms nodules on the roots of legume plants. There are about a dozen species of Rhizobjum which form association with different legume roots, e.g., R. leguminosarum, R. lupini, R. trifolii, R. phaseoli. The bacteria live freely in the soil but cannot fix nitrogen except for a strain. of Cowpea Rhizobium. They develop the ability to fix nitrogen only when they are present inside the root nodules. In the nodule cells, bacteria (bacteroids) lie in groups surrounded by membrane of the host which is lined by a pink-red pigment called leghaemoglobin. Presently cultures of Rhizobium specific for different crops are raised in the laboratory
Frankia, a nitrogen fixing mycelial bacterium (actinomycete), is associated symbolically with the root nodules of several nonlegume plants like Casuarina, Alnus (Alder) Myrica, Rubus etc. Leaves of á few plants (e.g., Ardisia) develop special internal cavities for providing space to symbiotic nitrogen fixing, bacteria, Xanthomopias and Mycobacterium. Such leaves are a constant source of nitrogen fertilizer to the soil.
3. Symbiotic Nitrogen Fixing Cyanobacteria. Azolla pinnata is aquatic fern which is often inoculated to paddy fields. It has a symbiotic association with nitrogen fixing cyanobacterium called Anabaena azollae.