Question

Name any one symbiotic nitrogen-fixing bacteria. How does it help in nitrogen fixation?

Answer 1

Molecular nitrogen is converted to ammonia in the process of nitrogen fixation. In the atmosphere, nitrogen is present as molecular di-nitrogen which is converted to ammonia by several means.

Complete answer: The nitrogen fixation is done by living organisms, then it is known as biological nitrogen fixation. Nitrogen-fixing bacteria or cyanobacteria are either free-living or symbiotic in nature. In symbiotic nitrogen fixation, two organisms live in a mutual relationship and fix nitrogen. Anabaena is a nitrogen fixing cyanobacteria which is in a symbiotic relationship with a water fern Azolla. Anabaena fixes nitrogen and increases the availability of nitrogen for the plant. Azolla in return, provides habitat for the cyanobacteria.
In leguminous plants, a special structure is formed in the roots for nitrogen fixation. It is called a root nodule. It is formed by the curling of root hair when the plant gets in contact with bacteria Rhizobium. Infection thread is formed and the bacteria invade the root hair. Some bacteria among this are modified into rod-shaped structures called bacteroides. In the final step, a vascular connection is also enabled between the nodule and the host. Root nodules primarily contain the enzyme nitrogenase. It catalyzes the conversion of nitrogen to ammonia. This ammonia produced is used for the synthesis of amino acids and finally into proteins by joining several amino acids. Root nodules also contain a special molecule called leghemoglobin. The enzyme nitrogenase is highly sensitive to oxygen. It needs anaerobic conditions for its function. Leg hemoglobin acts as an oxygen scavenger and protects the enzyme by providing an anaerobic environment.

Note: Gene responsible for nodule formation in legumes are known as plant nod genes. Nitrogen fixation occurs under the control of plant nod genes, bacterial ‘nod’ genes, ‘nif’ genes, and ‘fix’ genes. The three bonds between nitrogen atoms are broken and ammonia is formed in this process.