The synthesis of organic nitrogenous compounds from atmospheric nitrogen by certain microorganisms is called biological nitrogen fixation. Higher plants cannot directly utilize molecular nitrogen of the atmosphere. But certain micro-organisms can utilize atmospheric nitrogen. There are two types of nitrogen fixing micro-organisms: Asymbiotic and symbiotic.
(a) ASYMBIOTIC NITROGEN FIXATION
The free living nitrogen fixing organisms are called are asymbiotic – organisms. There are following organisms which fix the atmospheric nitrogen.
- Aerobic bacteria and anaerobic bacteria
They are saprophytic organisms. Therefore, they fix nitrogen only in the soil. This soil must have good supply of organic manure. The example of anaerobic bacteria is Clostridium and the example of aerobic bacteria is Azotobacter.
- Blue green algae (Cyanobacteria)
Blue green algae are autotrophic photosynthetic organisms. They grow in waterlogged and wet soil like paddy (rice) fields. Members of families Nostocales and Stigonematales are important nitrogen fixture. Some of its examples are:
(a) Anabaena is an important member. It consists of branched
filaments. These filaments arc made up of two types of cells
• Green photosynthetic cells: They fix the atmospheric carbon dioxide and release oxygen during pltotosynthesis.
• Heterocyst: They are slightly enlarged colourless cells. These
are non-photosynthetic. They can fix atmospheric nitrogen aerobically. Some of the blue green algae do not have heterocysts. But they can fix nitrogen. Their mechanism is not understood.
(b) Blue green algae living in higher plants: The blue-green algae are free living in the soil. Some blue green algae enter into the tissues of higher plants. They permanently live inside. For example Nostoc lives in the mucilage cavities of Anthoceras. Some anabaena lives in the leaf cavities of the xvater fern. The nitrogen fixing ability of the blue green algae increases in higher plants. They produce more number of heterocysts. Nitrogenous substances regularly pass from the alga to the higher plant and carbohydrates pass from plant to cyanobacteria. Therefore, it may be called a symbiotic association.
(c) Lichens: Nostoc and Scytonema develops symbiotic association with fungi. This association is called lichens. The algal components fix the molecular nitrogen in the form of organic compounds.
(b) SYMBIOTIC NITROGEN FIXATION
It is the most important metlioa of nitrogen fixation. These are of two types:
- Fungi (Actinomyettes)
Several species of Actinomycetes develop mycorrhizal association with the root of Casurina. Pinus and other plants. These mycorrhizae may be ectotropic and endotropic mycorrhizae. These fungi have ability to fix atmospheric nitrogen.
- Nodule forming Rhizobium species
It is most important nitrogen fixation. Its mechanism is fully known. A large number of leguminous plants like bean, pea, gram and soybean develop root nodules. All the plants of family Papilionacae develop nodules. 30 per cent of the species belongs to Caesalpinacae. Leguminous plants increase the soil fertility. German Bacteriologist, Beijerinck identified that Rhizobium infect the root system and they develop nodules.
Bacteria mainly fix atmospheric nitrogen in the form of amino acids. These amino acids are transported by transpiration stream to different parts of the plant. In turn, bacteria get their food (carbohydrates) from the legume plant. Therefore, this relationship is symbiosis. The nodules are not formed on the root system without bacteria. Similarly bacteria themselves can fix atmospheric nitrogen.
Nodules are formed by following process:
I. Infection thread formation: Nodule causing bacteria are present in
the soil. The root system of leguminous plants secretes some sugars and amino acids. It attracts the bacteria. The bacteria multiply near the root zone. They develop a sufficient bacterial population. Then they infect the root hairs. A number of changes take place in the root hair cell. Cytoplasmic streaming movement increases. The nucleus of the root hair become doubles in size. The cell wall loosened. The plasmalemma invaginates and engulfs the bacterium. The root hair becomes curled. The bacterium becomes threadlike inside the root hair This is called infection thread.
- Formation of nodule: The infection thread becomes branched.These branches grow from one cell to the adjacent cell. They extend up to the inner cortical cells of the root. The cortical cells now start dividing rapidly. The growth substances auxins and cytokinins are secreted by the bacteria. These substances stimulate this cell division. Thus a tumor like structure appears on the root. It is known as nodule.
- Bacterioids formation: The nodule grows and increases in size. The vascular tissues of the root extend to the tip of the nodule. The bacteria multiply rapidly in the host cell of the root nodule. The infection threads are changed into globular oval cells. These are known as bacterioids. Nitrogen fixation is carried out in the bacterioids.4. Leghaemoglobin formation: The nodule is pink in colour. The pink colour is due to the presence of pigment leghaemoglobin. This pigment is synthesized by the host cells in response to bacterial infection. It is similar to haemoglobin of red blood cells of mammals. As it is present in leguminous plants, so it is called leghaemoglobin. Significance of leghaemoglobin:This is very important pigment. The ability of nitrogen fixation depends upon contents of this pigment. Older nodules lose this pigment. They become green or brown. They fail to fix nitrogen. Leghaemoglobin is an oxygen carrier. Thus oxygen becomes available for rapid respiration and ATP is synthesized in the nodule. Nitrogen fixation is a reductive process. It requires a large amount of energy. Respiration supplies this energy. The leghaemoglobin supply oxygen for respiration.