Describe in brief the role of microbes in Nitrogen fixation.
Q. Describe in brief the role of microbes in Nitrogen fixation.
Ans. Nitrogen Fixation: The molecular nitrogen (N2) is found in the atmosphere about 78% as a relatively inert gas. This nitrogen cannot be used directly by higher plants. The plants assimilate only fixed forms of nitrogen e.g., nitrate and ammonium ions. The nitrogen (N2) reacts with certain elements or compounds under suitable conditions to form fixed form of nitrogen, the process referred to as nitrogen fixation. Thus, the phenomenon of conversion of free nitrogen into nitrogenous salts to make it available for absorption by plants. The nitrogen fixation is divided into following two types :
I. Physical Nitrogen Fixation : Atmosphere free nitrogen combines with the xygen due to electric discharge and thunder and forms nitric oxide. The nitric oxides are again oxidized with oxygen to form nitrogen peroxide.
The nitrogen peroxide combines with water during rains to form nitrous acid and nitric acid. The acid falls on the ground alongwith rain water and react with alkaline radicals to produce water soluble nitrates and nitrites.
The nitrates are soluble in water and are directly absorbed by the plants.
II. Biological Nitrogen Fixation: The process is carried by two types of microorganism-those which are free living or asymbiotic and those which live in close symbiotic association with other plants. However, a third category of microorganisms which fix nitrogen in association with roots of grasses and cereals has been reported. It is called associative symbiotic nitrogen fixation.
1. Asymbiotic biological nitrogen fixation : A number of free living nitrogen fixing organisms and blue-green algae live in soil. These include a number of aerobic and anaerobic bacteria e.g., Azotobacter, Derxia, Nostoc, Anabaena, Pullularia and yeasts.
The asymbiotic free living nitrogen fixers are quite primitive. These organisms fix nitrogen more actively under poor aeration, provided no hydrogen gas is being produced.
Step of N2 fixation: The overall steps involved in N₂ fixation through asymbiotic organisms are as follows:
(i) The pyruvic acid, an end product of glycolysis, acts as an electron donor in most of the cases (however, other electron donating substrates, viz, sucrose, glucose, hydrogen etc., have also been reported to operate) in N2 fixation.
The pyruvic acid undergoes a phosphoroelastic breakdown releasing acetyl phosphate, CO2 and H2. The reaction occurs in two steps requiring the enzymes pyruvic acid dehydrogenase and phosphotransacetylase is follows:
(ii) Acetyl phosphate is converted to acetate releasing a molecule of ATP from ADP which acts as a source of energy in the nitrogen fixing system.
Acetyl Phosphate + ADP → Acetate + ATP
The A can be made available in the nitrogen fixing system through oxidative or pnotophosphorylation.
(iii) The source of electrons (viz, pyruvic acid, hydrogen etc.) reduces ferredoxin which acts as electron carrier. The ferredoxin reduces Fe-protein component (dinitrogenase reductase) of enzyme nitrogenase by donating its electron. The H+ is taken up from the medium.
(iv) Finally, ATP molecules interact and bind to reduced Fe-protein in presence of Mg+2, so that the reduced Fe-protein becomes activated and finally reduces Mo-Fe protein component of nitrogenase.
(v) The reduced Mo-Fe protein then reduces N2 to NH3. The electrons and protons are passed to the bound nitrogen stepwise, during which the nitrogen remains bound to enzyme.
Fig. Scheme for Sequential Reduction of Nitrogen to Ammonia in Nitrogenase System
(vi) The product of N2 fixation is ammonia which is very rapidly assimilated into cell material. In most of the cases it is transformed into glutamic acid and then translocated through vascular tissues to other parts of the plant.
Associative symbiotic nitrogen fixation : Certain bacteria, living in close contact with the roots of cereals (e.g. Azospirillum brasilense) and grasses transition zone between soil and root (the rhizosphere) and sometimes enter the roots. Some of the fixed nitrogen is absorbed by the roots and in return the bacteria get nourishment from the carbohydrates released by the roots.
2. Symbiotic biological nitrogen fixation : Where two organisms live together symbiotically and the microbial partner fixes atmospheric nitrogen are called symbiotic biological nitrogen fixation. This type of nitrogen fixation can be grouped into following three categories :
Nitrogen fixation through nodule formation: Symbiotic nitrogen fixers in leguminous plants inhabit small, knob-like protuberance, called root nodules, on the roots of plants. These root nodules vary considerably in their shape and size. They may be spherical, elongated, flat and grooved or may have finger-like projections.
The nodule Formation: The genus Rhizobium includes many species, e.g. Rhizobium leguminosarium (in pea), R. trifolli (in clover), R. phaseoli (in beans).
Rhizobium multiply very fast near the roots of specific, leguminous plants because the roots secrete some special types of lectins (carbohydrate containing proteins) over their surfaces, which help in the ecognition and attachment of specific rhizobial cells.
The lectins interact selectively with the carbohydrate receptors located on the surfaces of the rhizobial cells. At the region of interaction mainly lies between the root tip and the young root hair, nodules are formed.
The Rhizobium cells then enter into the host. Prior to entry, the bacterial cells secrete some hormone like substances which causes deformation and curling of young root hair. It is followed by the formation of tubular infection thread in the root hair cell.
As soon as they enter into the root hair cell, a new cell wall is formed. The bacteria in the mucopolysaccharide substance in the tube starts multiplying. The infection thread grows beyond the root hair cell and penetrates upto the inner cortical cells where the bacteria are released. They generally stay outside the plasma membrane of these cortical cells.
The bacterial cells multiply and colonize inside the host cells until the available space is filled. They then become dormant as so called bacteroides. The surrounding tissue proliferates to form the nodule. The bacteroids float in a reddish pigment called leghaemoglobin found in the cytoplasm of host cells. This pigment helps in N, fixation.
Steps of N₂ fixation: The overall steps involved in N₂ fixation are as follows:
- > The sucrose molecules are translocated from leaves to the roots where they are converted to glucose and fructose by enzyme invertase. Both glucose and fructose are converted to glucose-6-phosphate, by glycolytic enzymes present in bacteroids.
- The glucose-6-phosphate probably acts as a substrate for donating hydrogen (H+ + e–) to N2 fixing system. It is converted to 6phosphogluconic acid and in presence of enzyme glucose-6-phosphate dehydrogenase. The coenzyme NADP+ gets reduced in this conversion.
Glucose-6-phosphate + NADP + H2O → 6-phosphogluconic acid + NADPH+H+
- NADPH acts as a hydrogen carrier. It donates electron to ferredoxin in presence of enzyme NADP ferredoxin oxidoreductase and release H+ to the medium.
- The reduced ferredoxin then reduces Mo-Fe-protein component of nitrogenase.
Fig. Symbiotic Biological N2 Fixation
- The reduced Mo-Fe protein then reduces N2 to ammonia. The electrons and protons are passed to the bound nitrogen stepwise, during which the nitrogen remains bound to enzyme. Sequential reduction of N2 to NH3 by enzyme nitrogenase is similar to asymbiotic N2 fixation.
- The product of N2 fixation is ammonia which is taken up by host plant and immediately assimilated into organic forms (viz., amino acids, amides or ureids) and then translocated through vascular tissues to other parts of the plant.
Nitrogen fixation in non-leguminous plants: There have been various plants of different families (other than Leguminosae) reported to produce root nodules, which Perform N₂ fixation. e.g., Parasponia in association with Rhizobium, Casuarina equisetifolia in association with Frankia; Alnus in association with Frankia.
There are a large number of examples where root nodules are not formed but symbiotic nitrogen fixation takes place. e.g., roots of Digitaria, Sorghum and maize associated with Spirillum notatum. Azolla in association with Anabaena azolla, etc.
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