Nitrogen Cycle

Nitrogen Fixation and Nitrogen Fixing Organisms

Few living organisms can utilize nitrogen available in the air.

The reduction of nitrogen to ammonia by the living organisms is called biological nitrogen fixation.

The enzyme nitrogenase is capable of nitrogen reduction present in eukaryotes such microbes are called nitrogen-fixing organisms.

The process of nitrogen fixation is carried out by various micro-organisms called diazotrophs.

Some are free-living soil bacteria such as azotobacter, cyanobacteria, and symbiotic bacteria e.g. rhizobium.

The rhizobium bacteria invade the roots of leguminous plants and form root nodules where nitrogen fixation takes place.

Nitrogen is cycled from the environment to organisms and back to the environment by several parts. Five types of micro-organisms usually play a key role in the nitrogen cycle.

Nitrogen Fixing Bacteria

The chief symbiotic nitrogen fixers are rhizobium found in the roots of leguminous plants.

Cyanobacteria are some species of Azotobacter are asymbiotic nitrogen fixers.

They have the ability to take free nitrogen from the atmosphere and to convert it into soluble nitrates which are taken by roots of higher plants.

Putrefying Microbes

They breakdown plants and animals and convert it into ammonia compounds that are released in the soil where they can be used by other types of micro-organisms.

These are bacterias actinomycetes and saprophytic fungi found in the soil.

The process of decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.

Nitrite Microbes

Ammonium compounds are converted into nitrites by the activities of Nitrosomonas, Nitrosospira, Nitrococcus, Nitrosolobus, bacteria. Nitrates are soluble in water.

Nitrate Microbes

These bacteria such as Nitrococcus, Nitrosospira, Nitrocystis, Nitrobacter convert nitrites into nitrates which are soluble in water and can be used by plants.

Denitrifying Microbes

Nitrogen is lost from the cycle the biological process of denitrification. Some fungi and bacteria convert nitrates or ammonium compounds into free nitrogen e.g. pseudomonas.

The nitrogen cycle in nature is called a perfect cycle because it keeps the overall amount of nitrogen in the atmosphere, soil, and water at a constant value.

Excessive commercial fixation of Nitrogen a nitrogen fertilizer is increasing the risk of imbalance in the nitrogen cycle day today.

The increased amount of nitrogen in the rivers and lakes enhance the growth of phytoplankton and causing the phenomena of eutrophication. This process is responsible for the depletion of oxygen from the water body.

Vehicles release large quantities of nitrogen oxide out of this most of is converted into nitrogen dioxide in the atmosphere by combining with ozone.

Nitrogen dioxide is the major component of smog in urban areas which causes acid rain.

Excessive nitrogen dioxide in the atmosphere also causes the conversion of ozone into oxygen gas.

Biological Nitrogen Fixation of Nitrogen-Fixing Organisms

The reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation.

The nitrogenase enzyme is responsible for nitrogen reduction which is basically present in prokaryotes and called nitrogen fixers.

Mechanism of Biological Nitrogen Fixation

Nodule formation is the sequence of multiple interactions between rhizobium and roots of legume plants.

Rhizobia multiply and colonize the surrounding roots.

After the rhizobia get attached to the epidermis and roots hair cells.

The root hair curl and the bacteria invade the root hair.

An infection thread is produced in the cortex of the root and the bacteria are released from a threat into the cells which differentiate into the nitrogen-fixing cells. In this way, nodules are formed.

It has a direct vascular with the host for the exchange of nutrients.

The nodule has all necessary biochemical components such as enzyme nitrogenase and leghaemoglobin.

The enzyme is a Mo-Fe and catalyzes the conversion of atmospheric nitrogen to ammonia.

Nitrogen fixation occurs near the center of nodules.

The legume provides carbohydrates to the bacteria. Bacteria obtain electron by the oxidation of carbohydrates and reduce the nitrogen to ammonium ions.

These ammonium ions are converted into nitrogenous compounds that are absorbed by the surrounding plant cells and translocated to other regions.

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