Nitrogen fixation, natural and synthetic, is essential for all forms of life because nitrogen is required to biosynthesize basic building blocks of plants, animals, and other life forms, e. Therefore, nitrogen fixation is essential for agriculture and the manufacture of fertilizer. Microorganisms that fix nitrogen are bacteria called diazotrophs.
The role of soil bacteria in the Nitrogen cycle : Nitrogen transitions between various biologically useful forms. Some higher plants, and some animals termites , have formed associations symbioses with diazotrophs.
Diazotrophs are microbes. They are intensively studied by microbiologists. Biological nitrogen fixation was discovered by the German agronomist Hermann Hellriegel and Dutch microbiologist Martinus Beijerinck.
Biological nitrogen fixation BNF occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas N 2. There is only one known family of enzymes that accomplishes this process.
All nitrogenases have an iron — and sulfur-containing cofactor that includes a heterometal complex in the active site e. In most species, this heterometal complex has a central molybdenum atom.
However, in some species it is replaced by a vanadium or iron atom. Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen. Many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with proteins.
For thousands of years farmers were aware that plants belonging to the legume family, such as peas and soy beans, promoted crop growth when planted with other non-legumes such as wheat. This led to the practice of crop rotation, a practice which can be traced back to techniques recorded in Roman literature.
Martinus Beijerinck : Work done by Martinus Beijerinck was key to the discovery of rhizobia, symbiotic bacteria found on the roots of legumes and responsible for nitrogen fixation. While the ancient Romans were aware of the improved results gained through crop rotation, they did not know that these benefits were brought about through the replenishment of nitrogen in the soil.
Later people knew legumes did replenish nitrogen in the soil, but did not know how atmospheric N 2 was converted into ammonium NH 3 by legumes until research done in the 19 th century.
Hermann Hellriegel , a noted German agricultural chemist, discovered that leguminous plants took atmospheric nitrogen and replenished the ammonium in the soil through the process now known as nitrogen fixation.
He found that the nodules on the roots of legumes are the location where nitrogen fixation takes place. Hellriegel did not determine what factors in the root nodules carried out nitrogen fixation.
Martinus Willem Beijerinck March 16, — January 1, , a Dutch microbiologist and botanist, explored the mechanism responsible, discovering that the root nodules contained microbes. He further demonstrated that these microbes were bacteria, which he named rhizobia. These rhizobia perform the chemical processes of nitrogen fixation.
In addition to having discovered this biochemical reaction vital to soil fertility and agriculture, Beijerinck is responsible for the discovery of this classic example of symbiosis between plants and bacteria. The bacteria in the root nodules are needed to provide nitrogen for legume growth, while the rhizobia are dependent on the root nodules as a environment to grow. The conversion of N 2 to NH 3 depends on a complex reaction, essential to which are enzymes known as nitrogenases.
Distinguish between component I and II of the nitrogenase enzyme and its role in biological nitrogen fixation. This type of reaction results in N 2 gaining electrons see above equation and is thus termed a reduction reaction. The exact mechanism of catalysis is unknown due to the technical difficulties biochemists have in actually visualizing this reaction in vitro, so the exact sequence of the steps of this reaction are not completely understood. Finally, all organisms need a steady supply of nitrogen because it is a building block of DNA and RNA, and of proteins.
These are vital macromolecules that organisms need to build their genetic info DNA and RNA and to carry out many of the processes of life proteins.
What is the importance of nitrogen fixation? Biology Microorganisms Bacteria. Johnny L. Once a lake has undergone eutrophication, it is even harder to do damage control. Algaecides can be expensive, and they also do not correct the source of the problem: the excess nitrogen or other nutrients that caused the algae bloom in the first place!
Another potential solution is called bioremediation , which is the process of purposefully changing the food web in an aquatic ecosystem to reduce or control the amount of phytoplankton.
For example, water managers can introduce organisms that eat phytoplankton, and these organisms can help reduce the amounts of phytoplankton, by eating them! The nitrogen cycle is a repeating cycle of processes during which nitrogen moves through both living and non-living things: the atmosphere, soil, water, plants, animals and bacteria.
In order to move through the different parts of the cycle, nitrogen must change forms. In the atmosphere, nitrogen exists as a gas N 2 , but in the soils it exists as nitrogen oxide, NO, and nitrogen dioxide, NO 2 , and when used as a fertilizer, can be found in other forms, such as ammonia, NH 3 , which can be processed even further into a different fertilizer, ammonium nitrate, or NH 4 NO 3.
There are five stages in the nitrogen cycle, and we will now discuss each of them in turn: fixation or volatilization, mineralization, nitrification, immobilization, and denitrification. In this image, microbes in the soil turn nitrogen gas N 2 into what is called volatile ammonia NH 3 , so the fixation process is called volatilization. Leaching is where certain forms of nitrogen such as nitrate, or NO 3 becomes dissolved in water and leaks out of the soil, potentially polluting waterways.
In this stage, nitrogen moves from the atmosphere into the soil. To be used by plants, the N 2 must be transformed through a process called nitrogen fixation. Fixation converts nitrogen in the atmosphere into forms that plants can absorb through their root systems. A small amount of nitrogen can be fixed when lightning provides the energy needed for N 2 to react with oxygen, producing nitrogen oxide, NO, and nitrogen dioxide, NO 2.
These forms of nitrogen then enter soils through rain or snow. Nitrogen can also be fixed through the industrial process that creates fertilizer. This form of fixing occurs under high heat and pressure, during which atmospheric nitrogen and hydrogen are combined to form ammonia NH 3 , which may then be processed further, to produce ammonium nitrate NH 4 NO 3 , a form of nitrogen that can be added to soils and used by plants.
Most nitrogen fixation occurs naturally, in the soil, by bacteria. In Figure 3 above , you can see nitrogen fixation and exchange of form occurring in the soil. Some bacteria attach to plant roots and have a symbiotic beneficial for both the plant and the bacteria relationship with the plant [ 6 ].
The bacteria get energy through photosynthesis and, in return, they fix nitrogen into a form the plant needs. The fixed nitrogen is then carried to other parts of the plant and is used to form plant tissues, so the plant can grow. Other bacteria live freely in soils or water and can fix nitrogen without this symbiotic relationship.
These bacteria can also create forms of nitrogen that can be used by organisms. This stage takes place in the soil. Nitrogen moves from organic materials, such as manure or plant materials to an inorganic form of nitrogen that plants can use. This becomes important in the second stage of the nitrogen cycle. Much of this nitrogen is provided to cropping systems in the form of industrially produced nitrogen fertilizers.
Use of these fertilizers has led to worldwide, ecological problems, such as the formation of coastal dead zones. Biological nitrogen fixation, on the other hand, offers a natural means of providing nitrogen for plants. It is a critical component of many aquatic, as well as terrestrial ecosystems across our biosphere. References and Recommended Reading Appleby, C. Leghemoglobin and Rhizobium respiration. Annual Review of Plant Physiology 33 , Article History Close. Share Cancel. Revoke Cancel.
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