The process by which bacteria convert nitrogen gas (N2) from the air into ammonia (NH3) is called nitrogen fixation. Nitrogen fixation is a vital biological process that transforms atmospheric nitrogen into a form usable by plants and other organisms.
There are two primary types of nitrogen-fixing bacteria:
Free-living nitrogen-fixing bacteria: These bacteria live independently in the soil or water. Examples include bacteria from the genera Azotobacter and Azospirillum. They possess the enzyme nitrogenase, which catalyzes the conversion of atmospheric nitrogen to ammonia.
Symbiotic nitrogen-fixing bacteria: These bacteria form a mutualistic relationship with certain plants, such as legumes (e.g., soybeans, peas, and clover). The most well-known symbiotic nitrogen-fixing bacteria are from the genus Rhizobium. These bacteria reside within specialized structures called nodules that develop on the roots of the host plants. Inside the nodules, the bacteria convert nitrogen gas into ammonia, which is then used by the host plant for growth and development. In return, the plant provides the bacteria with carbohydrates and a suitable environment.
The process of nitrogen fixation involves the following steps:
Nitrogenase production: The nitrogen-fixing bacteria synthesize the enzyme nitrogenase, which is responsible for the conversion of nitrogen gas to ammonia.
Oxygen protection: Nitrogenase is highly sensitive to oxygen and can be inhibited by its presence. To protect nitrogenase, free-living nitrogen-fixing bacteria have specialized mechanisms to reduce or eliminate oxygen in their environment. Symbiotic nitrogen-fixing bacteria, on the other hand, reside within nodules, which provide a low-oxygen environment.
Conversion of nitrogen gas: Nitrogenase breaks the strong triple bond between nitrogen molecules (N2) and converts them into ammonia (NH3) through a series of enzymatic reactions. This process requires a large amount of energy and ATP (adenosine triphosphate), which is supplied by the bacteria or the host plant.
Utilization by plants: The produced ammonia can be utilized by plants as a nitrogen source. In plants, ammonia is further assimilated into organic compounds such as amino acids, proteins, and nucleic acids.
Overall, nitrogen fixation is a crucial biological process that contributes to the availability of nitrogen in ecosystems, promoting plant growth and supporting the nitrogen cycle.