Select The Organisms That Are Able To Perform Nitrogen Fixation

Nitrogen fixation, the remarkable process of converting atmospheric nitrogen (N₂) into ammonia (NH₃), is essential for life on Earth. This transformation makes nitrogen available to plants and other organisms that cannot directly utilize atmospheric nitrogen. While plants are the primary beneficiaries, they themselves cannot perform this process. Instead, they rely on a select group of microorganisms with the unique ability to break the strong triple bond in N₂ molecules. Let's delve into the fascinating world of these nitrogen-fixing organisms, exploring their diverse types, mechanisms, and ecological significance.

The Biological Nitrogen Fixation Process

Before identifying the organisms, it's crucial to understand the process of nitrogen fixation itself.

The Nitrogenase Enzyme: The key to nitrogen fixation lies in a complex enzyme called nitrogenase. This enzyme, found exclusively in nitrogen-fixing microorganisms, catalyzes the reduction of atmospheric nitrogen into ammonia.
Energy Requirement: Nitrogen fixation is an energy-intensive process. Microorganisms require a significant amount of ATP (adenosine triphosphate), the cellular energy currency, to power the nitrogenase enzyme.
Oxygen Sensitivity: Nitrogenase is extremely sensitive to oxygen. The presence of oxygen can irreversibly damage the enzyme, rendering it inactive. Consequently, nitrogen-fixing organisms have evolved various strategies to protect nitrogenase from oxygen exposure.

Categories of Nitrogen-Fixing Organisms (Diazotrophs)

Organisms capable of nitrogen fixation are called diazotrophs. These are all prokaryotes (bacteria and archaea). They can be categorized based on their lifestyle and relationship with plants:

Free-Living (Non-Symbiotic) Diazotrophs: These organisms live independently in the soil or water and fix nitrogen without direct interaction with other organisms.
Symbiotic Diazotrophs: These organisms form a mutually beneficial relationship with plants, providing them with fixed nitrogen in exchange for carbon and a protected environment.
Associative Diazotrophs: This group falls somewhere between free-living and symbiotic. They live in close proximity to plant roots and may provide some nitrogen benefit, but the relationship is not as tightly integrated as in true symbiosis.

Free-Living (Non-Symbiotic) Diazotrophs:

These bacteria and archaea operate independently in various environments. Key examples include:

Aerobic Bacteria:
- Azotobacter: Azotobacter is a well-studied genus of aerobic, free-living bacteria commonly found in soil. They are characterized by their large size and ability to form cysts, which are resistant to desiccation. Azotobacter protects nitrogenase from oxygen by employing a high respiration rate, consuming oxygen rapidly and maintaining a low oxygen concentration within the cell. They also produce a slime layer that further limits oxygen diffusion.
- Azotomonas: Similar to Azotobacter, Azotomonas are aerobic bacteria found in soil and water.
- Beijerinckia: Beijerinckia are acid-tolerant aerobic bacteria found in acidic soils. They employ a unique mechanism to protect nitrogenase, producing a thick capsule that limits oxygen diffusion.
- Derxia: Another genus of aerobic, free-living nitrogen fixers found in soil.
Anaerobic Bacteria:
- Clostridium: Clostridium is a genus of anaerobic bacteria commonly found in soil and sediments. They are obligate anaerobes, meaning they cannot survive in the presence of oxygen. Clostridium species fix nitrogen in anaerobic conditions, avoiding the oxygen sensitivity of nitrogenase.
- Klebsiella pneumoniae: While Klebsiella pneumoniae is often associated with opportunistic infections, some strains are capable of nitrogen fixation under anaerobic conditions. This bacterium protects nitrogenase through anaerobic respiration.
Facultative Anaerobes:
- Bacillus: Some species of Bacillus can fix nitrogen under either aerobic or anaerobic conditions. Bacillus species may employ different mechanisms for nitrogenase protection depending on the oxygen availability.
Cyanobacteria (Blue-Green Algae):
- Anabaena: Anabaena are photosynthetic cyanobacteria that can fix nitrogen. They often form symbiotic relationships with aquatic plants like Azolla. To protect nitrogenase from oxygen produced during photosynthesis, Anabaena develop specialized cells called heterocysts. Heterocysts are thick-walled cells that lack photosystem II, the oxygen-evolving component of photosynthesis. This creates an anaerobic environment within the heterocyst, allowing nitrogen fixation to occur.
- Nostoc: Similar to Anabaena, Nostoc are filamentous cyanobacteria that can fix nitrogen and form symbiotic relationships with various organisms. They also develop heterocysts for nitrogenase protection.
Archaea:
- Methanococcus: Some species of methanogenic archaea, such as Methanococcus maripaludis, have been shown to fix nitrogen. These archaea are typically found in anaerobic environments like marine sediments. The exact mechanisms of nitrogenase protection in these archaea are still being investigated.

Symbiotic Diazotrophs:

These bacteria engage in mutually beneficial relationships with plants. The most well-known example is the association between rhizobia and legumes.

Rhizobia and Legumes:
- Rhizobium, Bradyrhizobium, Azorhizobium, Sinorhizobium, Mesorhizobium: These genera of bacteria are collectively known as rhizobia. They form a symbiotic relationship with leguminous plants (e.g., beans, peas, soybeans, alfalfa). The rhizobia infect the roots of the legume, inducing the formation of specialized structures called nodules. Within the nodules, the rhizobia differentiate into bacteroids, which are the active nitrogen-fixing form. The plant provides the bacteroids with carbon and a protected environment, while the bacteroids fix nitrogen and supply it to the plant. This symbiotic relationship is crucial for nitrogen availability in agricultural systems. The process involves a complex exchange of molecular signals between the plant and the bacteria, ensuring the specificity of the interaction.
Frankia and Non-Legumes:
- Frankia: Frankia is a genus of filamentous bacteria that form symbiotic relationships with a diverse group of non-leguminous plants, including alder (Alnus), sweet fern (Comptonia), and bayberry (Myrica). Similar to the rhizobia-legume symbiosis, Frankia induces the formation of root nodules, where nitrogen fixation occurs. Frankia can fix nitrogen under a wide range of environmental conditions, making it an important contributor to nitrogen availability in various ecosystems.
Cyanobacteria and Plants:
- Anabaena-Azolla symbiosis: As mentioned earlier, Anabaena can form a symbiotic relationship with the aquatic fern Azolla. Azolla provides Anabaena with a protected environment within its leaf cavities, while Anabaena fixes nitrogen and makes it available to the fern. This symbiosis is widely used in rice cultivation as a natural fertilizer.

Associative Diazotrophs:

These bacteria live in close proximity to plant roots, potentially enhancing nitrogen availability.

Azospirillum: Azospirillum is a genus of bacteria that colonizes the roots of various plants, including grasses and cereals. While they do not form nodules like rhizobia, they can enhance plant growth by fixing nitrogen and producing plant growth-promoting substances. The exact mechanisms of nitrogen fixation and plant growth promotion by Azospirillum are still being investigated.
Acetobacter: Some species of Acetobacter are found associated with sugarcane and other plants. They can fix nitrogen within the plant tissues, contributing to the plant's nitrogen nutrition.
Herbaspirillum: Similar to Azospirillum, Herbaspirillum colonizes the roots and stems of various plants and can fix nitrogen.

Mechanisms of Nitrogenase Protection:

Nitrogen-fixing organisms have evolved diverse strategies to protect the oxygen-sensitive nitrogenase enzyme. These include:

Conformational Protection: Some bacteria, like Azotobacter vinelandii, protect nitrogenase by physically binding it to a protein that undergoes a conformational change under high oxygen concentrations, inhibiting its activity. This mechanism is reversible and allows the enzyme to become active again when oxygen levels decrease.
Respiratory Protection: Certain aerobic bacteria, such as Azotobacter, exhibit a high respiration rate, rapidly consuming oxygen and creating a microaerobic environment within the cell. This helps to minimize oxygen exposure to nitrogenase.
Slime Layer Production: Some bacteria produce a thick slime layer or capsule that acts as a physical barrier, limiting oxygen diffusion into the cell. This is observed in Azotobacter and Beijerinckia.
Heterocyst Formation: Cyanobacteria like Anabaena develop specialized cells called heterocysts, which lack photosystem II and have thickened cell walls to restrict oxygen diffusion.
Anaerobic Conditions: Anaerobic bacteria, like Clostridium, thrive in oxygen-free environments, completely avoiding the oxygen sensitivity of nitrogenase.
Leghemoglobin: In the rhizobia-legume symbiosis, the plant produces a protein called leghemoglobin, which binds oxygen and maintains a low oxygen concentration within the nodules. This allows nitrogen fixation to occur efficiently without inhibiting nitrogenase.

Ecological Significance:

Nitrogen-fixing organisms play a crucial role in the global nitrogen cycle and the functioning of various ecosystems:

Primary Input of Fixed Nitrogen: Biological nitrogen fixation is the primary natural mechanism for converting atmospheric nitrogen into a usable form for plants and other organisms.
Soil Fertility: Nitrogen-fixing organisms enhance soil fertility by increasing the availability of nitrogen, a key nutrient for plant growth.
Plant Productivity: Nitrogen fixation directly contributes to plant productivity, supporting agriculture and natural ecosystems.
Ecosystem Stability: Nitrogen-fixing organisms play a vital role in maintaining the stability and health of ecosystems by ensuring a sufficient supply of nitrogen.
Bioremediation: Some nitrogen-fixing organisms have the potential to be used in bioremediation, helping to clean up contaminated soils and water.

Agricultural Applications:

The knowledge of nitrogen-fixing organisms has significant implications for agriculture:

Biofertilizers: Nitrogen-fixing bacteria, such as rhizobia and Azospirillum, are used as biofertilizers to enhance crop yields and reduce the need for synthetic nitrogen fertilizers.
Crop Rotation: Legumes are often used in crop rotation systems to increase soil nitrogen levels and improve the productivity of subsequent crops.
Sustainable Agriculture: Promoting nitrogen fixation in agricultural systems is a key strategy for sustainable agriculture, reducing reliance on synthetic fertilizers and minimizing environmental impacts.

Conclusion:

Nitrogen fixation is a remarkable biological process carried out by a diverse group of microorganisms. These organisms, including free-living bacteria, symbiotic bacteria, and archaea, have evolved unique mechanisms to overcome the challenges of nitrogenase protection. Their contribution to the global nitrogen cycle and the health of ecosystems is immense. Understanding the diversity, mechanisms, and ecological significance of nitrogen-fixing organisms is crucial for developing sustainable agricultural practices and ensuring the long-term health of our planet. Further research into these fascinating organisms will undoubtedly unlock new opportunities for improving nitrogen management and promoting sustainable development.