Model Answer
0 min readIntroduction
Nitrogen, though abundant in the atmosphere as inert N<sub>2</sub> gas, is often a limiting nutrient for plant growth as most organisms cannot directly utilize it. Biological Nitrogen Fixation (BNF) is a critical biological process by which certain prokaryotic microorganisms convert atmospheric nitrogen into usable forms, primarily ammonia (NH<sub>3</sub>), which plants can then assimilate for synthesizing vital biomolecules like proteins and nucleic acids. This natural process is indispensable for maintaining ecosystem productivity and soil fertility, offering a sustainable alternative to synthetic nitrogen fertilizers whose excessive use contributes to environmental pollution and greenhouse gas emissions. Among the various types of BNF, symbiotic nitrogen fixation stands out due to its profound agricultural and ecological importance.
What is Biological Nitrogen Fixation?
Biological Nitrogen Fixation (BNF) is the biochemical process mediated by specialized microorganisms (diazotrophs) that transform atmospheric dinitrogen (N2) gas into ammonia (NH3). This conversion is catalyzed by the enzyme complex nitrogenase. The overall reaction is:
N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16 Pi
This process is energetically intensive, requiring a significant amount of ATP, which is supplied by the host plant in symbiotic associations. BNF is a cornerstone of the global nitrogen cycle, making nitrogen available to the biosphere and supporting primary productivity.
Mechanism of Symbiotic Nitrogen Fixation
Symbiotic nitrogen fixation involves a mutualistic relationship between nitrogen-fixing microorganisms and plants, where both partners benefit. The most well-studied and agriculturally significant example is the association between leguminous plants (e.g., peas, soybeans, clover) and bacteria of the genus Rhizobium (collectively known as rhizobia).
Stages of Symbiotic Nitrogen Fixation (Rhizobium-Legume Symbiosis):
- Signaling and Attraction:
- Leguminous roots release chemical signals called flavonoids into the rhizosphere.
- These flavonoids attract specific Rhizobium bacteria present in the soil.
- In response, Rhizobium produces Nod factors (Nodulation factors), which are lipo-oligosaccharides, triggering changes in the plant root.
- Root Hair Infection and Curling:
- Nod factors induce curling of the root hair cells of the host plant.
- The bacteria then attach to the curled root hair and invade it.
- This leads to the formation of an infection thread, a tubular structure that guides the bacteria through the root hair cell and into the cortical cells of the root.
- Nodule Formation:
- As the infection thread reaches the cortical cells, the bacteria are released into these cells, but remain enclosed within a plant-derived membrane, forming symbiosomes.
- The infected cortical cells undergo rapid division and enlargement, leading to the formation of specialized structures called root nodules.
- Within the mature nodule, the rhizobia differentiate into pleomorphic, nitrogen-fixing forms called bacteroids.
- Nitrogen Fixation and Ammonia Assimilation:
- Inside the bacteroids, the nitrogenase enzyme complex converts atmospheric N2 into ammonia (NH3).
- The nitrogenase enzyme is extremely sensitive to oxygen and is irreversibly inactivated in its presence. To protect nitrogenase, the plant synthesizes a protein called leghemoglobin (leghaemoglobin).
- Leghemoglobin, similar to hemoglobin in animal blood, binds oxygen, creating a micro-anaerobic environment essential for nitrogenase activity, while still allowing enough oxygen for the bacteroids' respiration (which provides the ATP for fixation).
- The fixed ammonia (NH3) is rapidly protonated to ammonium (NH4+) and then assimilated by the plant into amino acids (e.g., glutamine, aspartate) and other nitrogenous compounds, which are then transported to other parts of the plant. The plant supplies carbohydrates from photosynthesis to the bacteroids as an energy source.
Examples of Symbiotic Nitrogen Fixation:
Symbiotic nitrogen fixation is not limited to legumes. Other plant-microbe associations also exist.
- Legume-Rhizobium Symbiosis:
- Plants: Peas, soybeans, alfalfa, clover, lentils, chickpeas, groundnuts, beans.
- Bacteria: Rhizobium species (e.g., Rhizobium leguminosarum, Bradyrhizobium japonicum), Mesorhizobium, Azorhizobium.
- Non-leguminous Plant Symbiosis (Actinorhizal Symbiosis):
- Some non-leguminous plants form root nodules with the actinomycete bacterium Frankia. These plants are often pioneer species in nutrient-poor soils.
- Plants: Alder trees (Alnus sp.), Casuarina, Sea Buckthorn (Hippophae), Sweet Gale (Myrica gale).
- Cyanobacteria Symbiosis:
- Certain cyanobacteria (formerly blue-green algae), such as Anabaena and Nostoc, form symbiotic associations with plants.
- Examples: Azolla (a water fern) with Anabaena azollae; Cycads with Nostoc in their coralloid roots; some bryophytes like Anthoceros with Nostoc.
The pink or reddish color of healthy root nodules is due to the presence of leghemoglobin, indicating active nitrogen fixation. This process is crucial for sustainable agriculture, reducing the reliance on synthetic fertilizers and improving soil health.
Conclusion
Biological nitrogen fixation, particularly symbiotic nitrogen fixation, is a cornerstone of global food security and ecological balance. By converting atmospheric nitrogen into bioavailable forms, it significantly enhances soil fertility and plant productivity. The intricate mechanism involving host plant signals, microbial responses, nodule formation, and the oxygen-scavenging action of leghemoglobin highlights a remarkable evolutionary collaboration. Promoting and optimizing these natural processes, through practices like crop rotation and biofertilizer use, can substantially reduce the environmental footprint of agriculture, contributing to a more sustainable and resilient food system for the future.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.