Biological Nitrogen Fixation by Legumes

Summary

This video explains the process of biological nitrogen fixation (BNF) by legumes, highlighting its significance in plant growth. While nitrogen is abundant in Earth's atmosphere, plants can't directly use it in its gaseous form. Legumes like beans and peas form a symbiotic relationship with rhizobia bacteria, which convert atmospheric nitrogen into ammonia, a form plants can use. The process requires significant energy from the plant, but it enables legumes to grow without relying on synthetic fertilizers. Biological nitrogen fixation is crucial for sustainable agriculture, with soil bacteria fixing a large portion of nitrogen for legumes.

Takeaways

• 😀 Nitrogen is a critical limiting nutrient for plant growth, making up 78% of the Earth's atmosphere, but it is not directly available to plants in its natural form (N2).
• 😀 The Haber-Bosch process is commonly used to convert nitrogen (N2) from the atmosphere into ammonia, which is then used in fertilizers for plants.
• 😀 Biological nitrogen fixation (BNF) is another way plants, particularly legumes, can obtain nitrogen through a symbiotic relationship with rhizobia bacteria.
• 😀 BNF was first discovered by Martin S. Beijerinck in 1901 and involves the conversion of atmospheric nitrogen into a form usable by plants.
• 😀 Legumes like beans and peas host rhizobia bacteria in their root systems, which help convert atmospheric nitrogen (N2) into ammonia.
• 😀 Only about 20% of legumes are known to fix nitrogen through their symbiotic relationship with rhizobia bacteria.
• 😀 The process starts when legumes release flavonoids from their roots, which attract rhizobia bacteria to the root hairs.
• 😀 Once the rhizobia attach to the root hairs, the plant curls the root hair around the bacteria, trapping them and forming an infection thread.
• 😀 The infection thread eventually forms a nodule, where the bacteria can use nitrogenase enzymes to break down N2 into ammonia, which is then utilized by the plant.
• 😀 This process requires significant energy from the plant, approximately 16 ATP molecules, highlighting the high energy cost of nitrogen fixation.
• 😀 Although energy-intensive, legumes prefer to fix nitrogen from the soil (in the form of nitrate or ammonia) rather than relying solely on biological nitrogen fixation.
• 😀 Biological nitrogen fixation is vital for plant growth, with soil bacteria responsible for fixing between 25% to 90% of total legume nitrogen.

Q & A

What is the importance of nitrogen in plant growth?

Nitrogen is a critical limiting nutrient in both natural and agricultural systems. It is essential for plant growth, as it helps in the formation of proteins, enzymes, and other important cellular components.

Why is nitrogen gas in the atmosphere not available to plants?

Although nitrogen makes up 78% of the Earth's atmosphere, it is in the form of nitrogen gas (N2), which is not directly usable by plants due to its inert nature and inability to be absorbed by plant roots.

How is nitrogen typically provided to plants in agricultural systems?

In agricultural systems, nitrogen is usually provided through the Haber-Bosch process, a chemical process that converts atmospheric nitrogen (N2) into ammonia, which is then used in fertilizers applied to the soil.

What is biological nitrogen fixation (BNF)?

Biological nitrogen fixation (BNF) is the process in which nitrogen gas from the atmosphere is converted into a form that plants can use, typically ammonia. This is achieved by symbiotic bacteria, such as rhizobia, found in the roots of legumes.

Who discovered biological nitrogen fixation, and when?

Biological nitrogen fixation was first discovered in 1901 by Martin S. Beijerinck, a Dutch microbiologist.

What role do legumes play in biological nitrogen fixation?

Legumes, such as beans and peas, have a symbiotic relationship with nitrogen-fixing bacteria (rhizobia). The bacteria fix nitrogen from the atmosphere into ammonia, which the legumes can then use for growth and protein synthesis.

How do legumes attract rhizobia to their roots?

Legumes release flavonoids from their roots, which attract rhizobia bacteria to the root hairs. This initiates the process of attachment and infection, eventually forming a nitrogen-fixing nodule.

What happens when rhizobia attach to the root hairs of legumes?

Once rhizobia attach to the root hairs, the root hair curls around the bacteria, trapping it. The root then forms an infection thread, which eventually leads to the development of a nodule containing the bacteria.

What enzyme is used by rhizobia to convert nitrogen into ammonia, and why is this process energy-intensive?

Rhizobia use an enzyme called nitrogenase to convert nitrogen gas (N2) into ammonia (NH3). This process is energy-intensive because it requires a significant amount of ATP from the plant, specifically 16 ATP molecules, to convert nitrogen and hydrogen into ammonia.

What percentage of nitrogen in legumes is fixed by biological nitrogen fixation?

Biological nitrogen fixation accounts for approximately 25% to 90% of the total nitrogen in legumes, depending on various factors such as environmental conditions and soil health.