Nitrogen Fixation and Nitrogen Metabolism | Botany Optional for UPSC PDF Download

Introduction

In the realm of living organisms, carbon, hydrogen, and oxygen reign supreme as the most abundant elements. However, a lesser-known but equally vital constituent is nitrogen. Nitrogen plays a crucial role in essential components such as proteins, hormones, amino acids, and vitamins. This article delves into the intricate world of nitrogen metabolism, elucidating the nitrogen cycle and its biological fixation, all while offering illustrative examples to enhance our understanding.

The Nitrogen Cycle

The cornerstone of nitrogen metabolism is the nitrogen cycle, a relentless exchange of nitrogen between three primary reservoirs: the atmosphere, soil, and biomass. This cycle is fundamental as nitrogen is a limited resource in the soil, contested for by both plants and microbes, making it a pivotal nutrient for agricultural and natural ecosystems.

Atmospheric Pool

Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃). This can occur through three main methods:

• Biological nitrogen fixation involves living organisms that reduce nitrogen to ammonia.
• Industrial nitrogen fixation involves human activities such as industrial combustions, automobile exhausts, forest fires, and power-generating stations as sources of nitrogen.
• Electrical nitrogen fixation occurs when natural forces like lightning and ultraviolet radiation provide the energy needed to convert nitrogen to nitrogen oxides.

Soil Pool

Atmospheric nitrogen fixed through these processes eventually makes its way into the soil. Nitrogen in the soil is then taken up by plants and used for growth.

Biomass Pool

When plants and animals die, the organic nitrogen within them undergoes decomposition, leading to the release of ammonia. This process is known as ammonification and returns nitrogen back to the soil. Some ammonia evaporates and re-enters the atmosphere, while a significant portion is converted into nitrate by soil bacteria.

• Nitrification is the process by which ammonia is oxidized to nitrite and then to nitrate by specific bacteria (Nitrosomonas, Nitrococcus, and Nitrobacter).
• These reactions are crucial for making nitrogen available to plants in the form of nitrates. Plants absorb nitrate and transport it to their leaves, where it is reduced to ammonia and incorporated into amino acids.

Denitrification

In addition to nitrification, there is a process called denitrification, which is carried out by bacteria like Pseudomonas and Thiobacillus. Denitrification converts nitrates in the soil back into atmospheric nitrogen, closing the nitrogen cycle loop.

Biological Nitrogen Fixation

Biological nitrogen fixation is a vital process in which specific prokaryotic organisms convert atmospheric nitrogen (N₂) into ammonia (NH₃). This conversion is made possible by the enzyme nitrogenase, which is exclusively present in prokaryotes capable of this process. These nitrogen-fixing microorganisms are referred to as N₂-fixers and can either be free-living or engage in symbiotic relationships with plants.
Examples of free-living N₂-fixers include Azotobacter, Bacillus, Anabaena, and Nostoc.

Symbiotic Biological Nitrogen Fixation

In symbiotic biological nitrogen fixation, certain plants, particularly legumes such as sweet pea, garden pea, and lentils, form a mutualistic relationship with nitrogen-fixing bacteria known as Rhizobium. This association results in the formation of specialized structures called nodules on the plant's roots. Another group of nitrogen-fixing microbes called Frankia can also establish similar nitrogen-fixing nodules on the roots of non-leguminous plants.

Nodule Formation

The process of nodule formation involves several interactions between the host plant's roots and Rhizobium:

• Rhizobia multiply and attach themselves to the root's epidermal and root-hair cells.
• The root hair curls, allowing the bacteria to invade, create an infection thread, and reach the root's cortex, initiating nodule formation.
• The infection thread releases the bacteria into the plant's cells, leading to the differentiation of specialized nitrogen-fixing cells within the nodule.
• The nodule contains the enzyme nitrogenase, which converts atmospheric nitrogen into ammonia, a critical nitrogen compound for plants. This conversion requires energy, which is provided by the host plant's cell respiration.
• To protect nitrogenase from oxygen, which can inhibit its function, the nodule contains leg-hemoglobin, an oxygen-scavenger.

Fate of Ammonia

The ammonia generated during nitrogen fixation is protonated to form ammonium ions (NH₄⁺) at physiological pH levels. While plants can accumulate nitrate and ammonium ions, excessive ammonium ions can be toxic to them. Therefore, plants use ammonium ions to synthesize amino acids through specific metabolic pathways:

• Reductive amination: Ammonia reacts with α-ketoglutaric acid in the presence of the enzyme glutamate dehydrogenase to form glutamic acid.
• Transamination: In this process, the amino group of one amino acid is transferred to the keto group of a keto acid, facilitated by the enzyme transaminase. Important plant amides like asparagine and glutamine are derived from this process.

The resulting amino acids, which contain valuable nitrogen, are transported through the xylem to different parts of the plant to support various growth and metabolic processes.