Key Role of Bacteria in the Nitrogen Cycle

The delicate balance of substances that is important for maintaining life is an important area of research, and the balance of nitrogen in the environment is no exception. When plants lack nitrogen, they become yellowed, with stunted growth, and produce smaller fruits and flowers. Farmers may add fertilizers containing nitrogen to their crops, to increase crop growth. Without nitrogen fertilizers, scientists estimate that we would lose up to one third of the crops we rely on for food and other types of agriculture. But we need to know how much nitrogen is necessary for plant growth, because too much can pollute waterways, hurting aquatic life.

Nitrogen is required by all living organisms for the synthesis of proteins, nucleic acids and other nitrogen containing compounds. The Earth’s atmosphere contains almost 80 % nitrogen gas. It cannot be used in this form by most living organisms until it has been fixed, that is reduced (combined with hydrogen), to ammonia.

The nitrogen cycle is a series of processes that convert nitrogen gas to organic substances and back to nitrogen in nature. It is a continuous cycle that is maintained by the decomposers and nitrogen bacteria. The nitrogen cycle can be broken down into four types of reaction and micro-organisms play roles in all of these.

Nitrogen fixation describes the conversion of nitrogen into nitrate, a form that can be used by plants. This is carried out by volcanic action, lightning, industrial activity and by some bacteria. Nitrogen-fixing bacteria can be found free in the soil or live in the root nodules of legumes (peas. beans, clover). The bacteria use the plant as a carbohydrate source of energy, protection & shelter (anaerobic conditions too) and the plant uses the nitrate produced by the bacterium (e.g. of mutualism).

Nitrogen enters the living world by way of bacteria and other single-celled prokaryotes, which convert atmospheric nitrogen—text N_2N2start text, N, end text, start subscript, 2, end subscript—into biologically usable forms in a process called nitrogen fixation. Some species of nitrogen-fixing bacteria are free-living in soil or water, while others are beneficial symbionts that live inside of plants.
Nitrogen-fixing microorganisms capture atmospheric nitrogen by converting it to ammonia—text {NH}_3NH3start text, N, H, end text, start subscript, 3, end subscript—which can be taken up by plants and used to make organic molecules. The nitrogen-containing molecules are passed to animals when the plants are eaten. They may be incorporated into the animal’s body or broken down and excreted as waste, such as the urea found in urine.

Nitrogen doesn’t remain forever in the bodies of living organisms. Instead, it’s converted from organic nitrogen back into text N_2N2start text, N, end text, start subscript, 2, end subscript gas by bacteria. This process often involves several steps in terrestrial—land—ecosystems. Nitrogenous compounds from dead organisms or wastes are converted into ammonia—text {NH}_3NH3start text, N, H, end text, start subscript, 3, end subscript—by bacteria, and the ammonia is converted into nitrites and nitrates. In the end, the nitrates are made into text N _2N2start text, N, end text, start subscript, 2, end subscript gas by denitrifying prokaryotes.
Bacteria play a central role: Nitrogen-fixing bacteria, which convert atmospheric nitrogen to nitrates. Bacteria of decay, which convert decaying nitrogen waste to ammonia. Nitrifying bacteria, which convert ammonia to nitrates/nitrites.


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Tahira Nadeem

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Nathan Nguyen
2 years ago

Good job Tahira!

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