Nitrogen Fixation

All living organisms require nitrogen compounds for survival. Unfortunately, most of these organisms cannot absorb nitrogen from its most abundant source—the atmosphere. Atmospheric nitrogen consists of N₂ molecules, which are very unreactive due to the strong nitrogen-nitrogen triple bond. However, a few organisms can overcome this problem through a process known as nitrogen fixation, illustrated in the figure below.

A flow chart is shown. A cow, grass, and a tree are shown in the center of the diagram. Downward-facing arrows lead from them to the phrase, “Decomposers ( aerobic and anaerobic bacteria and fungi ).” A downward-facing arrow leads to a space-filing model with one blue atom bonded to four white atoms. The model is labeled, “Ammonium ( N H subscript 4 ).” A right-facing arrow leads from this molecule to another molecule that is composed of a blue atom bonded to two red atoms. The model is labeled, “Nitrites ( N O subscript 2 superscript negative sign ).” Below this arrow is a picture of a circle with two rod-shaped structures. It is labeled, “Nitrifying bacteria.” Above the nitrites label is an upward-facing arrow leading to a blue atom single-bonded to three red atoms. The model is labeled, “Nitrates ( N O subscript 3 superscript negative sign ).” Next to this arrow is a picture of a circle with two rod-shaped structures labeled, “Nitrifying bacteria.” The nitrates label has a double-headed, upward-facing arrow that leads to two pictures: one of the roots of the tree which is labeled, “Assimilation,” and one leading to a picture of a circle with four oval-shaped structures labeled, “Denitrifying bacteria.” A left-facing arrow leads from this bacteria to a molecule made up of two atoms triple-bonded together and labeled, “Atmospheric nitrogen ( N subscript 2 ).” This molecule is connected to a downward-facing, double-headed arrow that leads to an image showing yellow filaments on a black background and a picture of a circle with four rod-shaped structures labeled, “Nitrogen-fixing soil bacteria.” An arrow leads from a picture of a plant’s roots to the yellow filaments and then to a photo of a circle with four oval-shaped structures labeled, “Nitrogen-fixing bacteria in root nodules.”

All living organisms require nitrogen. A few microorganisms are able to process atmospheric nitrogen using nitrogen fixation. (credit “roots”: modification of work by the United States Department of Agriculture; credit “root nodules”: modification of work by Louisa Howard)

Nitrogen fixation is the process where organisms convert atmospheric nitrogen into biologically useful chemicals. To date, the only known kind of biological organisms capable of nitrogen fixation are microorganisms. These organisms employ enzymes called nitrogenases, which contain iron and molybdenum. Many of these microorganisms live in a symbiotic relationship with plants, with the best-known example being the presence of rhizobia in the root nodules of legumes.

This lesson is part of:

Metals, Metalloids, and Nonmetals

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