The model provided by the sulfur cycle can be extended to other biologically important elements. Nitrogen is the fourth most abundant element in the human body, contributing 4% by weight as a component of proteins, nucleic acids, and other compounds. As is true of sulfur, primary producers incorporate inorganic forms of nitrogen into biomolecules, and we get the nitrogen we need from our food. As we’ll see, however, there is a twist.

Let’s examine the biological nitrogen cycle (Fig. 26.11). Carbon is a minor component of the atmosphere and sulfur only a trace constituent, but the air we breathe consists mostly (78%) of nitrogen gas (N₂). Indeed, most of the nitrogen at Earth’s surface resides in the atmosphere. Although nitrogen is plentiful, the nitrogen available to primary producers in many environments is limited. The reason is that plants cannot make use of nitrogen gas and neither can algae. Only certain bacteria and archaeons can reduce N₂ to ammonia (NH₃), a form of nitrogen that can be incorporated into biomolecules.

The process of converting N₂ into a biologically useful form such as ammonia is called nitrogen fixation, and it is one of the most important reactions ever evolved by organisms. Farmers plant soybeans to regenerate nitrogen nutrients in soils, but it is not the soybean that fixes nitrogen. Rather, soybean roots have nodules that harbor nitrogen-fixing bacteria (Fig. 26.12). That is, plants like soybean have entered into an intimate partnership with nitrogen-fixing bacteria to obtain the biologically useful forms of nitrogen needed for growth (Chapter 29).

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The nitrogen cycle, then, begins with nitrogen fixation, and it continues through many additional metabolisms, most of them confined to bacteria and archaeons. Ammonia released by the decomposition of dead cells can be taken up by primary producers, but commonly it is oxidized to nitrate (NO₃–) by chemoautotrophic bacteria, a process called nitrification (see Fig. 26.11). This process works efficiently in both soil and water, and so nitrate is the most common form of nitrogen available to life in both environments.

Anaerobic respiration completes the cycle by returning N₂ to the atmosphere. Some bacteria can use nitrate as an electron acceptor in respiration, a process known as denitrification (see Fig. 26.11). Because denitrification is so efficient at returning nitrogen to the atmosphere as nitrogen gas, nitrogen fixation must continually make nitrogen available for organisms. Without nitrogen fixation, the nitrogen cycle would slowly wind down, and the productivity of Earth’s biota—that is, of all its organisms—would wind down with it. This is why nitrogen fixation is such an important process.

Until the 1990s, the nitrogen cycle was described—completely, it was thought—in terms of the metabolic reactions just discussed. Then, a new and unexpected form of nitrogen metabolism was discovered in the oceans. A number of prokaryotic microorganisms function as chemoautotrophs by reacting ammonium ion (NH₄+) with nitrite (NO₂–) to gain energy. The reaction is shown here:

NH₄+ + NO₂– → N₂ + 2H₂O

Note that this is a redox reaction, like other reactions involved in energy metabolism. In this case, the ammonium ion is oxidized to nitrogen gas, and nitrite is reduced to nitrogen gas, with water produced as a by-product. The reaction, called anammox (anaerobic ammonia oxidation), provides only a small amount of energy, but it supports many bacteria and archaeons in oxygen-depleted parts of the ocean, providing a major route by which fixed nitrogen returns to the atmosphere as N₂ (see Fig. 26.11).

The biological nitrogen cycle, then, begins and ends with prokaryotic metabolisms. Bacteria and archaeons fix N₂ into a biologically available form, nitrifying bacteria oxidize ammonia to nitrate, and still other prokaryotes gain energy from denitrification and anammox, returning N₂ to the air. Unlike the carbon and sulfur cycles, the nitrogen cycle does not rely at any point on elements stored as minerals in rocks and sediments. Most of Earth’s nitrogen was released to the atmosphere as gas early in our planet’s history, and so movement into sediments and out of volcanoes contributes relatively little to the cycle. Humans, however, contribute in important ways, most notably through the industrial synthesis of nitrate fertilizer (Chapter 49).

Quick Check 5 How would the nitrogen cycle operate in the absence of bacteria and archaeons?