Nitrogen is one of the most essential elements for life. It is a key ingredient of proteins and of DNA itself. And it is astonishingly abundant—the air is roughly four-fifths nitrogen gas. Yet despite being surrounded by it, most living things cannot use the nitrogen in the air at all. The story of how life solves this paradox is the nitrogen cycle.

Surrounded by an Unusable Resource

Here is the paradox. The atmosphere is full of nitrogen gas. But nitrogen gas is held together by an extremely strong chemical bond between its two atoms—one of the strongest in nature.

This bond is so strong that nitrogen gas is, for most organisms, chemically inert and unusable. Plants cannot absorb it; animals cannot use it. Life is, in effect, starving for nitrogen while immersed in an ocean of it. The atmospheric nitrogen must first be converted into a usable form.

Nitrogen Fixation: Unlocking the Air

The crucial step that unlocks atmospheric nitrogen is called nitrogen fixation. It is the process of breaking that powerful bond and converting nitrogen gas into a chemically usable form.

This is a difficult feat, and in nature it is accomplished mainly by certain microbes—specialized bacteria with the rare ability to "fix" nitrogen. Some of these live freely in soil and water; others live in close partnership with plants, famously in the roots of legumes, supplying their plant hosts with usable nitrogen in exchange for nourishment.

These nitrogen-fixing microbes are, quietly, among the most important organisms on Earth. They are the gateway through which the air's vast nitrogen supply enters the living world.

Passing Through Life

Once nitrogen has been fixed into a usable form, it can move through ecosystems. Plants take up usable nitrogen from the soil and build it into their proteins and other molecules. Animals obtain nitrogen by eating plants or other animals.

In this way, fixed nitrogen passes along the food chain, incorporated and reincorporated into living tissue—much as carbon does.

The Return to the Air

For the cycle to be complete, nitrogen must eventually be returned to the atmosphere.

When organisms produce waste and when they die, decomposer microbes break down the nitrogen-containing molecules, releasing nitrogen back into the soil in various forms. And a further group of microbes carries out the final step, converting nitrogen compounds back into nitrogen gas, which returns to the atmosphere.

The loop is then closed, ready to begin again with fixation.

A Cycle Run by Microbes

The most striking feature of the nitrogen cycle is how much of it depends on microbes. Fixation, the transformations within the soil, and the final return to the air are all driven largely by bacteria. The grand cycle of an element essential to all life is, in large part, microbial work.

The Hidden Engine

The nitrogen cycle is a hidden engine. It runs quietly, mostly underground and out of sight, mostly by organisms too small to see. Yet without it, the proteins and DNA of every living thing could not be built. Understanding the nitrogen cycle reveals one of the deepest truths of biology: that life on the grand scale depends on the unseen labor of microbes, and that the nature we admire is sustained by chemistry we rarely notice.