Nitrogen is fundamental to all life, forming the structural basis of DNA, RNA, and proteins. The major reservoir of this element is the atmosphere, which is composed of roughly 78% nitrogen gas (\(\text{N}_2\)). However, this atmospheric form is chemically inert due to a strong triple bond, making it unusable by most organisms. Nitrogen fixation is the process that converts this inert atmospheric nitrogen into reactive compounds, like ammonia (\(\text{NH}_3\)), that can be absorbed by living things.
Biological Organisms Responsible
The vast majority of natural nitrogen capture is performed exclusively by certain types of prokaryotes. Only these bacteria and archaea possess the necessary enzymatic machinery to break the triple bond of atmospheric nitrogen. These nitrogen-fixing microbes, often called diazotrophs, are categorized by their relationship with other organisms.
One well-known group is the symbiotic fixers, such as the genus Rhizobium, which partner with plants in the legume family (peas, beans, and clover). The bacteria invade the plant roots, stimulating the formation of specialized structures called nodules. Within these nodules, the bacteria receive carbohydrates for energy and fix atmospheric nitrogen into ammonia for the plant’s use.
Other nitrogen-capturing organisms are free-living, existing independently in the soil and aquatic environments. Examples include the aerobic soil bacterium Azotobacter and certain cyanobacteria, such as Anabaena. Cyanobacteria are significant in aquatic systems, fixing nitrogen in specialized cells called heterocysts to shield the process from oxygen. These microbial communities are responsible for approximately 90% of all natural nitrogen fixation on the planet.
The Mechanism of Nitrogen Fixation
Nitrogen fixation is one of the most energy-intensive reactions in nature. It is catalyzed by a large, complex enzyme called nitrogenase, which exists solely in the nitrogen-fixing prokaryotes. This enzyme complex works by sequentially adding electrons and protons to the nitrogen molecule. The reaction requires a significant input of cellular energy, consuming at least 16 molecules of adenosine triphosphate (ATP) for every molecule of nitrogen gas converted.
The nitrogenase enzyme is extremely sensitive to oxygen, which irreversibly damages its iron-sulfur clusters and prevents it from functioning. To overcome this challenge, nitrogen-fixing organisms have evolved various protection strategies. In legume root nodules, the plant produces leghemoglobin, which binds to oxygen and maintains a low-oxygen environment. Free-living organisms employ strategies like high respiration rates to consume oxygen quickly or forming protective slime layers.
The result of the nitrogenase enzyme’s action is the reduction of atmospheric nitrogen (\(\text{N}_2\)) to two molecules of ammonia (\(\text{NH}_3\)). This ammonia is the first stable, usable form of nitrogen produced by the fixation process. Once formed, the ammonia is rapidly incorporated into amino acids and other nitrogen-containing compounds, making the essential element available to the broader food web.
Atmospheric and Industrial Capture
Nitrogen can also be captured and converted into reactive forms through non-biological processes. Atmospheric fixation occurs naturally through high-energy events like lightning strikes. The intense heat and energy of a lightning bolt provide enough power to break the triple bond of the nitrogen molecule.
When the nitrogen atoms react with oxygen in the air, they form various nitrogen oxides (\(\text{NO}_x\)), which then dissolve in rainwater. This process deposits small amounts of nitrogen compounds onto the soil, where they can be utilized by plants. This natural phenomenon, while less significant than biological fixation, represents a continuous abiotic source of fixed nitrogen.
The most impactful non-biological method of nitrogen capture is the industrial Haber-Bosch process. Developed in the early 20th century, this process combines atmospheric nitrogen with hydrogen gas under extremely high temperatures and pressures using a catalyst to produce ammonia. The massive scale of the Haber-Bosch process has been transformative, as the resulting ammonia is the primary ingredient in synthetic fertilizers that support the global food supply. However, the industrial process consumes approximately 2% of the world’s total energy supply, contrasting sharply with the efficient biological method.