The Nitrogen Cycle

All life requires nitrogen, as it is the building block of proteins (molecules made of amino acids that function at the cellular level that perform many different reactions to sustain life) and deoxyribonucleic acid (DNA), the information-storage unit of cells that codes for genes and proteins. Nitrogen is a very abundant element, making up 80% of Earth’s atmosphere in its gaseous (N2) phase. However, most organisms require nitrogen in the form of the nitrate ion (NO3-) or the ammonium ion (NH4+) and cannot use the atmospheric nitrogen directly. Because nitrogen is a relatively unreactive gas, few processes convert molecular nitrogen to nitrate or ammonium. The major source of conversion of molecular nitrogen to these compounds is through bacterial nitrogen fixation-the process of converting molecular nitrogen to ammonium or nitrate.

One such bacterium is the Rhizobia, an organism that “teams” up with different species of leguminous plants to form what’s known as a symbiotic relationship. Essentially, the Rhizobia attach to the root systems of the legume and convert atmospheric nitrogen (N2) into amino acids that the plant will then use to build proteins and grow. In return, the plant will provide carbohydrates (sugars used for energy storage) to the Rhizobia and a place to attach. When the plant dies and decomposes, the nitrogen from its tissues is made available to other plants and organisms.


However, nitrate and ammonium does not continue to concentrate in the environment as time goes on. Other species of bacteria and archaea use nitrate (NO3-) as electron acceptors during cellular respiration, which causes it to be converted back to molecular nitrogen (N2) in a process known as denitrification. This cycle in which nitrogen is circulated from its molecular form to biologically usable forms is called the nitrogen cycle. The circulation of nitrogen keeps a delicate balance to the amount of nitrogen kept in the soil and in organisms.


Below is a short stop-motion video of the process at work.