Which organisms are primarily responsible for atmospheric nitrogen fixation?

Atmospheric nitrogen fixation occurs primarily through the action of certain bacteria and archaea, which possess the unique ability to convert nitrogen gas (N2) from the atmosphere into ammonia (NH3), a form that can be utilized by living organisms. This process is essential for the nitrogen cycle, as nitrogen is a vital nutrient necessary for the synthesis of amino acids, proteins, and nucleic acids.

Many free-living and symbiotic bacteria are involved in nitrogen fixation. For example, the genus Rhizobium forms nodules on legume roots and provides them with ammonia, while cyanobacteria also fix nitrogen in aquatic environments. Archaea, particularly certain methanogens, contribute to nitrogen fixation in extreme environments.

In contrast, plants and animals do not fix atmospheric nitrogen themselves; they depend on nitrogen-fixing organisms to convert atmospheric nitrogen into usable forms. Similarly, fungi and algae do not perform nitrogen fixation in a significant way; while some algae, like cyanobacteria, can fix nitrogen, the majority of nitrogen-fixing activities are performed by specialized bacteria. Protozoa and viruses play different roles in ecosystems and do not contribute to the process of nitrogen fixation. Thus, the primary responsibility for atmospheric nitrogen fixation lies with bacteria and archaea.