Nitrogen fixation is the biochemical process that converts atmospheric nitrogen gas into biologically usable forms such as ammonia or nitrate.
Biochemical mechanisms
Biological nitrogen fixation is carried out by certain bacteria and archaea that possess the nitrogenase enzyme complex. Nitrogenase is composed of the dinitrogenase reductase (Fe protein) and dinitrogenase (MoFe protein) that work together to reduce inert N₂ to ammonia. This reaction requires a large input of energy: at least 16 molecules of ATP and a strong reducing power per molecule of nitrogen reduced. The enzyme’s active site contains iron and molybdenum or vanadium cofactors that facilitate electron transfer. Nitrogenase is highly sensitive to oxygen, so nitrogen fixers employ strategies to protect the enzyme. Heterocysts in filamentous cyanobacteria create micro-anaerobic compartments, legume root nodules produce leghemoglobin to bind oxygen, and free-living aerobes increase respiratory rates or produce protective slime layers. Some organisms also reduce nitrogen to hydrogen gas as a by‑product. The fixed ammonia is assimilated into glutamine and glutamate for incorporation into amino acids and nucleotides, completing the assimilation pathway.
Ecological and agricultural significance
Nitrogen fixation is a cornerstone of the global nitrogen cycle, balancing the large loss of usable nitrogen through denitrification and leaching. Symbiotic associations between legumes and Rhizobium or Bradyrhizobium bacteria form nodules on plant roots where nitrogen fixation occurs, supplying the host with reduced nitrogen while receiving carbohydrates and a protected environment. Actinorhizal plants form similar associations with Frankia species, and many cyanobacteria fix nitrogen in aquatic environments, contributing to primary productivity. Free-living bacteria such as Azotobacter, Clostridium and some archaea also fix nitrogen independently in soils and sediments. These biological processes contribute more nitrogen to ecosystems than lightning or industrial sources. In agriculture, legume cultivation and green manures harness symbiotic nitrogen fixation to improve soil fertility and reduce the need for synthetic fertilizers. Industrial nitrogen fixation via the Haber-Bosch process produces ammonia for fertilizers but consumes significant fossil fuel energy and contributes to greenhouse gas emissions.
Nitrogen fixation links atmospheric nitrogen to the biosphere, sustaining life by providing the building blocks for proteins and nucleic acids. Understanding and managing this process is essential for food security and environmental sustainability.
Related Terms: Nitrogenase, Rhizobium, Symbiosis, Nitrogen cycle, Legume