nitrogenfixer

Nitrogen fixer, or biological nitrogen fixer, is an organism capable of converting atmospheric nitrogen (N2) into ammonia or related bioavailable forms, enabling incorporation of nitrogen into organic molecules. This biological nitrogen fixation is a key step in the global nitrogen cycle and supports plant growth in nitrogen-poor soils. The process is carried out by certain bacteria and archaea and occurs in two main contexts: symbiotic associations with plants and free-living or associative fixation in the soil or water.

Symbiotic nitrogen fixers include rhizobia, which infect legume roots and form root nodules where the bacteria receive carbohydrates from the plant and protect nitrogenase from oxygen while fixing N2. Frankia bacteria form nodules on actinorhizal plants such as alder. Free-living or associative nitrogen fixers include genera such as Azotobacter, Clostridium, and cyanobacteria (like Anabaena and Nostoc), which fix nitrogen in the absence of a plant partner or near the plant rhizosphere. The core enzymatic machinery is nitrogenase, a complex of dinitrogenase and its iron-molybdenum cofactor, which reduces N2 to ammonia using ATP and electrons. Because nitrogenase is highly oxygen-sensitive, organisms have strategies to maintain low intracellular oxygen or protect the enzyme, such as leghemoglobins in nodules or heterocysts in cyanobacteria.

Ecological and agricultural significance: biological nitrogen fixation supplements soil nitrogen, reducing the need for synthetic fertilizers and supporting crop yields in legumes and in ecosystems with limited mineral nitrogen. In horticulture and agriculture, inoculants containing Rhizobium or other fixers are used to enhance legume crops.