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Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen (N2) in the atmosphere is converted into ammonia (NH3). Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.

Microorganisms (Diazotrophs) that fix nitrogen

  • Cyanobacteria
  • Azotobacteraceae
  • Rhizobia
  • Frankia

Rhizobia are Gram-negative soil bacteria with the ability to establish a N2-fixing symbiosis on legume roots and on the stems of some aquatic legumes. During this interaction bacteroids, as rhizobia are called in the symbiotic state, are contained in intracellular compartments within a specialized organ, the nodule, where they fix N2. Rhizobia within nodules in their root systems, produce nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil. Similarly, Frankia, Gram-positive soil bacteria induce the formation of nitrogen-fixing nodules in actinorhizal plants.