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How do nitrogen-fixing bacteria make atmospheric nitrogen useful to plants?
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Atmosphere
The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.
Although the atmosphere is composed of approximately 78 percent nitrogen gas, most living organisms cannot utilize it in this inert form. Nitrogen-fixing bacteria bridge this gap by transforming atmospheric nitrogen into fixed nitrogen, which consists of inorganic compounds that plants can absorb and use for development.12
These bacteria facilitate this process through the use of nitrogenase, an enzyme that catalyzes the reduction of molecular nitrogen into ammonia. This conversion occurs in two primary ways:3
- Symbiotic bacteria: Microorganisms such as Rhizobium invade the root hairs of host plants, forming root nodules. Within these structures, the bacteria convert nitrogen into ammonia, which the host plant directly utilizes.2
- Free-living bacteria: Organisms like Azotobacter and certain cyanobacteria perform this fixation independently in the environment, contributing to the nitrogen cycle.2
Once converted into ammonia or nitrates, the nitrogen becomes available for plants to synthesize essential molecules like DNA, proteins, and chlorophyll.1