- Nitrogen-fixing bacteria are specialized microorganisms capable of transforming atmospheric nitrogen gas (N₂)—which most living organisms cannot use—into ammonia (NH₃) or ammonium ions (NH₄⁺), which are forms that plants can readily absorb and utilize.
- These bacteria play a central role in the nitrogen cycle, making them essential for maintaining soil fertility and supporting plant growth. Without their activity, ecosystems and agriculture would struggle to meet the nitrogen demands of living organisms.
- There are two main categories of nitrogen-fixing bacteria: free-living (non-symbiotic) and symbiotic. Free-living nitrogen fixers, such as Azotobacter, Clostridium, and many cyanobacteria (like Anabaena and Nostoc), reside independently in the soil or aquatic environments. These bacteria fix nitrogen on their own, often under aerobic or anaerobic conditions, depending on the species. For example, Azotobacter is aerobic and thrives in well-aerated soils, while Clostridium is anaerobic and prefers oxygen-poor environments.
- The symbiotic nitrogen-fixing bacteria form mutually beneficial relationships with specific host plants. The most well-known examples are species of Rhizobium, Bradyrhizobium, and Sinorhizobium, which associate with leguminous plants (like peas, beans, lentils, and clover). These bacteria invade the root hairs of their host plant and stimulate the formation of root nodules, where they reside and convert atmospheric nitrogen into ammonia in exchange for carbohydrates and a protected environment. This symbiosis significantly enriches the soil with nitrogen compounds, reducing the need for synthetic fertilizers and benefiting neighboring plants.
- The enzyme responsible for nitrogen fixation in all these bacteria is nitrogenase, which catalyzes the reduction of N₂ into ammonia. However, nitrogenase is extremely sensitive to oxygen, which can inhibit its function. Symbiotic bacteria overcome this by producing protective molecules like leghemoglobin within the root nodules, which bind to oxygen and maintain a low-oxygen environment, allowing nitrogenase to function effectively.
- In aquatic ecosystems, cyanobacteria also play a critical role in nitrogen fixation. Some of these, like Trichodesmium, are important in marine environments, particularly in nutrient-poor regions where nitrogen fixation supports the base of the food web. In rice paddies and wetlands, Anabaena and other filamentous cyanobacteria help sustain nitrogen levels in flooded soils.
- From an agricultural perspective, nitrogen-fixing bacteria are invaluable. They are harnessed in biofertilizers to naturally enhance soil nitrogen content, promoting sustainable farming practices and reducing dependency on chemical nitrogen fertilizers. This not only cuts costs for farmers but also helps prevent environmental issues like eutrophication caused by fertilizer runoff into water bodies.
