The main role of nitrogen-fixing bacteria in ecosystems is to convert atmospheric nitrogen (N₂) into a form usable by plants, such as ammonium or nitrate. This process, known as nitrogen fixation, is essential for plant growth, as nitrogen is a crucial nutrient for synthesizing proteins and nucleic acids. These bacteria, often found in the root nodules of legumes or freely in the soil, enhance soil fertility and promote plant diversity. By supporting the nitrogen cycle, nitrogen-fixing bacteria contribute significantly to ecosystem productivity and health.