The primary source of nitrogen fixation in ecosystems is typically certain bacteria, including those found in the root nodules of legumes as well as free-living bacteria in the soil. These microorganisms convert atmospheric nitrogen (N2) into forms that plants can utilize, such as ammonia or related compounds. This process is essential for synthesizing important biological molecules like proteins and nucleic acids, making nitrogen fixation a critical step in the nutrient cycle. Without it, many ecosystems would face nutrient limitations, affecting both plant and animal life.