The process by which nitrogen is converted into forms usable by living organisms is known as nitrogen fixation. In nature, this process is primarily carried out by certain bacteria, including those in the genus Rhizobium, which form symbiotic relationships with legume plants. Nitrogen fixation transforms atmospheric nitrogen (N2) into ammonia (NH3), which can be incorporated into amino acids and nucleotides. This process is crucial for the nitrogen cycle, ensuring that nitrogen is accessible to plants and, subsequently, to animals that rely on plants for food.