Which statement defines nitrogen fixation?

• A. Conversion of atmospheric N2 into ammonia by bacteria or industrial processes.
• B. Movement of nitrogen through the food chain.
• C. Conversion of ammonia into nitrate.
• D. Release of nitrogen gas from the ocean.

Answer: (A)

Nitrogen fixation is the process of converting inert atmospheric nitrogen (N2) into a form that living organisms can use, typically ammonia (NH3) or ammonium (NH4+). This transformation is carried out by specialized bacteria—often in root nodules of legumes or as free-living cyanobacteria in aquatic systems—and can also occur industrially (Haber-Bosch) to produce fertilizer. By turning N2 into ammonia, nitrogen becomes available for incorporation into amino acids, nucleotides, and other biomolecules, fueling growth. The statement that best defines nitrogen fixation describes this conversion of atmospheric N2 into ammonia by bacteria or industrial processes, which is the essential step that makes nitrogen biologically usable. The other options describe different parts of the nitrogen cycle—nutrient movement through the food chain, nitrification (ammonia to nitrate), or the release of nitrogen gas from ocean systems—not fixation itself.