Which of these organisms use nitrogen directly from the air?

• A. Blue-Green Algae
• B. Fungi
• C. Bacteria
• D. Animals

Answer: (A)

Nitrogen fixation is the process of converting atmospheric N2 into a form that organisms can use to build vital molecules like proteins and nucleic acids. Blue-green algae, or cyanobacteria, can fix nitrogen directly from the air. They use the enzyme nitrogenase to convert N2 into ammonia, and many species keep this process safe from oxygen by localizing it in specialized cells called heterocysts that protect the enzyme during photosynthesis. This allows them to obtain nitrogen straight from the atmosphere rather than relying on fixed nitrogen compounds in the soil or environment. Fungi and animals, and most bacteria, obtain nitrogen from already fixed forms (organic matter or mineral nitrogen), not directly from air. So blue-green algae are the organisms that use nitrogen directly from the air.