Which type of organisms are capable of fixing atmospheric nitrogen (N_2) into a biologically usable form?

• A. Certain prokaryotes, such as bacteria and archaea
• B. All plants
• C. Fungi
• D. All animals

Answer: (A)

Verified step by step guidance

1. Understand the concept of nitrogen fixation: Nitrogen fixation is the process by which atmospheric nitrogen (N₂), which is inert and unusable by most organisms, is converted into biologically usable forms such as ammonia (NH₃). This process is essential for the synthesis of amino acids and nucleotides in living organisms.
2. Identify the organisms capable of nitrogen fixation: Nitrogen fixation is primarily carried out by certain prokaryotes, including bacteria and archaea. These organisms possess the enzyme nitrogenase, which facilitates the conversion of N₂ into NH₃.
3. Recognize examples of nitrogen-fixing prokaryotes: Some examples include free-living bacteria like Azotobacter and symbiotic bacteria such as Rhizobium, which form mutualistic relationships with leguminous plants. Cyanobacteria, a type of photosynthetic bacteria, are also capable of nitrogen fixation.
4. Eliminate incorrect options: Plants, fungi, and animals cannot fix atmospheric nitrogen directly because they lack the nitrogenase enzyme. Instead, they rely on nitrogen-fixing prokaryotes or other sources of biologically available nitrogen.
5. Conclude the correct answer: Based on the information above, the organisms capable of fixing atmospheric nitrogen are certain prokaryotes, such as bacteria and archaea.