Which organisms are primarily responsible for transforming atmospheric nitrogen (\(N_2\)) into a form that is usable by plants?

• A. Protozoa
• B. Fungi
• C. Nitrogen-fixing bacteria
• D. Green algae

Answer: (C)

Verified step by step guidance

1. Understand the concept of nitrogen fixation: Nitrogen fixation is the process by which atmospheric nitrogen (\(N_2\)), which is inert and unusable by most organisms, is converted into ammonia (\(NH_3\)) or other nitrogenous compounds that can be utilized by plants.
2. Identify the organisms capable of nitrogen fixation: Nitrogen-fixing bacteria are specialized microorganisms that possess the enzyme nitrogenase, which allows them to break the strong triple bond in \(N_2\) and convert it into usable forms like ammonia.
3. Recognize the symbiotic relationship: Many nitrogen-fixing bacteria, such as *Rhizobium*, form symbiotic relationships with the roots of leguminous plants, creating nodules where nitrogen fixation occurs. Other free-living bacteria, like *Azotobacter*, also perform nitrogen fixation independently.
4. Eliminate incorrect options: Protozoa, fungi, and green algae do not have the ability to fix atmospheric nitrogen into a usable form for plants. These organisms may play other roles in ecosystems but are not involved in nitrogen fixation.
5. Conclude the correct answer: Nitrogen-fixing bacteria are the primary organisms responsible for transforming atmospheric nitrogen (\(N_2\)) into a form usable by plants, making them essential for maintaining the nitrogen cycle in ecosystems.