Which of the following is primarily responsible for converting atmospheric nitrogen (\(N_2\)) into a form usable by plants and animals?

• A. Nitrogen-fixing bacteria
• B. Fungi
• C. Photosynthetic algae
• D. Herbivorous insects

Answer: (A)

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 and animals.
2. Recognize the role of nitrogen-fixing bacteria: These specialized bacteria, such as those in the genus *Rhizobium*, live in the soil or in symbiotic relationships with the roots of certain plants (e.g., legumes). They possess the enzyme nitrogenase, which facilitates the conversion of \(N_2\) into ammonia (\(NH_3\)).
3. Eliminate incorrect options: Fungi, photosynthetic algae, and herbivorous insects do not have the ability to fix atmospheric nitrogen. Fungi primarily decompose organic matter, photosynthetic algae produce oxygen and organic compounds through photosynthesis, and herbivorous insects consume plants but do not contribute to nitrogen fixation.
4. Connect nitrogen fixation to the nitrogen cycle: Nitrogen-fixing bacteria play a critical role in the nitrogen cycle, ensuring that nitrogen is available in forms that plants can absorb (e.g., ammonium ions \(NH_4^+\) or nitrate ions \(NO_3^-\)). This supports the growth of plants, which are the base of the food chain.
5. Conclude that nitrogen-fixing bacteria are the correct answer: Based on the explanation above, nitrogen-fixing bacteria are primarily responsible for converting atmospheric nitrogen (\(N_2\)) into a form usable by plants and animals.