What is the role of nitrogen-fixing bacteria in the nitrogen cycle?

• A. They break down dead organic matter to release nitrogen back into the atmosphere.
• B. They convert atmospheric nitrogen (\(N_2\)) into ammonia (\(NH_3\)), making nitrogen available to plants.
• C. They convert ammonia (\(NH_3\)) into nitrate (\(NO_3^-\)) through nitrification.
• D. They absorb excess nitrogen from the soil to prevent toxicity.

Answer: (B)

Verified step by step guidance

1. Nitrogen-fixing bacteria play a crucial role in the nitrogen cycle by converting atmospheric nitrogen (\(N_2\)), which is inert and unavailable to most organisms, into ammonia (\(NH_3\)). This process is called nitrogen fixation.
2. Atmospheric nitrogen (\(N_2\)) is highly stable due to its triple bond, making it inaccessible to plants. Nitrogen-fixing bacteria, such as those in the genus Rhizobium, possess the enzyme nitrogenase, which facilitates the conversion of \(N_2\) into \(NH_3\).
3. The ammonia (\(NH_3\)) produced by nitrogen-fixing bacteria can be further utilized by plants directly or converted into other nitrogen compounds, such as nitrate (\(NO_3^-\)), through subsequent processes like nitrification.
4. Nitrogen fixation is essential for maintaining the nitrogen cycle because it provides a usable form of nitrogen for plants, which is a critical nutrient for their growth and development.
5. This process is symbiotic in some cases, as nitrogen-fixing bacteria often form mutualistic relationships with plants, particularly legumes, by inhabiting root nodules and providing nitrogen in exchange for carbohydrates and a suitable environment.