How do nitrogen-fixing bacteria help cycle nitrogen through ecosystems?

• A. They convert nitrate into nitrogen gas (N2), returning nitrogen to the atmosphere.
• B. They convert atmospheric nitrogen gas (N2) into biologically usable forms such as ammonia that can be incorporated into organic molecules.
• C. They decompose cellulose to release carbon dioxide while directly producing atmospheric nitrogen (N2).
• D. They oxidize ammonia to nitrite and nitrate, providing nitrogen mainly in its most oxidized form.

Answer: (B)

Verified step by step guidance

1. Understand the nitrogen cycle and the role of nitrogen-fixing bacteria within it. Nitrogen is essential for all living organisms as it is a key component of amino acids, proteins, and nucleic acids.
2. Recognize that atmospheric nitrogen gas (N2) is abundant but not directly usable by most organisms because of the strong triple bond between nitrogen atoms, making it inert.
3. Identify that nitrogen-fixing bacteria have the unique ability to convert atmospheric nitrogen gas (N2) into ammonia (NH3) or related compounds through a process called nitrogen fixation. This makes nitrogen biologically available.
4. Note that the ammonia produced by nitrogen-fixing bacteria can be incorporated into organic molecules by plants and other organisms, thus entering the biological nitrogen cycle.
5. Distinguish nitrogen fixation from other processes such as nitrification (oxidation of ammonia to nitrite and nitrate) and denitrification (conversion of nitrate back to N2), which are carried out by different groups of bacteria.