In the context of the importance of microorganisms, what substance is fixed from the atmosphere by certain bacteria (e.g., nitrogen-fixing bacteria)?

• A. Ozone ($O_3$)
• B. Carbon dioxide ($C{O}_2$)
• C. Oxygen gas ($O_2$)
• D. Nitrogen gas ($N_2$)

Answer: (D)

Verified step by step guidance

1. Understand the concept of nitrogen fixation: Certain bacteria, known as nitrogen-fixing bacteria, have the unique ability to convert atmospheric nitrogen gas (N\_2) into a form usable by living organisms, such as ammonia (NH\_3).
2. Recognize that atmospheric nitrogen gas (N\_2) is abundant but chemically inert, meaning most organisms cannot use it directly for biological processes.
3. Identify that nitrogen-fixing bacteria possess the enzyme nitrogenase, which catalyzes the conversion of N\_2 into ammonia, thus 'fixing' nitrogen from the atmosphere into a biologically accessible form.
4. Differentiate nitrogen gas (N\_2) from other atmospheric substances like ozone (O\_3), carbon dioxide (CO\_2), and oxygen gas (O\_2), which are not fixed by these bacteria in the same way.
5. Conclude that the substance fixed from the atmosphere by nitrogen-fixing bacteria is nitrogen gas (N\_2), which is essential for synthesizing amino acids, nucleotides, and other vital biomolecules.