Question

Given below are two statements:
Statement I: Nitrogenase enzyme is sensitive to molecular oxygen.
Statement II: Bacteria of genus Azospirillum fix nitrogen under microaerophilic conditions.
In the light of above statements, choose the most appropriate answer from the options given below:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is correct but Statement II is false
• D. Statement II is correct but Statement I is false

Hint:

Organisms use various methods to protect nitrogenase from oxygen:
1. Heterocysts (in Cyanobacteria).
2. Leghemoglobin (in legume nodules).
3. High respiration rates (in Azotobacter).
4. Microaerophily (in Azospirillum).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question tests the understanding of biological nitrogen fixation, the nature of the nitrogenase enzyme complex, and the specific physiological requirements of nitrogen-fixing bacteria like Azospirillum.

Step 2: Detailed Explanation:

• Sensitivity of Nitrogenase (Statement I): The nitrogenase enzyme is responsible for breaking the triple bond of $N_2$ gas to form ammonia. This enzyme consists of two metalloproteins: the Fe-protein (dinitrogenase reductase) and the MoFe-protein (dinitrogenase). Both components are extremely sensitive to molecular oxygen ($O_2$), which irreversibly poisons the Fe-S clusters in the enzyme. This is a universal characteristic of all known nitrogenases. Therefore, Statement I is true.

• Physiology of Azospirillum (Statement II): Azospirillum is a genus of free-living, associative nitrogen-fixing bacteria commonly found in the rhizosphere of grasses. Since they are aerobic organisms but possess an oxygen-sensitive nitrogenase, they have evolved to fix nitrogen only under microaerophilic conditions. Microaerophilic refers to environments where the oxygen concentration is significantly lower than that of the atmosphere (typically 1-5% $O_2$). This level is low enough to prevent nitrogenase inactivation but high enough to support aerobic respiration. Therefore, Statement II is true.

• Integration: Both statements highlight the biochemical constraint of nitrogen fixation (oxygen sensitivity) and the ecological strategy used by certain bacteria to overcome it.
Final Answer:
Since both the sensitivity of the enzyme and the microaerophilic requirement of Azospirillum are scientifically accurate, both statements are true.