Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R). 
• [Assertion (A):] Inoculated Azospirillum culture did not show growth in Nitrogen free malate broth incubated at 37 °C under shaking conditions. 
• [Reason (R):] Azospirillum requires microaerophilic conditions for nitrogen fixation. Aerobic conditions inhibit nitrogen fixation. 
In light of the above statements, choose the correct answer from the options given below. 
 

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

The nitrogenase enzyme is universally sensitive to oxygen.
Organisms use different strategies to protect it:
- Azospirillum prefers microaerophilic environments.
- Cyanobacteria use specialized cells called heterocysts.
- Legumes produce oxygen-binding leghemoglobin in their nodules.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Azospirillum is a genus of free-living, associative nitrogen-fixing bacteria found in the rhizosphere of grasses and cereals.
Like other diazotrophs, they rely on the enzyme complex nitrogenase to reduce atmospheric nitrogen (N$_2$) into ammonia.

Step 3: Detailed Explanation:
Let us evaluate the Assertion and Reason statements:
- Assertion (A) is true: If Azospirillum is inoculated into a nitrogen-free medium (such as nitrogen-free malate broth) and incubated under continuous shaking conditions, it will not grow.
Continuous shaking increases oxygen dissolution in the liquid medium, creating highly aerobic conditions.
Without a source of fixed nitrogen in the medium, the bacteria must fix atmospheric nitrogen to grow.
However, the high oxygen levels caused by shaking prevent nitrogen fixation, halting bacterial growth.
- Reason (R) is true: The nitrogenase enzyme complex is highly sensitive to oxygen and is irreversibly inactivated by it.
To protect this enzyme, Azospirillum requires microaerophilic conditions (low oxygen levels, typically \(1\text{--}2%\)) to fix nitrogen.
Highly aerobic conditions inhibit nitrogenase activity, preventing nitrogen fixation.
- Evaluating the connection: The bacterium fails to grow in nitrogen-free medium under shaking conditions because shaking introduces high levels of oxygen.
These aerobic conditions inactivate nitrogenase and prevent nitrogen fixation, which the bacterium needs to grow in the absence of added nitrogen.
Therefore, the Reason is the correct explanation of the Assertion.

Step 4: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A), matching Option (A).