Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
• [Assertion (A):] Methane is the major gas generated during anaerobic digestion of organic materials. 
• [Reason (R):] Methanotrophs are the key players in anaerobic digestion of organic matter. 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:

Be careful with similar-sounding microbiological terms:
- Methanogens: Produce methane under strict anaerobic conditions (domain Archaea).
- Methanotrophs: Consume/oxidize methane under aerobic conditions (domain Bacteria).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Anaerobic digestion is a biological process where microorganisms break down biodegradable organic matter in the absence of oxygen.
This process is used to treat organic wastes and generate biogas, a renewable energy source.

Step 3: Detailed Explanation:
Let us analyze the Assertion and Reason statements:
- Assertion (A) is true: During anaerobic digestion, complex organic matter is broken down into simple molecules and fermented.
In the final stage (methanogenesis), specialized microbes convert acetic acid, hydrogen, and carbon dioxide into biogas.
Biogas consists primarily of methane (CH$_4$, typically \(50-70%\)) and carbon dioxide (CO$_2$, \(30-45%\)).
Thus, methane is the major gas generated.
- Reason (R) is false: The microorganisms responsible for producing methane during the final step of anaerobic digestion are called methanogens.
Methanogens are obligate anaerobic Archaea (such as Methanobacterium and Methanosarcina).
In contrast, methanotrophs are aerobic bacteria that utilize methane as their sole carbon and energy source, oxidizing it back to carbon dioxide:
\[ \text{CH}_4 + 2\text{O}_2 \xrightarrow{\text{Methanotrophs}} \text{CO}_2 + 2\text{H}_2\text{O} \] Methanotrophs require oxygen to consume methane, so they are not involved in anaerobic digestion.
Therefore, Assertion (A) is true, but Reason (R) is false.

Step 4: Final Answer:
The correct option is (A) is true but (R) is false, matching Option (C).