Which bacterium is capable of utilising the most abundantly available gas in the atmosphere for one of their metabolic pathways, but cannot utilise the second most abundantly available gas for another metabolic pathway?

Options

A. Pseudomonas
B. Clostridium
C. Rhodomicrobium
D. Xanthomonas

Answer

### Step-by-Step Solution:

1. **Identify the Most Abundant Gas in the Atmosphere**:
- The most abundant gas in the atmosphere is Nitrogen (N₂), which makes up about 78% of the air we breathe.

2. **Identify the Second Most Abundant Gas**:
- The second most abundant gas in the atmosphere is Oxygen (O₂), which constitutes about 21% of the atmosphere.

3. **Determine the Bacterium that Utilizes Nitrogen**:
- The question asks for a bacterium that can utilize Nitrogen for one of its metabolic pathways. Clostridium is a genus of bacteria known for its ability to fix Nitrogen, meaning it can convert atmospheric Nitrogen into a form that can be used by living organisms.

4. **Determine the Bacterium's Relationship with Oxygen**:
- Clostridium is classified as an anaerobic bacterium, which means it thrives in environments without Oxygen. In fact, it cannot survive in the presence of Oxygen, which means it cannot utilize Oxygen for any of its metabolic pathways.

5. **Conclusion**:
- Therefore, the bacterium that can utilize Nitrogen but cannot utilize Oxygen is **Clostridium**.

### Final Answer:
- The bacterium is **Clostridium**.