An anaerobic bacterium capable of atmospheric N-fixation is:

Options

A. Clostridium
B. Azotobacter
C. Rhodospirillum
D. Chlorobium

Explanation

To solve the question "An anaerobic bacterium capable of atmospheric N-fixation is:", we will follow these steps:

### Step 1: Understand the Terms
- **Anaerobic Bacteria**: These are bacteria that can survive and grow in environments devoid of oxygen.
- **Nitrogen Fixation**: This is the process by which nitrogen (N2) from the atmosphere is converted into ammonia (NH3) or related compounds in soil, which can then be utilized by plants.

### Step 2: Identify Nitrogen-Fixing Bacteria
- We need to identify which bacteria are capable of fixing atmospheric nitrogen and are also anaerobic.

### Step 3: Evaluate the Options
1. **Clostridium**: This is a genus of bacteria that is strictly anaerobic and is known for its ability to fix nitrogen.
2. **Azotobacter**: This is a genus of bacteria that is aerobic, meaning it requires oxygen to survive, so it cannot be the answer.
3. **Rhodospirillum**: This bacterium is facultative anaerobic, meaning it can survive in both the presence and absence of oxygen, but it is not strictly anaerobic.

### Step 4: Conclusion
- Based on the evaluation, **Clostridium** is the only anaerobic bacterium listed that is capable of atmospheric nitrogen fixation.

### Final Answer:
The anaerobic bacterium capable of atmospheric nitrogen fixation is **Clostridium**.

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