A free-living aerobic and nonphotosynthetic nitrogen-fixing bacterium is

Options

A. Anabaena
B. Clostridium
C. Azotobacter
D. Rhizobium

Answer

To solve the question regarding a free-living aerobic and non-photosynthetic nitrogen-fixing bacterium, we will analyze the characteristics of the options provided.

### Step-by-Step Solution:

1. **Understand the Terms**:
- **Free-living**: This means the bacterium can exist independently, without relying on a host organism.
- **Aerobic**: This indicates that the bacterium requires oxygen for growth and survival.
- **Non-photosynthetic**: This means the bacterium does not perform photosynthesis to obtain energy.

2. **Analyze the Options**:
- **Anabena**: This is a free-living nitrogen-fixing bacterium, but it is also known for its symbiotic relationships and is photosynthetic. Thus, it does not fit the criteria.
- **Clostridium**: This bacterium is free-living but is anaerobic, meaning it does not require oxygen and therefore does not fit the aerobic requirement.
- **Azotobacter**: This bacterium is free-living, aerobic, and non-photosynthetic. It is well-known for its ability to fix nitrogen in the soil, making it a strong candidate.
- **Rhizobium**: While this bacterium is known for nitrogen fixation, it is primarily a symbiotic bacterium that forms relationships with legumes, so it does not meet the free-living criterion.

3. **Conclusion**:
- Based on the analysis, the only bacterium that fits all the criteria of being free-living, aerobic, and non-photosynthetic while also being a nitrogen fixer is **Azotobacter**.

### Final Answer:
The correct answer is **Azotobacter**.

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