Which of the following is free-living aerobic non-photosynthetic nitrogen fixing bacterium

Options

A. Nostoc
B. Rhodospirillum
C. Rhizobium
D. Azotobacter

Answer

To solve the question "Which of the following is a free-living aerobic non-photosynthetic nitrogen-fixing bacterium?", we can follow these steps:

### Step 1: Understand the Terms
- **Free-living**: Refers to organisms that do not form symbiotic relationships with other organisms.
- **Aerobic**: Organisms that require oxygen for respiration.
- **Non-photosynthetic**: Organisms that do not perform photosynthesis to obtain energy.
- **Nitrogen-fixing**: Refers to the ability of certain bacteria to convert atmospheric nitrogen into a form that can be used by plants.

### Step 2: Analyze the Options
1. **Nostoc**: This is a cyanobacterium that is photosynthetic and can fix nitrogen, but it is not free-living as it often forms symbiotic relationships.
2. **Rhodospirillum**: This is a purple non-sulfur bacterium that is photosynthetic and can also fix nitrogen, so it does not fit the criteria.
3. **Rhizobium**: This bacterium forms a symbiotic relationship with leguminous plants, making it a symbiotic nitrogen-fixing bacterium, not free-living.
4. **Azotobacter**: This is a free-living bacterium that is aerobic, non-photosynthetic, and capable of fixing nitrogen.

### Step 3: Identify the Correct Answer
Based on the analysis, the only bacterium that meets all the criteria of being free-living, aerobic, non-photosynthetic, and nitrogen-fixing is **Azotobacter**.

### Conclusion
The correct answer is **Azotobacter**.

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