Which of the following is free-living aerobic non-photosynthetic nitrogen fixing bacterium (A) Anabaena (B) Azospirillium (C) Azotobacter (D) Rhizobium

Options

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

Answer

To determine which of the following is a free-living aerobic non-photosynthetic nitrogen-fixing bacterium, we will analyze each option provided.

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into a form that plants can use, typically ammonia (NH3). This process is essential for plant growth as they cannot utilize atmospheric nitrogen directly.

2. **Classifying Nitrogen-Fixing Bacteria**:
- Nitrogen-fixing bacteria can be categorized into two main types:
- **Symbiotic bacteria**: These form associations with plants (e.g., Rhizobium, Azospirillum, and Anabaena).
- **Non-symbiotic (free-living) bacteria**: These do not form associations with plants and can be further classified into photosynthetic and non-photosynthetic types.

3. **Evaluating the Options**:
- **(A) Anabaena**: This is a nitrogen-fixing cyanobacterium that can exist in both symbiotic and non-symbiotic forms. It is photosynthetic.
- **(B) Azospirillum**: This is a nitrogen-fixing bacterium that is primarily symbiotic, often associated with the roots of plants.
- **(C) Azotobacter**: This is a free-living, aerobic, non-photosynthetic nitrogen-fixing bacterium. It does not require a plant host and is known for its ability to fix nitrogen in the soil.
- **(D) Rhizobium**: This is a symbiotic nitrogen-fixing bacterium that forms nodules on the roots of leguminous plants.

4. **Conclusion**:
- Based on the classifications and characteristics of each bacterium, the correct answer is **(C) Azotobacter**, as it is a free-living, aerobic, non-photosynthetic nitrogen-fixing bacterium.

**Final Answer**: (C) Azotobacter

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