Which of the following is not a free living `N_2`- fixing bacterium?

Options

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

Answer

To determine which of the following is not a free-living nitrogen-fixing bacterium, we need to analyze each option provided: Azotobacter, Rhizobium, Bacillus, and Rhodospirillum.

### Step-by-Step Solution:

1. **Understand Nitrogen-Fixing Bacteria**:
- Nitrogen-fixing bacteria are capable of converting atmospheric nitrogen (N₂) into fixed nitrogen compounds that can be utilized by plants.

2. **Types of Nitrogen-Fixing Bacteria**:
- There are two main types of nitrogen-fixing bacteria:
- **Free-living bacteria**: These bacteria do not form symbiotic relationships with plants. Examples include Azotobacter and Clostridium.
- **Symbiotic bacteria**: These bacteria form close associations with plants, particularly legumes. An example is Rhizobium.

3. **Analyze Each Option**:
- **Azotobacter**: This is a well-known free-living nitrogen-fixing bacterium.
- **Rhizobium**: This bacterium is primarily known for its symbiotic relationship with leguminous plants, making it not a free-living nitrogen-fixing bacterium.
- **Bacillus**: Some species of Bacillus can fix nitrogen and are considered free-living.
- **Rhodospirillum**: This is also a free-living nitrogen-fixing bacterium.

4. **Conclusion**:
- Among the options provided, **Rhizobium** is the bacterium that is not free-living, as it forms a symbiotic relationship with leguminous plants.

### Final Answer:
The bacterium that is not a free-living nitrogen-fixing bacterium is **Rhizobium**.

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