One of the free-living, anaerobic nitrogenfixer is

Options

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

Answer

**Step-by-Step Solution:**

1. **Understanding Anaerobic Nitrogen Fixers**:
- Anaerobic nitrogen fixers are organisms that can convert atmospheric nitrogen (N₂) into ammonia (NH₃) without the presence of oxygen.

2. **Identifying Free-Living Nitrogen Fixers**:
- Free-living nitrogen fixers are those that do not form symbiotic relationships with plants. They exist independently in the soil or water and can fix nitrogen on their own.

3. **Reviewing Examples of Nitrogen Fixers**:
- Azotobacter: This is a free-living nitrogen-fixing bacterium, but it is aerobic, meaning it requires oxygen.
- Rhizobium: This bacterium is also a nitrogen fixer, but it is symbiotic, forming relationships with legume roots and also requires oxygen.
- Bajonchia: Similar to Azotobacter and Rhizobium, it is aerobic and does not fit the criteria for anaerobic nitrogen fixers.

4. **Identifying the Correct Organism**:
- Rhodospirillum: This is a gram-negative, purple non-sulfur bacterium that is facultatively anaerobic. It can fix nitrogen in the absence of oxygen, making it a suitable candidate for a free-living anaerobic nitrogen fixer.

5. **Conclusion**:
- The answer to the question is Rhodospirillum, as it is a free-living, anaerobic nitrogen-fixing bacterium.

**Final Answer**: Rhodospirillum

---