One of the free-living, anaerobic nitrogenfixer is

Options

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

Answer

To solve the question "One of the free-living, anaerobic nitrogen fixers is," we will analyze each option provided and determine which one fits the criteria of being a free-living anaerobic nitrogen fixer.

### Step-by-Step Solution:

1. **Understanding the Question**:
- We need to identify a nitrogen-fixing organism that is both free-living and anaerobic.

2. **Analyzing Option A: Azotobacter**:
- Azotobacter is known to be a free-living nitrogen fixer. However, it requires oxygen to grow, which makes it aerobic.
- **Conclusion**: Azotobacter is not an anaerobic nitrogen fixer.

3. **Analyzing Option B: Bajornikia**:
- Bajornikia is also an aerobic nitrogen fixer. It utilizes a range of multi-carbon compounds for its growth.
- **Conclusion**: Bajornikia is not an anaerobic nitrogen fixer.

4. **Analyzing Option C: Rhodospirillum**:
- Rhodospirillum is a gram-negative purple non-sulfur bacterium that is facultatively anaerobic. This means it can grow in both the presence and absence of oxygen.
- It is capable of nitrogen fixation, converting atmospheric nitrogen into ammonia with the help of nitrogenase enzyme.
- **Conclusion**: Rhodospirillum is a free-living, facultative anaerobic nitrogen fixer.

5. **Analyzing Option D: Rhizobium**:
- Rhizobium forms a symbiotic relationship with the roots of legume plants and is an aerobic nitrogen fixer. It is not free-living in the soil as it relies on the host plant for its growth.
- **Conclusion**: Rhizobium is not a free-living anaerobic nitrogen fixer.

6. **Final Answer**:
- Based on the analysis, the correct answer is **C: Rhodospirillum**. It is the only option that is a free-living anaerobic nitrogen fixer.