Free-living nitrogen-fixing aerobic bacterium is

Options

A. Rhodospirillum
B. Anabaena
C. Rhizobium
D. Beigerinckia

Answer

To solve the question regarding the free-living nitrogen-fixing aerobic bacterium, we will analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which nitrogen in the atmosphere is converted into ammonia or related nitrogenous compounds in soil and water. This is crucial for plants as they cannot utilize atmospheric nitrogen directly.

2. **Identifying Free-Living Bacteria**:
- Free-living nitrogen-fixing bacteria are those that do not form symbiotic relationships with plants but can still fix nitrogen independently.

3. **Analyzing the Options**:
- **Option 1: Rhodospirillum**:
- Rhodospirillum is known to be a free-living bacterium, but it is anaerobic, meaning it does not require oxygen for its metabolic processes. Therefore, this option is incorrect.

- **Option 2: Anabaena**:
- Anabaena is a cyanobacterium that is known for its ability to fix nitrogen, but it is primarily a symbiotic organism, often found in association with plants. Thus, this option is also incorrect.

- **Option 3: Rhizobium**:
- Rhizobium is a well-known nitrogen-fixing bacterium that forms symbiotic relationships with leguminous plants. Although it can fix nitrogen, it does so primarily in anaerobic conditions within root nodules, making this option incorrect as well.

- **Option 4: Bijurinkia**:
- Bijurinkia is recognized as a free-living nitrogen-fixing aerobic bacterium. It can fix nitrogen in the presence of oxygen, which makes this option the correct answer.

4. **Conclusion**:
- After evaluating all the options, the correct answer to the question "Free-living nitrogen-fixing aerobic bacterium is" is **Bijurinkia**.

### Final Answer:
The correct option is **Bijurinkia**.