Which one is correct about the free living Rhizobium sp. and its enzyme?

Options

A. Bacteria are aerobic and nitrogenase are active
B. Bacteria are anaerobic and nitrogenase is active (operational)
C. Bacteria are aerobic and nitrogenase is inactive
D. Bacteria are anaerobic and nitrogenase is inactive

Answer

### Step-by-Step Solution:

1. **Understanding Rhizobium**:
- Rhizobium is a genus of bacteria known for its ability to fix nitrogen. It forms symbiotic relationships with leguminous plants, where it resides in root nodules.

2. **Free-Living Stage**:
- In its free-living stage, Rhizobium exists independently in the soil and is capable of nitrogen fixation. This means it can convert atmospheric nitrogen (N₂) into ammonia (NH₃), which is a form that plants can utilize.

3. **Enzyme Involved**:
- The key enzyme involved in this nitrogen fixation process is called nitrogenase. This enzyme is crucial for the conversion of nitrogen gas into ammonia.

4. **Aerobic vs. Anaerobic Conditions**:
- Rhizobium is classified as an aerobic bacterium, meaning it requires oxygen for its metabolic processes. However, when it is in the root nodules of plants, it operates in a more anaerobic environment due to the presence of leghemoglobin, which helps to maintain low oxygen levels.

5. **Evaluating the Options**:
- The options provided in the question include statements about the bacteria being aerobic or anaerobic and whether the nitrogenase enzyme is active or inactive.
- The correct statement is that Rhizobium is aerobic and its nitrogenase enzyme is active when it is in the free-living stage.

6. **Conclusion**:
- The correct answer is that Rhizobium is an aerobic bacterium and its nitrogenase enzyme is active, particularly when it is fixing nitrogen in the root nodules of leguminous plants.