Oxygen is harmful for

Options

A. ferns
B. nitrogen fixing bacteria
C. chara
D. mango tree

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks which of the given options oxygen is harmful for. The options are ferns, nitrogen-fixing bacteria, chara, and the mango tree.

2. **Identifying the Organisms:**
- **Ferns, chara, and mango tree**: These are plants that require oxygen for their metabolic processes.
- **Nitrogen-fixing bacteria**: These are bacteria that convert atmospheric nitrogen into a form usable by plants. An example is Azotobacter.

3. **Role of Oxygen in Nitrogen Fixation:**
- Nitrogen-fixing bacteria like Azotobacter utilize an enzyme called nitrogenase to perform nitrogen fixation.
- This enzyme is crucial for the conversion of nitrogen gas (N2) into ammonia (NH3), which plants can use.

4. **Effect of Oxygen on Nitrogenase:**
- Oxygen is harmful to nitrogen-fixing bacteria because it destroys the nitrogenase enzyme.
- In the presence of oxygen, these bacteria cannot carry out nitrogen fixation effectively.

5. **Anaerobic Conditions:**
- Nitrogen-fixing bacteria thrive in anaerobic (absence of oxygen) conditions.
- They may also adapt by reducing oxygen levels or binding oxygen with proteins like leghemoglobin to protect nitrogenase.

6. **Conclusion:**
- Since oxygen is detrimental to the function of nitrogen-fixing bacteria due to its destructive effect on nitrogenase, the correct answer to the question is that oxygen is harmful for **nitrogen-fixing bacteria**.

**Final Answer:**
Oxygen is harmful for nitrogen-fixing bacteria (Option B).

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