The process of nitrogen-fixation by bacteria does not take place in the presence of

Options

A. molecular form of hydrogen
B. elemental form of oxygen
C. water
D. elemental form of nitrogen

Explanation

**Step-by-Step Solution:**

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process by which atmospheric nitrogen (N₂), which is not directly usable by plants, is converted into a usable form, such as nitrates (NO₃⁻). This process is essential for plant growth.

2. **Role of Bacteria**: Certain bacteria, such as Azotobacter and Rhizobium, are responsible for nitrogen fixation. Azotobacter exists freely in the soil, while Rhizobium forms symbiotic relationships with leguminous plants (like peas and beans) in root nodules.

3. **Conversion Process**: These bacteria convert atmospheric nitrogen into water-soluble nitrates, which plants can absorb and utilize for growth.

4. **Enzymes Involved**: The key enzymes involved in nitrogen fixation are called nitrogenases. These enzymes are crucial for the conversion of nitrogen gas into a usable form.

5. **Effect of Oxygen**: Nitrogenase enzymes are very sensitive to oxygen. The presence of oxygen can destroy these enzymes, making nitrogen fixation impossible. Many nitrogen-fixing bacteria either grow in anaerobic conditions (without oxygen) or have mechanisms to reduce oxygen levels around them.

6. **Conclusion**: Therefore, nitrogen fixation by bacteria does not take place in the presence of oxygen.

**Final Answer**: The process of nitrogen fixation by bacteria does not take place in the presence of oxygen.

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