Which of the following are denitrifying bacteria?

Options

A. A. Nitrosomonas and Pseudomonas
B. B. Nitrobacter and Pseudomonas
C. C. Rhizobium and Clostridium
D. D. Pseudomonas and Clostridium

Explanation

To determine which of the following are denitrifying bacteria, we will analyze each option provided in the question. Denitrifying bacteria are those that convert nitrates into nitrogen gas, thereby reducing the nitrogen content in the soil.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **Denitrifying Bacteria**: These bacteria convert nitrates (NO3-) into nitrogen gas (N2), which is released into the atmosphere.
- **Nitrifying Bacteria**: These bacteria convert ammonia (NH3) into nitrates (NO3-). They are involved in the nitrogen cycle by fixing nitrogen.

2. **Analyze the Options**:
- **Option A: Nitrosomonas and Pseudomonas**:
- **Nitrosomonas**: Known to be a nitrifying bacterium.
- **Pseudomonas**: Known to be a denitrifying bacterium.
- Conclusion: This option is incorrect because it includes a nitrifying bacterium (Nitrosomonas).

3. **Option B: Nitrovacter and Pseudomonas**:
- **Nitrovacter**: Also a nitrifying bacterium.
- **Pseudomonas**: Denitrifying bacterium.
- Conclusion: This option is incorrect because it includes a nitrifying bacterium (Nitrovacter).

4. **Option C: Rhizobium and Clostridium**:
- **Rhizobium**: Known to be a nitrifying bacterium (nitrogen-fixing).
- **Clostridium**: Known to be a denitrifying bacterium.
- Conclusion: This option is incorrect because it includes a nitrifying bacterium (Rhizobium).

5. **Option D: Pseudomonas and Clostridium**:
- **Pseudomonas**: Denitrifying bacterium.
- **Clostridium**: Denitrifying bacterium.
- Conclusion: This option is correct as both are denitrifying bacteria.

### Final Answer:
The denitrifying bacteria among the options provided are **Pseudomonas and Clostridium**, which corresponds to **Option D**.

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