Which of the following bacteria reduce nitrate in soil into nitrogen?

Options

A. Nitrobacter
B. Nitrococcus
C. Thiobacillus
D. Nitrosomonas

Answer

To answer the question "Which of the following bacteria reduce nitrate in soil into nitrogen?", we will analyze the options provided: Nitrobacter, Nitrococcus, Thiobacillus, and Nitrosomonas.

### Step-by-Step Solution:

1. **Understanding Nitrogen Cycle**:
- The nitrogen cycle involves several processes, including nitrogen fixation, nitrification, and denitrification.
- Nitrogen fixation converts atmospheric nitrogen (N₂) into ammonia (NH₃).
- Nitrification is the process where ammonia is converted into nitrite (NO₂⁻) and then into nitrate (NO₃⁻) by nitrifying bacteria.

2. **Identifying Nitrifying Bacteria**:
- The bacteria involved in the nitrification process include:
- **Nitrosomonas**: Converts ammonia to nitrite.
- **Nitrobacter**: Converts nitrite to nitrate.
- **Nitrococcus**: Also involved in nitrification.
- These bacteria do not reduce nitrate to nitrogen; instead, they convert ammonia to nitrate.

3. **Understanding Denitrification**:
- Denitrification is the process where nitrate (NO₃⁻) is reduced to nitrogen gas (N₂) or nitrous oxide (N₂O).
- This process is crucial for returning nitrogen to the atmosphere.

4. **Identifying Denitrifying Bacteria**:
- The bacteria responsible for denitrification include:
- **Thiobacillus**: This bacterium can reduce nitrate to nitrogen gas.
- **Pseudomonas**: Another example of denitrifying bacteria.
- Therefore, Thiobacillus is one of the correct answers for the bacteria that reduce nitrate to nitrogen.

5. **Conclusion**:
- Among the options given, **Thiobacillus** is the bacterium that reduces nitrate in soil into nitrogen.
- The other options (Nitrobacter, Nitrococcus, and Nitrosomonas) are nitrifying bacteria and do not perform this reduction.

### Final Answer:
**Thiobacillus** is the bacterium that reduces nitrate in soil into nitrogen.