Which one of the following is denitrifying bacteria ?

Options

A. Nitrosomonas
B. Pseudomonas
C. Azotobacter
D. Nostoc

Answer

To determine which of the given bacteria is a denitrifying bacterium, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Denitrifying Bacteria**: Denitrifying bacteria are microorganisms that convert nitrates (NO3-) in the soil into nitrogen gas (N2), which is then released into the atmosphere. This process is known as denitrification.

2. **Identify the Given Options**: The options provided are:
- Nitrosomonas
- Pseudomonas
- Azotobacter
- Nostoc

3. **Analyze Each Option**:
- **Nitrosomonas**: This bacterium is involved in nitrification, which is the conversion of ammonia (NH3) into nitrites (NO2-) and then into nitrates (NO3-). Therefore, it is not a denitrifying bacterium.
- **Pseudomonas**: This is a known denitrifying bacterium. It plays a significant role in the denitrification process, converting nitrates into nitrogen gas.
- **Azotobacter**: This bacterium is a nitrogen-fixing bacterium, which means it converts atmospheric nitrogen into a form that plants can use. It is not involved in denitrification.
- **Nostoc**: Similar to Azotobacter, Nostoc is also a nitrogen-fixing bacterium and does not participate in denitrification.

4. **Conclusion**: Based on the analysis, the only bacterium from the options that is classified as a denitrifying bacterium is **Pseudomonas**.

### Final Answer:
The denitrifying bacterium among the options is **Pseudomonas**.