Which one of the following is not a nitrogen-fixing organism ?

Options

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

Answer

To determine which of the following organisms is not a nitrogen-fixing organism, we will analyze each option provided:

1. **Anabena**:
- Anabena is a genus of cyanobacteria that is known for its ability to fix nitrogen. It is a free-living organism that can convert atmospheric nitrogen into a form that can be used by plants. Therefore, Anabena is a nitrogen-fixing organism.

2. **Nostoc**:
- Nostoc is another genus of cyanobacteria that also has the capability to fix nitrogen. Like Anabena, it is free-living and plays a significant role in nitrogen fixation in various ecosystems. Thus, Nostoc is also a nitrogen-fixing organism.

3. **Azotobacter**:
- Azotobacter is a genus of free-living bacteria that can fix nitrogen. It is commonly found in soil and is known for its role in enriching soil fertility by converting atmospheric nitrogen into a usable form for plants. Therefore, Azotobacter is a nitrogen-fixing organism as well.

4. **Pseudomonas**:
- Pseudomonas is a genus of bacteria that is not involved in nitrogen fixation. Instead, Pseudomonas species are primarily known for their role in the biodegradation of organic pollutants and can also perform denitrification, which is the process of converting nitrates back into nitrogen gas. Thus, Pseudomonas is not a nitrogen-fixing organism.

Based on the analysis above, the organism that is not a nitrogen-fixing organism is **Pseudomonas**.

### Final Answer:
**Pseudomonas** is not a nitrogen-fixing organism.