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

Options

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

Explanation

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

### Step-by-Step Solution:

1. **Identify the Organisms**: The options given are Anabina, Nostoc, Azotobacter, and Pseudomonas.

2. **Analyze Anabina**:
- Anabina is a type of cyanobacteria.
- It is known to be a free-living organism that can fix nitrogen from the atmosphere.

3. **Analyze Nostoc**:
- Nostoc is also a cyanobacteria, similar to Anabina.
- It is capable of nitrogen fixation and is also free-living.

4. **Analyze Azotobacter**:
- Azotobacter is a genus of free-living nitrogen-fixing bacteria.
- It plays a significant role in the nitrogen cycle by converting atmospheric nitrogen into a form that plants can use.

5. **Analyze Pseudomonas**:
- Pseudomonas is a genus of bacteria that is primarily saprophytic.
- It does not fix nitrogen; instead, it is involved in the biodegradation of organic materials and pollutants.

6. **Conclusion**:
- Based on the analysis, Anabina, Nostoc, and Azotobacter are all nitrogen-fixing organisms.
- Pseudomonas, on the other hand, does not fix nitrogen and is the correct answer to the question.

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

---