Which one of the following is not a nitrogen-fixing organism ? (a) Anabaena (b) Nostoc (c) Azotobacter (d) Pseudomonas

Options

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

Explanation

To solve the question "Which one of the following is not a nitrogen-fixing organism?", we will analyze each option provided:

1. **Option A: Anabaena**
- Anabaena is a genus of filamentous cyanobacteria. It is known for its ability to fix nitrogen, particularly in association with certain plants like mosquito ferns. Therefore, Anabaena is a nitrogen-fixing organism.

2. **Option B: Nostoc**
- Nostoc is another genus of cyanobacteria that can fix nitrogen. It often forms symbiotic relationships with plants, such as cycads, and is found in coralloid roots. Hence, Nostoc is also a nitrogen-fixing organism.

3. **Option C: Azotobacter**
- Azotobacter is a free-living nitrogen-fixing bacterium found in soil. It is known for its role in fixing atmospheric nitrogen into a form that plants can use, such as in wheat and sugarcane. Thus, Azotobacter is a nitrogen-fixing organism.

4. **Option D: Pseudomonas**
- Pseudomonas is a genus of bacteria that is not primarily known for nitrogen fixation. Instead, it is involved in the process of denitrification, where it converts nitrates into nitrogen gas, thus releasing it into the atmosphere. Therefore, Pseudomonas is not a nitrogen-fixing organism.

After analyzing all the options, we conclude that the correct answer is **(d) Pseudomonas**, as it is not a nitrogen-fixing organism.

### Summary of the Solution:
- Anabaena: Nitrogen-fixing organism.
- Nostoc: Nitrogen-fixing organism.
- Azotobacter: Nitrogen-fixing organism.
- Pseudomonas: Not a nitrogen-fixing organism (involved in denitrification).

### Final Answer:
**(d) Pseudomonas**