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 can analyze each option provided:

1. **Anabena**: This is a genus of filamentous cyanobacteria that are known for their ability to fix nitrogen. They have specialized cells called heterocysts that facilitate this process.

2. **Nostoc**: Another genus of cyanobacteria, Nostoc is also capable of nitrogen fixation. They form colonies that can fix atmospheric nitrogen into a form usable by plants.

3. **Azotobacter**: This is a free-living nitrogen-fixing bacterium found in soil. It plays a significant role in the nitrogen cycle by converting atmospheric nitrogen into ammonia.

4. **Pseudomonas**: Unlike the previous three, Pseudomonas is not a nitrogen-fixing organism. Instead, it is known as a denitrifying bacterium, which means it converts nitrates and nitrites into nitrogen gas, thus releasing nitrogen back into the atmosphere.

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

### Final Answer:
**D. Pseudomonas**

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