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: Anabena, Nostoc, Azotobacter, and Pseudomonas.

### Step-by-Step Solution:

1. **Identify Nitrogen-Fixing Organisms:**
- Nitrogen-fixing organisms are those that can convert atmospheric nitrogen (N2) into a form that is usable by plants (like ammonia).

2. **Analyze the First Option: Anabena**
- Anabena is a genus of cyanobacteria that is known for its ability to fix nitrogen. It forms symbiotic relationships with certain plants, making it a nitrogen-fixing organism.

3. **Analyze the Second Option: Nostoc**
- Nostoc is another genus of cyanobacteria that also has the capability to fix nitrogen. It can exist in symbiotic relationships as well, confirming that it is a nitrogen-fixing organism.

4. **Analyze the Third Option: Azotobacter**
- Azotobacter is a free-living nitrogen-fixing bacterium. It is known for its ability to fix nitrogen in the soil, making it a nitrogen-fixing organism.

5. **Analyze the Fourth Option: Pseudomonas**
- Pseudomonas is a genus of bacteria that is primarily known for its role in causing diseases in plants and animals. It does not have the capability to fix nitrogen, which means it is not a nitrogen-fixing organism.

6. **Conclusion:**
- After analyzing all the options, we find that Pseudomonas is the only organism listed that does not fix nitrogen. Therefore, the correct answer is **Option D: Pseudomonas**.