Which of the following alga fixes atmospheric nitrogen in soil

Options

A. Anabaena
B. Ulothrix
C. Spirogyra
D. Fucus

Answer

To answer the question "Which of the following alga fixes atmospheric nitrogen in soil?", we will analyze the options provided and identify the correct one.

### Step-by-Step Solution:

1. **Identify the Options**:
The options given are:
- Anabena
- Phyllotrichs
- Spirogyra
- Fucus

2. **Understand Nitrogen Fixation**:
Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that can be utilized by living organisms, particularly plants. Certain algae and bacteria are known for their ability to fix nitrogen.

3. **Evaluate Each Option**:
- **Anabena**: This is a genus of blue-green algae (cyanobacteria) known for its ability to fix atmospheric nitrogen. Anabena often forms symbiotic relationships with certain plants, such as the fern Azolla, where it provides nitrogen to the plant.
- **Phyllotrichs**: This is a genus of filamentous green algae, but it does not have the capability to fix nitrogen.
- **Spirogyra**: This is a green algae known for its spiral chloroplasts and photosynthesis, but it does not fix nitrogen.
- **Fucus**: This is a genus of brown algae (seaweed) that is not involved in nitrogen fixation.

4. **Conclusion**:
Based on the evaluation, the only alga that is known to fix atmospheric nitrogen in soil is **Anabena**. Therefore, the correct answer is **Anabena**.

### Final Answer:
**Anabena** is the alga that fixes atmospheric nitrogen in soil.

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