A nitrogen-fixing microbe associated with Azolla in rice fields is :

Options

A. Spirulina
B. Anabaena
C. Frankia
D. Tolypothrix

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a nitrogen-fixing microbe that is associated with Azolla in rice fields.

2. **Identifying Key Terms**:
- **Nitrogen-fixing microbe**: This refers to microorganisms that can convert atmospheric nitrogen into a form that plants can use.
- **Azolla**: This is an aquatic pteridophyte that is commonly found in rice fields.

3. **Analyzing the Options**: The options provided are:
- Spirulina
- Anabina
- Francia
- Tolipothrix

4. **Recalling Information**:
- Azolla is known to harbor certain types of blue-green algae (cyanobacteria) that are capable of nitrogen fixation.
- Among the options, Anabina is recognized as a blue-green algae that is specifically associated with Azolla.

5. **Selecting the Correct Answer**: Based on the association of Azolla with nitrogen-fixing microbes, the correct answer is **Anabina**.

6. **Justifying the Answer**:
- Anabina is a nitrogen-fixing cyanobacterium that lives in symbiosis with Azolla.
- This relationship is beneficial as it helps in fixing atmospheric nitrogen, which is essential for plant growth, particularly in flooded rice fields.

7. **Conclusion**: Therefore, the nitrogen-fixing microbe associated with Azolla in rice fields is **Anabina**.

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