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

Options

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

Answer

To solve the question regarding the nitrogen-fixing microbe associated with Azolla in rice fields, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Azolla**:
- Azolla is a type of freshwater fern that grows in rice fields. It is known for its ability to coexist with rice plants without hindering their growth.

2. **Identify the Microbe**:
- The question specifically asks for a nitrogen-fixing microbe associated with Azolla. In this context, we need to identify which microbe has a symbiotic relationship with Azolla.

3. **Recognize the Symbiotic Relationship**:
- Azolla has a symbiotic relationship with a cyanobacterium known as Anabaena. This relationship is crucial because Anabaena is capable of fixing atmospheric nitrogen.

4. **Explain Nitrogen Fixation**:
- Nitrogen fixation is the process by which certain microbes convert nitrogen gas (N2) from the atmosphere into ammonia (NH3), which can be utilized by plants. In this case, Anabaena fixes nitrogen, and Azolla absorbs this ammonia.

5. **Impact on Agriculture**:
- The ammonia produced by Anabaena is used by Azolla, which in turn benefits rice crops. After the rice is harvested, Azolla decomposes and enriches the soil, maintaining its fertility.

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

### Final Answer:
The nitrogen-fixing microbe associated with Azolla in rice fields is **Anabaena**.

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