A nitrogen fixing microbe associated with the fern Azolla in rice fields is

Options

A. Frankia
B. Rhizobium
C. Spirunlina
D. Anabaena

Explanation

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

### Step-by-Step Solution:

1. **Identify the Organism**: Recognize that Azolla is a free-floating aquatic fern commonly found in rice fields.

2. **Understand the Symbiotic Relationship**: Azolla has a symbiotic relationship with certain nitrogen-fixing microbes. This means that these microbes live in close association with Azolla and provide benefits to each other.

3. **Determine the Nitrogen-Fixing Microbe**: The specific nitrogen-fixing microbe that is associated with Azolla is Anabaena. Anabaena is a genus of cyanobacteria that has the ability to fix atmospheric nitrogen.

4. **Explain the Benefits**: In this relationship, Anabaena fixes nitrogen from the atmosphere and provides it to the Azolla plant, which in turn offers carbohydrates and other nutrients to Anabaena. This mutualistic relationship enhances the nitrogen content in the soil, benefiting the growth of rice plants.

5. **Conclusion**: Based on the information, the correct answer to the question is option D: Anabaena.

### Final Answer:
The nitrogen-fixing microbe associated with the fern Azolla in rice fields is **Anabaena** (Option D).

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