Answer:

Introduction:
The fern Azolla is known for its ability to fix atmospheric nitrogen with the help of a symbiotic relationship with a nitrogen-fixing microbe. This microbe is called Anabaena, and it plays a crucial role in nitrogen fixation in rice fields.

Anabaena:
Anabaena is a filamentous cyanobacterium that can form a mutualistic symbiotic association with Azolla. It is commonly found in the water fern Azolla's leaf cavities, where it forms a symbiotic relationship with the plant. This association is beneficial for both Azolla and Anabaena.

Key Points:
- Azolla provides a protected environment and essential nutrients for Anabaena.
- Anabaena, in return, fixes atmospheric nitrogen and provides it to Azolla.
- Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into a form (ammonia, NH3) that can be utilized by plants.
- Azolla requires nitrogen for its growth and development, and Anabaena fulfills this requirement by fixing nitrogen.

Significance in Rice Fields:
Rice is a staple crop for millions of people worldwide, and nitrogen is an essential nutrient for its growth. However, the availability of nitrogen in the soil is often limited. The association between Azolla and Anabaena in rice fields has several benefits:

1. Nitrogen Fixation: Anabaena fixes atmospheric nitrogen and converts it into a usable form for plants. This helps in increasing the nitrogen content in the soil, making it available for rice plants.

2. Organic Fertilizer: Azolla-Anabaena association acts as a natural, organic source of nitrogen fertilizer. It reduces the reliance on synthetic fertilizers, which can have negative environmental impacts.

3. Weed Suppression: The dense growth of Azolla in rice fields helps in suppressing weed growth. This reduces the competition for nutrients and resources, benefiting the rice crop.

4. Biological Control: Azolla also acts as a host for beneficial microorganisms, including those that can control pests and diseases in rice fields.

Conclusion:
In conclusion, the nitrogen-fixing microbe associated with the fern Azolla in rice fields is Anabaena. This association plays a vital role in nitrogen fixation, providing an organic source of nitrogen fertilizer and promoting sustainable rice cultivation.