A : Azotodesmic lichens are biofertilisers enriching nitrogen contents in soil.
R : This ability is due to the presence of heterocystous blue-green algae as phycobiont component.

Options

A. If both assertion & reason are true and the reason is the correct explanation of the assertion
B. If both assertion & reason are true but the reason is not the correct explanation of the assertion
C. If assertion is true statement but reason is false
D. If both assertion and reason are false statements

Answer

### Step-by-Step Solution:

1. **Understanding Azotodesmic Lichens**:
- Azotodesmic lichens are a type of lichen that play a significant role in enriching the nitrogen content of the soil. Lichens are symbiotic associations between fungi and algae or cyanobacteria.

2. **Role of Phycobiont**:
- In azotodesmic lichens, the phycobiont (the algal component) is typically a cyanobacterium. Cyanobacteria are known for their ability to perform photosynthesis and fix atmospheric nitrogen.

3. **Heterocysts in Cyanobacteria**:
- Cyanobacteria possess specialized cells called heterocysts. These cells are crucial for nitrogen fixation as they provide an anaerobic environment necessary for the nitrogen-fixing enzyme, nitrogenase, to function.

4. **Nitrogen Fixation Process**:
- The heterocysts in cyanobacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can then be utilized by plants and enrich the soil with nitrogen, acting as a natural biofertilizer.

5. **Conclusion**:
- Therefore, the assertion that azotodesmic lichens are biofertilizers enriching nitrogen content in the soil is correct. The reason provided, which states that this ability is due to the presence of heterocystous blue-green algae (cyanobacteria) as the phycobiont component, is also correct. The reason explains the assertion accurately.

6. **Final Answer**:
- Both the assertion and reason are true, and the reason is the correct explanation for the assertion.