Name the organisms which are free living but act as symbionts can fix atmospheric nitrogen.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for organisms that are free-living but can also act as symbionts and have the ability to fix atmospheric nitrogen.

2. **Identifying the Organism**: One of the primary organisms that fits this description is **Anabaena**. Anabaena is a genus of cyanobacteria that can exist independently in the environment but also forms symbiotic relationships with certain plants.

3. **Explaining the Symbiotic Relationship**: Anabaena can form a symbiotic association with plants, particularly with a type of fern known as the **mosquito fern** (Azolla). In this relationship, Anabaena helps the fern by fixing atmospheric nitrogen, which is essential for plant growth.

4. **Importance of Nitrogen Fixation**: Nitrogen is a crucial nutrient for plants, and they cannot utilize atmospheric nitrogen directly. Through the process of nitrogen fixation, Anabaena converts atmospheric nitrogen into forms that plants can absorb and use for their development.

5. **Conclusion**: Thus, the organism that is free-living but can act as a symbiont and fix atmospheric nitrogen is **Anabaena**.