The relationship between water fern Azolla and cyanobacterium Anabaena is symbiotic.

Explanation:

Symbiotic Relationship:
A symbiotic relationship is a close and long-term interaction between two different species, where both species benefit from the association. In the case of Azolla and Anabaena, they share a mutualistic symbiotic relationship.

Azolla:
Azolla is a small aquatic fern that floats on the surface of water bodies. It has a unique characteristic of hosting a specific species of cyanobacteria called Anabaena in specialized cavities present in its leaves.

Anabaena:
Anabaena is a filamentous cyanobacterium that can perform photosynthesis and fix atmospheric nitrogen. It is able to fix nitrogen due to the presence of specialized cells called heterocysts. These heterocysts provide a favorable environment for nitrogen fixation by creating an anaerobic condition.

The Symbiotic Relationship:
1. Nitrogen Fixation: Anabaena provides a significant benefit to Azolla by fixing atmospheric nitrogen and converting it into a form (ammonia) that can be utilized by plants. This is crucial for Azolla's growth and development as it allows them to obtain a vital nutrient (nitrogen) that is often limited in aquatic environments.

2. Shelter and Nutrients: Azolla, on the other hand, provides a suitable habitat for Anabaena. The specialized cavities in Azolla's leaves provide sheltered and protected conditions for the cyanobacteria. Azolla also releases organic matter through its roots, which serves as a nutrient source for Anabaena.

3. Mutual Dependence: Both Azolla and Anabaena rely on each other for their survival and growth. Azolla benefits from the fixed nitrogen provided by Anabaena, which enhances its growth and allows it to outcompete other plants in nutrient-limited environments. Anabaena, in turn, benefits from the sheltered habitat and nutrient supply provided by Azolla.

In conclusion, the relationship between Azolla and Anabaena is a symbiotic one, where both species derive mutual benefits from each other's presence. Azolla provides a suitable habitat and nutrients for Anabaena, while Anabaena fixes nitrogen for Azolla's growth. This symbiotic association has ecological significance by contributing to nitrogen cycling and nutrient availability in aquatic ecosystems.