The algae that has a symbiotic relationship with reef builders (corals) is called zooxanthellae.

Here's why this relationship is so important:

* Zooxanthellae are dinoflagellates, a type of single-celled algae.

* They live inside the coral tissue, forming a mutualistic relationship.

* The coral provides the algae with:

* Protection from predators

* A stable environment with access to sunlight

* Nutrients like carbon dioxide and nitrogen

* The algae provide the coral with:

* Oxygen

* Sugars produced through photosynthesis (which is the main source of energy for the coral)

* Help with calcium carbonate deposition for coral skeleton growth

This symbiotic relationship is essential for the survival and health of coral reefs. It allows the corals to thrive in nutrient-poor tropical waters.