Reef substrate is composed of calcium carbonate from living and dead scleractinian corals. These corals have a symbiotic relationship with a single-celled algae known as zooxanthallae. B. Zooxanthallae 1. Zooxanthallae live symbiotically within coral polyp tissue. They assist coral in nutrient production through photosynthetic activities.
Corals receive coloration from the photosynthetic pigments of zooxanthallae.
When stressed, zooxanthallae may be expelled from coral tissue. If all zooxanthallae are lost, the coral will die. C. Importance of Reefs 1. Reef ecosystems have been compared to rain forests in terms of biodiversity and density of living organisms. Total area covered by coral reefs is approximately 240,000 square miles.