In the early life of a coral polyp it can adopt a single celled algea (actually dinoflagellates) called zooxanthellae. They are extremely small, with even a small coral colony harboring millions. It is these microbes that allow corals to build vast structures in a completely nutrient starved environment. These areas of ocean are so devoid of nutrients they are refered to as deserts. The zooxanthellae contain chlorophyll, and are so able to synthesize oxygen and energy rich products using sunlight carbondioxide and water by means of photosynthesis.
The symbiotic relasionship is highly complex. The algea exists in a stable environment safe from preditation and benefits from constant physical and chemical conditions, inside the cells of a living organism, where a beneficial exchange of nutrients and wastes is provided.
Despite being sheltered from direct sun light, zooxanthellae photosynthesize at nearly the same rate as free swimming spores. As well as protection the zooxanthellae benefit from dissolved nutrients that the polyp absorbs; nitrates, phosphates and ammonia given off by the coral metabolically.
In return the polyp gets energy rich products from the zooxanthellae from photosynthesis (mutualism). In theory a coral could survive without zooxanthellae, but in the nutrient starved reef conditons the coral would inevitably die. There are over 80 known strains of dinoflagellets, and are suited to specific corals and conditions. However a coral can expell a current strain of zooxanthellae for another which better meets its environmental requirements.
Extracted from (Aqaurium corals, Eric H. Borneman, 2001 pg47-55)
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