Symbiotic Relationship Between Coral and Algae
Coral and Zooxanthellae Need Each Other to Survive
May 20, 2009
The large structures people see and think of as coral are actually made up of thousands of coral organisms called coral polyps and the skeletons their predecessors have left behind. Corals are animals, but many kinds of coral depend on plants for survival. The plant so many corals need is a single-celled algae called zooxanthellae, that lives within the polyp. This symbiotic relationship, where two species benefit each other, is known as mutualism.
Corals Depend on Algae Photosynthesis
Algae offer corals the byproducts of their photosynthesis, which is why deep-water corals have no use for algae. However, virtually all tropical reef-building corals rely on zooxanthellae. The NOAA describes coral feeding behavior in their “Coral Reef Biology” page, on the Coral Reef Information System (CoRIS) website. In the coral algae symbiotic relationship the zooxanthellae fix large amounts of carbon through photosynthesis and pass on the carbon to the coral in the form of glycerol or glucose. These chemicals provide the coral with the energy and building blocks it needs to function. Scientists estimate that, depending on the species, corals receive 50% to 95% of their energy from this photosynthetic process.
In This Mutualism Coral Provide Algae Nutrients and a Place to Live
In the coral-algae mutualism the algae receive a place to live. “Coral Reef Biology” describes how most zooxanthellae live in the coral polyp’s tentacles and gastrodermal cells in the digestive tract.
Corals do not receive all their nutrients from algae. Many corals catch prey at night, using their tentacles to catch zooplankton and bring it to their mouths. This prey provides the polyps with nitrogen, an essential element needed for functioning. The polyp passes some of the nitrogen along to the algae, which also requires this element.
Global Warming is Affecting the Coral-Algae Symbiosis
In her April 2003 article “Coral bleaching –will global warming kill the reefs?”, Vanessa Woods describes how higher water temperatures force corals to kick out their indispensable algae. Warmer water temperatures block a photosynthetic reaction in algae that’s necessary for converting carbon dioxide into sugar. This causes a poisonous build-up in the algae which threatens the polyp. The polyp reacts by forcing out the zooxanthellae. Since the algae is actually responsible for the coral’s color, its exit causes the coral to turn a bleached white. Corals can recover if cooler temperatures return, but if they don’t the corals slowly starve to death without their zooxanthellae.
Coral reefs are not only beautiful, they are an important part of the ecosystem. Many fisheries and thus the diets of many people rely on the health of coral reefs. It is thus important to ensure the coral-zooxanthellae symbiosis continues.
