How Do Coral Reefs Depend on Photosynthesis?

A coral reef depends on sunlight.

Coral reefs, those beautiful structures found in shallow tropical oceans, play an important role in their ecosystems. Besides being home for the coral itself, these reefs provide a protective habitat for countless sea organisms. Among these organisms are the reef-building corals themselves and zooxanthellae, a type of photosynthetic algae. These two organisms depend on each other for their survival.

Coral

Corals are animals that join to live as a colony. One reef is actually composed of hundreds of thousands of individual coral organisms called polyps. A coral polyp consists of a stomach and a mouth surrounded by tentacles that catch food and dispel wastes. Corals also secrete calcium carbonate, which forms the calyx, the hard skeleton that a coral sits in. The individual polyps of a colony are connected by a thin band of living tissue and the hard stony shells join to create the reef.

The Role of Zooxanthellae

Each coral polyp is the host organism for photosynthetic algae called zooxanthellae. Photosynthetic means that the algae require sunlight to produce food, so coral reefs are found in shallow, clear water. However, the algae also require water and carbon dioxide, which are provided by the coral as by-products of its cellular respiration. In turn, the algae produce glucose (sugar), glycerol (fat), amino acids (used in making protein) and oxygen, all of which are needed by the coral to make its food.

Mutual Benefits

Both the coral and the zooxanthellae need this mutually beneficial relationship to survive. Coral provides protection, water and carbon dioxide for the algae, while the algae give the coral the nutrients it needs to produce food. In fact, the coral is so dependent on the algae that it acts in many ways like a plant, such as living where there is sufficient sunlight. Coral colonies that become stressed due to environmental conditions may expel, or blow, all the zooxanthellae, a process known as coral bleaching. When this happens, the coral soon dies.

Reef Building

A coral polyp secretes calcium carbonate, formed with the help of the zooxanthellae, to make the hard stony calyx. This acts as an external skeleton, providing protection from predators. From time to time, a polyp raises itself up off the floor of its calyx and secretes a new floor, leaving a small space between the new and old base. Over thousands of years, a colony of coral polyps secretes enough calcium carbonate to build a reef.

Threats to Coral Reefs

Coral reefs face a number of dangers that can damage, or even kill, whole colonies. Greatest among these is anything that blocks the sunlight essential for photosynthesis. Runoff from agricultural areas may contain fertilizers high in the nitrates that certain algae types need, creating a large colony of algae that compacts together as it floats on the water's surface. This mass, known as an algae bloom, can block sunlight to a coral reef under the surface, depriving the zooxanthellae of the means for photosynthesis. Reefs are also damaged or smothered by garbage, careless fishing practices, violent weather and prolonged low-water levels.

References

• National Oceanic and Atmospheric Administration (NOAA): Corals
• NOAA: Zooxanthellae
• Exploring the Environment: Coral Reefs