Algae and the Reef

Of the two main types of plants found in the coral reef environment (algae and seagrasses), algae is probably the most important to the health of the reef. Algae vary in size from unicellular plants such as zooxanthellae to multicellular forms, commonly called seaweeds. Different pigments in the algae are responsible for the characteristic colors of certain types, such as red, brown, and green algae. For various reasons, the presence of both unicellular and multicellular algae is critical to the growth and maintenance of the reef ecosystem.

Perhaps the most commonly known type of algae is zooanthellae, single-celled green algae. Zooanthellae live within the tissues of coral and allow hard corals to build their stony homes by assisting them in the production of calcium carbonate. Through the process of photosynthesis, the zooanthellae provide the coral with oxygen, essential to the survival of all living things. The rigid environmental requirements of the zooanthellae are the explanation behind the narrow temperature range that corals can live in. When the temperature of water rises from such causes as global warming, coral polyps expel their population of zooanthellae, and consequently, the coral and algae die.

Reef Brief is a weekly column published in the San Pedro Sun
Of multicellular algae, the two main types are coralline and calcareous. Coralline algae are plants that contain calcium carbonate in their tissue, made up of masses of fine thread-like filaments that spread out over the reef rock surface. The encrusting filaments trap sediments of sand, as well as cement the particles of sand together. In this way, coralline algae strengthen and support the coral reef structure. Even after a storm hits and many coral colonies have been broken, coralline algae quickly bind the pieces back together. Unlike the encrusting quality of coralline algae, calcareous algae, on the other hand, tend to grow upright. They too produce calcium carbonate (limestone) and a unique quality of this type of algae is that when the algae dies, sand from the limestone is produced. As a result, about fifty percent of the sand found on our beaches is created. Algae is not only important to coral reefs; as a primary producer in the food chain, many fish depend on it as well. Many fish feed on the algae, which helps the reef when there is an overproduction of algae. Parrotfish, for example, were once thought to be destructive, feeding on large areas of the reef, but now it is understood that Parrotfish actually help keep the algae production of the reef in check. In another relationship with algae, male Damselfish use red algae to prepare a nest on the reef on which the female can lay her eggs.

Everything is the sea strives to live in balance with each other. This applies to algae as well. If there is not enough algae, the coral will die because it will not be able to acquire food and oxygen. If there is too much algae, the coral will be choked to death. It is this balance of the ecosystem that is currently being threatened by changes taking place on Earth, most notably, global warming. An imbalance in algae will inevitably lead to a chain of events that will change the coral reef ecosystem forever.

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