Two important symbiotic relationships concerning algae are the Lichens and coral reefs. Things that live in symbiosis have relationships that benefit at least one of the participants. There are three common types of symbiosis: Parasitic, in which only one entity benefits, may depend fully on the other for survival, and is usually harmful to its host. Commensal, in which only one entity benefits and the other is unaffected. And mutualistic, in which both participants provide the other with something beneficial. The two types of symbiosis described below are mutualistic.
Coral reefs owe their striking beauty to the algae living among them, whose photosynthetic pigments allow for a wide range of colorful compositions on the ocean floor. Coral, although it has the appearance of an upside down jellyfish, are actually classified as Cnidarians in the Animal Kingdom. They consist of multitudes of genetically similar polyps all existing next to one another. These multicellular organisms, usually only a few millimeters across, are responsible for building such wonders as the Great Barrier Reef, the world's largest coral reef, located off the shores of Australia. The calcium carbonate skeletons of coral build up slowly over hundreds of years and give these stacks of tiny marine organisms their pleasantly shaped homes we see today.
Unicellular algae, known as zooxanthellae, inhabit the tissues of coral. Each Cnidarian polyp contains a single, photosynthetic algal cell. The algae undergo photosynthesis and produce oxygen and energy rich nutrients, like fat and carbohydrates, that the coral needs. This is why coral usually grows in depths less than 100 meters. Because light is needed in order for their algal symbiots to achieve photosynthesis. In return for the nutrients they receive, the polyps provide nitrates, phosphates, and carbon dioxide to the algae cells.
During times of stress, the polyp may expel the algae. This is referred to as coral bleaching due to the loss of algae pigment responsible for the coral's bright colors. Causes may include higher concentrations of carbon dioxide due to global warming, a slight increase in water temperature, or too much fresh water dispersed in the area. This may also affect other sea creatures, both large and small, who depend on the coral reefs for food or shelter.
A Lichen is another symbiotic relationship between algae and fungi. A Lichen is a growth that may look like a simple plant, but is instead a symbiosis formed when a fungal tissue surrounds an algal cell. The Lichen takes nutrients from whatever it happens to be growing on and hands them off to the algae. The Lichen also provides the algae with water needed for photosynthesis and can help its algal counterpart in arid conditions. The algal cells produce nutrients for the fungus through photosynthesis, even in conditions where there is no other source of nutrients, like on a rock surface on the frozen tundra. Lichens are a food source for reindeer, some squirrels, insects, and slugs. Some Lichens extracts are also used for medicinal purposes as well as dyes and soaps.
Both coral reefs and Lichens are sensitive to their environment. Coral bleaching or dying populations of Lichens can be an indication that the surrounding ecosystem may be in danger.