The algae zooxanthellae are not coral, but they make coral reefs possible. They live within the body of coral polyps and busily photosynthesize, which the coral themselves, being animals, cannot do. This is why reef-building coral polyps, which do not need sunlight themselves, only live in the top 50 meters of the ocean. The algae that live within them need to be in the photic zone, the zone where they can harvest the energy of the sun.
It is a symbiotic relationship, one from which both partners benefit. The algal cells carry out photosynthesis and produce nutrients, and their excess nutrients go to the coral polyps, enabling them to live and grow. The coral provide shelter and protection to the zooxanthellae, as well as nitrogen and phosphorus from their waste, which might be thought of as fertilizer. They also provide carbon dioxide, which is needed for photosynthesis.
Without their symbionts, their algal partners, the coral would grow too slowly to produce their splendid reefs. About 90% of coral nutrition comes from algae. These organisms need each other.
Unfortunately, when the coral are stressed they shed or consume their zooxanthellae. This is coral bleaching. There are fewer of the golden brown algae or their photosynthetic pigments decline. (The cnidarian itself, the coral, may lose its pigment too.) At the worst, the result is a bony white reef which may soon die. Stress may be caused by extra low tides, by intensive solar radiation in shallow water, or by temperature extremes. The coral and the algae must live together in an undisturbed, moderate environment for the reef to thrive.
Lichens are another well-known example of a symbiotic relationship involving algae. The algae involved in this relationship are generally a one-celled green alga like Trexbouxia, Myrmecia, or unpronounceable Pseudotrexbouxia. (Cyanobacteria take the place of alga in some lichen.) The fungus partner may be a Basidiomycete or an Ascomycete.
Lichen is composed of fungal filaments that have grown around and into the interspersed cells of the alga. Lichen grows on soil, rock, structures, and trees. They are not parasites of the trees, because they do not take anything from them. They do sometimes secrete substances that break down rock, creating new soil. They live almost anywhere, including the tundra. In arctic regions they form almost the only groundcover in some regions, together with moss and liverworts. Herbivores graze upon them there. Extremely hardy, lichen can dry out to the point of coming apart in flakes and still revive when moistened.
Yet they are extremely vulnerable to air pollution. Some lichens are now used to assess air quality because of this sensitivity.
There is an amphibian in a symbiotic relationship as well. The spotted salamander, Ambystoma maculatum, has a symbiotic relationship with a kind of algae found almost nowhere in nature but in jelly capsules. A jelly capsule is the scientific way of describing a mass of salamander eggs. The alga Oophila amblystomatis, whose name means "loves salamander eggs", lives inside the jelly capsule of the salamander.
The female salamander deposits eggs in a jelly mass attached to vegetation in a pool. The jelly provides a protective environment for the embryos, but makes it difficult for oxygen to reach them through the thick mass in which they are incubating. Most eggs in nature allow gas exchange, even hard-shelled bird eggs. The "eggs" of these salamanders though, are not very permeable at all. However, the Oophila algae solve the problem. Living in the egg mass with embryos, the algae benefit from carbon dioxide that the developing salamanders exhale and nitrogen-bearing ammonia they excrete, while the embryo benefits from the oxygen the algae produce by photosynthesis, and from their removal of waste products. The algae also tint the egg mass green.
These are only three examples of symbiosis that includes algae. Other algae form unions with the protists Forminifera, an association which produces a fine-shelled sea bottom deposit known as living sands. Algae also live in symbiosis with certain sea sponges and jellyfish in partnerships also susceptible to bleaching. In fact, all these intricate relationships are delicate. Variations in temperature or the presence of pollution can easily eliminate such fascinating creatures from the world.