What we see as one thing, coral, is actually a partnership between an animal and a single-celled critter (more formally known as a protist, or more specifically, zooxanthella). The animal part is like a tiny sea anemone that builds the hard part of the coral. They look a bit like plants, but they eat, so they are animals. The protist does photosynthesis. Plants do this too, but plants are made of more than one cell (theyre multicellular). Corals get their bright colors from the protists. Protists have brightly-colored pigments to absorb light for their photosynthesis.

When the partnership works well, the protist gets a valuable nutrients from the coral. The urine of the coral contains nitrogen, which the protist needs. The coral also releases CO₂ (carbon dioxide), which the protist uses to do photosynthesis. It takes the CO₂ and water and uses the energy from the sun to make sugar. The coral use this sugar. Both partners benefit, so we call this mutualism.

When conditions get bad, like when it gets too hot or cold, or when the tiny animals that the coral eat are scarce, the partnership can break down. The protists reproduce quickly and keep the sugar for themselves. The coral then uses chemicals to make them leave. This leaves them without the bright colors, so we say they are bleached.

Can you think of other partnerships in the natural world?

Yes, increased temperatures can lead to coral bleaching.

Many corals (and some other sea animals) have single-celled algae that live in their bodies. These algae are a kind of dinoflagellate; otherspecies of dinoflagellate live freely and cause the phosphorescence or"glow" that you can sometimes see in the ocean at night as the water moves around, and some times large populations of dinoflagellates can cause "red tides" that kill a lot of fish. The dinoflagellates that live in corals help the tiny coral animals, however--because they arephotosynthetic, they make extra food to give to their coral hosts, and they may help the coral make its mineral skeleton.

When corals become stressed, however, the dinoflagellates can die or be expelled, and this is what is called "bleaching." Corals can become bleached for many different reasons, but one of the most important is higher temperatures (as you said). Although bleached corals can sometimes survive and regain their dinoflagellates, bleached corals grow and reproduce more slowly and are more likely to die. Coral bleaching has become much more common beginning in the 1980s, and with human-induced global warming represents a major threat to one of the Earth's grandest ecosystems.

Yes, coral is definitely bleached by higher temperatures! I actually wrote an article all about coral bleaching for The Santa Barbara Independent last September, and talked about temperature affecting coral health in it -- this is the relevant part to your question:

"The foundation of coral reefs is an ancient symbiotic relationship between coral and microscopic algae. The single-celled, golden-brown algae, called zooxanthellae (genus Symbiodinium), are less than 0.01 millimeters in diameter, yet they are a fundamental part of most coral reef systems. The relationship between the coral and the zooxanthellae is mutualistic: Both gain something from it. This relationship works so well that, based on fossils, its been around for over 240 million years (since the Triassic period). While this relationship has endured the test of time, it is possibly being challenged more now than ever before, and, with it, the whole structure of coral reefs worldwide.

Over the last 20 to 30 years, there has been a significant increase in coral bleaching. Bleaching occurs when the water temperatures increase only two to three degrees Fahrenheit above the normal temperatures. For reasons not fully understood, this causes the zooxanthellae, which are normally bound in the corals endodermis cells, to be expelled from the coral. If the coral does not get the zooxanthellae back within a month, the coral dies. The remaining coral skeleton is white, or bleached. In 1998, an El Nino year, around 16 percent of the coral reefs worldwide were destroyed by bleaching. In the August 27, 2010, issue of the magazine Science, Clive Wilkinson, a reef expert at Australias Reef and Rainforest Research Center in Townsville, predicted that this year may be worse than 1998, which would be especially disastrous for the many reefs that have not yet recovered from 1998. If temperatures continue to increase, as some experts predict they will, in about 25 to 60 years most reefs will be damaged too greatly to ever recover.

However, there is some evidence to suggest that there may be zooxanthellae that are temperature tolerant, and more resistant to bleaching; this has been especially seen in the fast-growing coral Acropora."

Corals prefer to live in water temperatures that average around 82 degrees Fahrenheit and have minor fluctuations (highs of 85F and lows of 75F). To see the article in its entirety, you can go to this website:

Coral bleaching consists of the corals expelling the algae that live in their tissues that give the corals their color. Corals do this when stressed and high temperature is certainly one way this can happen, but not the only way. Not all corals respond the same way, either, and some coral die-offs are due to other effects such as infections (e.g. white band disease).