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Reef-building corals need warm, tropical or subtropical waters. Generally, most tropical or subtropical corals can't grow in oceans where the water temperature dips below 18°C (64°F) for extended periods in the winter. But how warm is too warm? In the 1980s, scientists discovered that corals start getting stressed if the water gets only 1°C warmer than the highest temperature expected in the summertime (Glynn and D'Croz, 1990). We call this temperature the "bleaching threshold" because the stress caused by warmer-than-normal water can cause the corals to bleach. But what happens to a coral when it bleaches? We mentioned earlier that Polly has tiny plants (algae) called zooxanthellae that grow in her body tissues. Normally, these zooxanthellae absorb energy from the sun and use it for photosynthesis. When the water gets too warm, the zooxanthellae can't use the sun's energy as efficiently. They turn this excess energy from sunlight into chemicals (toxins called "reactive oxygen species") that can cause damage to themselves and to Polly. So even though Polly normally needs the zooxanthellae, she has to get rid of them to survive this temperature stress. As a result, she will expel most of the zooxanthellae from her body. Her own body tissue is transparent, and the rock underneath her is white -- so what you see is her white skeleton instead of the normal golden-brown of the zooxanthellae. The entire coral soon looks pale or white, so we say that it looks "bleached". Can Polly the polyp survive the bleaching? Yes -- if the warm temperatures don't get too high or last too long, the coral bleaching may be mild and Polly can recover. Polly may gradually rebuild her necessary stock of zooxanthellae over several months and go on to live a healthy life. However, if the water temperature increases substantially above the bleaching threshold, and/or stays above the bleaching threshold for an extended period of time, significant coral bleaching can occur over a large area of the reef. We call this mass coral bleaching. If the mass coral bleaching persists or intensifies even further, entire colonies of Polly and her cousins can die. Why do the corals die? Corals without their symbiotic algae (zooxanthellae) are left without their main energy source, and there may be cellular damage from the reactive oxygen species. Even corals that survive bleaching are often likely to become infected with coral diseases, just like a stressed person is more likely to get sick. These diseases can also kill corals, sometimes many months after the bleaching. Each year, scientists learn more and more about why corals bleach, and why they are so sensitive to high temperatures and sunlight. This research helps us better understand ways to help corals survive or recover from bleaching, especially mass bleaching events driven by climate change. As the world continues to warm at an accelerated rate, we need to do all we can to keep coral reefs healthy. In the last section of this tutorial, we will discuss some of the resources that are available to help resource managers and the public take action to help protect their coral reefs from bleaching.
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