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listen to part of a lecture in an ecology class

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and at this point in the semester,

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it\'s time to turn our attention to marine ecosystems,

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and we\'ll begin by discussing coral reefs.

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Coral reefs are teeming with life,

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with organisms of all different types.

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In fact, in terms of their biodiversity, coral reefs are on a par with the tropical rainforests we were studying last week,

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but right now,

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we\'re going to focus on the corals themselves and on a certain microorganism that\'s intimately associated with them.

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Some people have a misconception about corals.

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They think that corals are plants,

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and some corals do look like plants, but actually,

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corals belong to the animal kingdom.

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They\'re tiny animals that live together in groups, forming huge colonies.

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Their outer skeletons are composed of calcium carbonate,

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and these exoskeletons, millions and millions of them, form the reef\'s physical structure,

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as I\'m sure you\'ve heard,

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the world\'s coral populations are decreasing dramatically, by almost 30% in the past 30 years.

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Most often the problem is a phenomenon known as coral bleaching.

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It\'s called bleaching because the affected corals lose their color,

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and it\'s related to the loss of that microorganism that I mentioned.

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Its name is zooxanthella.

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Zooxanthella is a type of algae which is present throughout a coral\'s tissue.

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So what do these zooxanthella Algae do?

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Actually, a number of things,

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but most important, they fulfill up to 90% of the coral\'s energy needs.

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They do this through photosynthesis.

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They convert sunlight into carbon the way plants do,

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and the corals use the carbon as food.

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It\'s a mutually beneficial arrangement.

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The zooxanthellae feed the coral,

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and the coral provide a home for the zooxanthellae.

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So coral bleaching,

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what happens in coral bleaching is

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you see zooxanthellae come in various different colors.

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That\'s why healthy corals are so colorful.

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But in coral bleaching,

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the zooxanthellae are expelled from the coral\'s body.

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Without zooxanthellae in their tissue,

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the corals are white.

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And why are the zooxanthellae expelled?

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Usually higher water temperatures are the culprit.

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When water temperatures get too high, the zooxanthellae photosynthetic apparatus starts to malfunction.

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It can\'t handle as much sunlight as before, and even though it\'s still producing carbon for the corals,

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it also produces toxic molecules that damage the coral tissue.

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The corals respond to this threat by expelling the zooxanthellae

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in coral bleaching.

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What we\'re actually seeing is corals defending themselves against toxic molecules.

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But you can see the problem, right?

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Corals need the carbon that the zooxanthellae produce

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when they expel these algae to protect themselves from tissue damage,

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they lose most of their food supply.

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It takes an increase of only a few degrees Celsius for the process to begin,

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which is bad news, because water temperatures are increasing globally,

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and there\'s not that much we can do to reverse that trend.

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There is good news, though,

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some coral communities that have undergone bleaching have recovered.

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They were repopulated by zooxanthellae that had developed a resistance to the effects of high water temperatures.

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So we\'re seeing the coral regenerate

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new young coral is spreading and replacing the coral that has died.

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This is a trend that could have. Remarkable implications for the future,

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but a lot of things can get in the way of coral reef recovery,

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and at the top of the list are human activities like overfishing and water pollution.

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For example, in the case of overfishing,

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overfishing upsets the delicate balance of the reef\'s ecosystem.

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You see some of the fish species that inhabit coral reefs, eat the plants and seaweeds that are a natural part of the reef ecosystem.

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The fish keep them under control.

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And when humans harvest too many fish from a reef community,

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these plants and seaweeds tend to proliferate

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and eventually cover over the corals completely,

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thereby preventing sunlight from getting through.

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And of course, the photosynthetic activities of the zooxanthellae require sunlight, right?

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Anyway, we can\'t do much to reverse water temperature increases, which is the primary underlying cause of coral bleaching,

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but over fishing and water pollution,

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these are problems we can address on a local as well as a global level, if we want to,

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it\'s by no means an unrealistic goal.

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