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1 .listen to part of a lecture in an ecology class
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2 .and at this point in the semester,
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3 .it\'s time to turn our attention to marine ecosystems,
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4 .and we\'ll begin by discussing coral reefs.
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5 .Coral reefs are teeming with life,
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6 .with organisms of all different types.
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7 .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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8 .but right now,
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9 .we\'re going to focus on the corals themselves and on a certain microorganism that\'s intimately associated with them.
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10 .Some people have a misconception about corals.
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11 .They think that corals are plants,
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12 .and some corals do look like plants, but actually,
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13 .corals belong to the animal kingdom.
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14 .They\'re tiny animals that live together in groups, forming huge colonies.
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15 .Their outer skeletons are composed of calcium carbonate,
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16 .and these exoskeletons, millions and millions of them, form the reef\'s physical structure,
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17 .as I\'m sure you\'ve heard,
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18 .the world\'s coral populations are decreasing dramatically, by almost 30% in the past 30 years.
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19 .Most often the problem is a phenomenon known as coral bleaching.
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20 .It\'s called bleaching because the affected corals lose their color,
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21 .and it\'s related to the loss of that microorganism that I mentioned.
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22 .Its name is zooxanthella.
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23 .Zooxanthella is a type of algae which is present throughout a coral\'s tissue.
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24 .So what do these zooxanthella Algae do?
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25 .Actually, a number of things,
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26 .but most important, they fulfill up to 90% of the coral\'s energy needs.
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27 .They do this through photosynthesis.
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28 .They convert sunlight into carbon the way plants do,
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29 .and the corals use the carbon as food.
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30 .It\'s a mutually beneficial arrangement.
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31 .The zooxanthellae feed the coral,
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32 .and the coral provide a home for the zooxanthellae.
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33 .So coral bleaching,
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34 .what happens in coral bleaching is
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35 .you see zooxanthellae come in various different colors.
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36 .That\'s why healthy corals are so colorful.
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37 .But in coral bleaching,
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38 .the zooxanthellae are expelled from the coral\'s body.
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39 .Without zooxanthellae in their tissue,
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40 .the corals are white.
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41 .And why are the zooxanthellae expelled?
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42 .Usually higher water temperatures are the culprit.
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43 .When water temperatures get too high, the zooxanthellae photosynthetic apparatus starts to malfunction.
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44 .It can\'t handle as much sunlight as before, and even though it\'s still producing carbon for the corals,
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45 .it also produces toxic molecules that damage the coral tissue.
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46 .The corals respond to this threat by expelling the zooxanthellae
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47 .in coral bleaching.
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48 .What we\'re actually seeing is corals defending themselves against toxic molecules.
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49 .But you can see the problem, right?
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50 .Corals need the carbon that the zooxanthellae produce
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51 .when they expel these algae to protect themselves from tissue damage,
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52 .they lose most of their food supply.
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53 .It takes an increase of only a few degrees Celsius for the process to begin,
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54 .which is bad news, because water temperatures are increasing globally,
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55 .and there\'s not that much we can do to reverse that trend.
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56 .There is good news, though,
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57 .some coral communities that have undergone bleaching have recovered.
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58 .They were repopulated by zooxanthellae that had developed a resistance to the effects of high water temperatures.
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59 .So we\'re seeing the coral regenerate
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60 .new young coral is spreading and replacing the coral that has died.
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61 .This is a trend that could have. Remarkable implications for the future,
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62 .but a lot of things can get in the way of coral reef recovery,
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63 .and at the top of the list are human activities like overfishing and water pollution.
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64 .For example, in the case of overfishing,
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65 .overfishing upsets the delicate balance of the reef\'s ecosystem.
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66 .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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67 .The fish keep them under control.
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68 .And when humans harvest too many fish from a reef community,
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69 .these plants and seaweeds tend to proliferate
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70 .and eventually cover over the corals completely,
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71 .thereby preventing sunlight from getting through.
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72 .And of course, the photosynthetic activities of the zooxanthellae require sunlight, right?
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73 .Anyway, we can\'t do much to reverse water temperature increases, which is the primary underlying cause of coral bleaching,
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74 .but over fishing and water pollution,
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75 .these are problems we can address on a local as well as a global level, if we want to,
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76 .it\'s by no means an unrealistic goal.
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