listen to part of a lecture in an ecology class and at this point in the semester, it\'s time to turn our attention to marine ecosystems, and we\'ll begin by discussing coral reefs. Coral reefs are teeming with life, with organisms of all different types. In fact, in terms of their biodiversity, coral reefs are on a par with the tropical rainforests we were studying last week, but right now, we\'re going to focus on the corals themselves and on a certain microorganism that\'s intimately associated with them. Some people have a misconception about corals. They think that corals are plants, and some corals do look like plants, but actually, corals belong to the animal kingdom. They\'re tiny animals that live together in groups, forming huge colonies. Their outer skeletons are composed of calcium carbonate, and these exoskeletons, millions and millions of them, form the reef\'s physical structure, as I\'m sure you\'ve heard, the world\'s coral populations are decreasing dramatically, by almost 30% in the past 30 years. Most often the problem is a phenomenon known as coral bleaching. It\'s called bleaching because the affected corals lose their color, and it\'s related to the loss of that microorganism that I mentioned. Its name is zooxanthella. Zooxanthella is a type of algae which is present throughout a coral\'s tissue. So what do these zooxanthella Algae do? Actually, a number of things, but most important, they fulfill up to 90% of the coral\'s energy needs. They do this through photosynthesis. They convert sunlight into carbon the way plants do, and the corals use the carbon as food. It\'s a mutually beneficial arrangement. The zooxanthellae feed the coral, and the coral provide a home for the zooxanthellae. So coral bleaching, what happens in coral bleaching is you see zooxanthellae come in various different colors. That\'s why healthy corals are so colorful. But in coral bleaching, the zooxanthellae are expelled from the coral\'s body. Without zooxanthellae in their tissue, the corals are white. And why are the zooxanthellae expelled? Usually higher water temperatures are the culprit. When water temperatures get too high, the zooxanthellae photosynthetic apparatus starts to malfunction. It can\'t handle as much sunlight as before, and even though it\'s still producing carbon for the corals, it also produces toxic molecules that damage the coral tissue. The corals respond to this threat by expelling the zooxanthellae in coral bleaching. What we\'re actually seeing is corals defending themselves against toxic molecules. But you can see the problem, right? Corals need the carbon that the zooxanthellae produce when they expel these algae to protect themselves from tissue damage, they lose most of their food supply. It takes an increase of only a few degrees Celsius for the process to begin, which is bad news, because water temperatures are increasing globally, and there\'s not that much we can do to reverse that trend. There is good news, though, some coral communities that have undergone bleaching have recovered. They were repopulated by zooxanthellae that had developed a resistance to the effects of high water temperatures. So we\'re seeing the coral regenerate new young coral is spreading and replacing the coral that has died. This is a trend that could have. Remarkable implications for the future, but a lot of things can get in the way of coral reef recovery, and at the top of the list are human activities like overfishing and water pollution. For example, in the case of overfishing, overfishing upsets the delicate balance of the reef\'s ecosystem. 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. The fish keep them under control. And when humans harvest too many fish from a reef community, these plants and seaweeds tend to proliferate and eventually cover over the corals completely, thereby preventing sunlight from getting through. And of course, the photosynthetic activities of the zooxanthellae require sunlight, right? Anyway, we can\'t do much to reverse water temperature increases, which is the primary underlying cause of coral bleaching, but over fishing and water pollution, these are problems we can address on a local as well as a global level, if we want to, it\'s by no means an unrealistic goal.