TOEFL Listening: ETS-TOEFL听力机经 - _2PD6P4CNVW_S3D3Q$

Lecture: Coral reefs: Narrator: Listen to part of a lecture in a marine biology class. Professor: Coral reefs are important marine ecosystems. These colorful undersea structures are actually built from secretions by coral animals. Recently though, reefs have been threatened by the problem of coral bleaching. To understand what coral bleaching is, you need to first understand the ecology of the corals themselves. So let's start with the coral animals, which is tiny with a single body cavity. Most reef-building corals, corals that actually form structures, have a symbiotic relationship with a type of algae called zooxanthellae. Zooxanthellae live in the tissue of the coral in which we might call the coral's gut. And this relationship between zooxanthellae and the coral is a mutualistic symbiosis which means that both organisms benefit from it. The zooxanthellae provide the coral with food6 and this food comes from the photosynthesis that the zooxanthellae undergo. Student: Excuse me, but I don't understand. These algae live inside the coral which lives in the water. So where do they get the light for photosynthesis? Professor: Well, most coral animals have transparent bodies and one of the requirements for coral is clear, shallow water so the zooxanthellae utilize the light that shine through the water and into the coral's body for photosynthesis. And corals get about 80% of their nutritional needs this way. So corals and zooxanthellae grow much faster and formation of coral structures is also going to occur much faster because of this. In fact, corals without zooxanthellae cannot form new structures. Now, there is another effect of the corals having zooxanthellae within them. This one is not actually a benefit but more of a side effect. The zooxanthellae give corals the wide variety of colors that we're used to seeing on coral reefs. As I've said, coral animal is actually fairly translucent; the colors we see come from the zooxanthellae. Okay, so the corals get nutrition to build reefs. What do the zooxanthellae get out of this relationship? They get some nutrients themselves from coral waste. Another benefit is that they're protective. Nothing can eat these tiny algae while they're living inside the coral. So now that we understand what's going on inside the coral animal, we can talk about coral bleaching which is a great sound of this symbiosis. A variety of environmental conditions can lead to coral bleaching. But the most common are high temperatures and high levels of light. If you think about it, high temperatures and high light levels are actually really good for photosynthesis, right? More photosynthesis, more food. Why should that be a problem? Well, when photosynthesis occurs too rapidly you get a buildup of what are called oxygen radicals within the coral. Now, oxygen radicals are unstable oxygen molecules and they bounce around inside the coral, causing damage to both coral and zooxanthellae. In reaction, the corals expel the zooxanthellae. Once that happens, what you see a white or bleached looking coral because the zooxanthellae hves left. If the high temperatures and the high light levels don't persist for very long, the coral can usually be repopulated with zooxanthellae and go back to normal functioning. It's when it's taken to the extremes that it's a problem. If the corals can't repopulate the zooxanthellae within about a month they're going to starve and die. But even if the coral does recover eventually, there can be some lasting effects according to something called tradeoff theory. Tradeoff theory suggest that organisms put different amounts of resources into different processes depending on how stressed they are. For corals in the aftermath of bleaching, all of their resources are going to go into recovering. They're not going to put any energy into growth and they're definitely not going to put any energy into reproduction. Coral bleaching events are increasing in frequency. Since 1979, recent fits of what we call mass bleaching events. In fact, there's no part of the world where bleaching has not been reported during the past few decades. So what's going to happen if bleaching continues at such a high frequency? We're definitely going to see a decline in coral overall. But we might also see some interesting changes in the species of corals populating reefs. For example, some coral species seem to be more resistant to bleaching than others. Corals that have a rounded, dome-like form, for example, are less prone to bleaching than blanching corals. So we're likely to see a shift to the resistant types of corals.