雅思阅读 065 — How Corals Fight Back against Warming Seas

改编自 Scientific American（2025-02，Rohini Subrahmanyam）。

🎧 课文朗读音频（约 2 分钟）

Reading Passage

Deep underneath the eastern tropical Pacific Ocean lies a dazzling landscape of undulating coral reefs coloured by photosynthetic algae, from which corals get their energy. But in the early 1980s an aquatic heat wave caused by the El Niño climate phenomenon led to a record-breaking mass-bleaching event, turning more than 90 per cent of these corals a pale, lifeless white. The algae — which had thrived inside their coral hosts for millions of years — could no longer bear to live within them.

Strong El Niño events warmed up the same Pacific waters in the late 1990s and again in 2015–2016, but scientists noticed that these heat waves did not affect the reefs as badly as the first. Diving after the latest event, University of Miami marine biologist Ana Palacio saw that some of the corals seemed to be resisting or recovering from the bleaching. Maybe, Palacio thought, they had found a way to adapt.

Many adult corals are tethered to the reefs they build. Swimming to cooler waters is not an option, making them particularly vulnerable to the changing climate. But corals are also resilient, and scientists are discovering how they adapt. Some corals switch out their algal tenants for more heat-resistant species. Others use rows of tiny hairs on their bodies to "fan" away excess harmful oxygen released by stressed-out algae. And certain baby corals modify their own metabolisms to withstand the warming waters.

When Palacio and her team examined coral reefs after the 2015–2016 heat wave, they found that particular corals called Pocillopora — the main reef-building species in the eastern tropical Pacific — seemed to have expelled the algae that usually resided within them and taken in other species more tolerant to heat.

"They start changing their algae community as the water becomes warmer and warmer, and they associate more and more with this thermotolerant algal symbiont called Durusdinium glynnii," Palacio explains. This species' name comes from the Latin word durus, meaning "rough" or "tough". Most symbiotic algae produce toxic levels of oxygen under heat stress, forcing the corals to evict them. But Durusdinium keeps its oxygen levels tolerable.

Yet corals do not always rely on their algal guests to avoid excessive oxygen; sometimes they can take matters into their own "hands". Rows of cilia — tiny, hairlike projections — act like corals' own personal ventilation system by fanning excess oxygen toward spots that lack it.

In 2022, marine biologists Cesar O. Pacherres and Soeren Ahmerkamp, then at the University of Bremen in Germany, showed that these fast-beating cilia create microscopic whirlpools in the water, swirling the oxygen around and preventing it from accumulating in any one spot. All corals have this ventilation system, but how much they use it varies between species.

Corals are not always stuck in place; their larvae float freely through the ocean before settling, which offers crucial opportunities for a species to shift to more hospitable waters or spread its heat-tolerant genes. Ariana Huffmyer, a marine biologist at the University of Washington, recently showed that coral larvae exposed to warm water for as little as three days in the laboratory alter their own metabolism to cope with heat stress and avoid bleaching.

Under stress, corals produce too much nitrogen. This excess causes the algae to divide rapidly, hoarding carbon and keeping it from their hosts. Huffmyer discovered that baby corals exposed to short periods of heat stress learn to keep the excess nitrogen to themselves, maintaining a stable symbiosis.

Pacherres cautions that such adaptations can protect an organism only to some extent. "If it's hot, we humans can sweat to alleviate the heat. But if it gets too hot, we die. At one point sweating is not enough."

Whatever heat-beating tools corals have can help scientists develop protection strategies. Baby corals that can withstand stress are especially important for conservation because they can travel between reefs and potentially share heat-tolerant genes in new locations.

Questions

Questions 1–5: TRUE / FALSE / NOT GIVEN

1. The 1980s El Niño event bleached more than 90% of corals in the eastern tropical Pacific.
2. Corals were affected just as severely by the 2015–2016 heat wave as by the 1980s event.
3. Pocillopora corals adopted heat-tolerant algae after the 2015–2016 event.
4. Durusdinium glynnii produces toxic oxygen under heat stress.
5. Adult corals can swim to cooler waters when temperatures rise.

Questions 6–10: Choose the correct letter, A, B, C or D.

6. The name Durusdinium derives from a Latin word meaning

   • A. "ocean"
   • B. "rough or tough"
   • C. "white"
   • D. "reef"

7. What function do cilia perform in corals?

   • A. They digest algae.
   • B. They fan away excess oxygen using microscopic whirlpools.
   • C. They attract prey.
   • D. They produce carbon.

8. Coral larvae exposed to warm water for three days in the lab

   • A. immediately died
   • B. changed their metabolism to cope with heat stress
   • C. grew larger than usual
   • D. developed new algae species

9. Under heat stress, excess nitrogen causes algae to

   • A. produce more oxygen
   • B. divide rapidly and hoard carbon
   • C. leave the coral immediately
   • D. become smaller

10. Pacherres's comparison with human sweating illustrates that

    • A. corals can adapt indefinitely
    • B. adaptation has limits beyond which organisms cannot survive
    • C. humans are more resilient than corals
    • D. heat is not a serious threat

Questions 11–13: Complete the sentences. Choose NO MORE THAN TWO WORDS.

11. Bleaching occurs when stressed corals expel their __________ partners.
12. Cilia create microscopic __________ that distribute excess oxygen.
13. Baby corals that survive heat stress are valuable because they can share heat-tolerant __________ between reefs.

Answers

1. TRUE
2. FALSE (影响减轻了)
3. TRUE
4. FALSE (它保持氧气水平在可耐受范围)
5. FALSE (成体珊瑚固定在礁上)
6. B
7. B
8. B
9. B
10. B
11. algal / symbiotic
12. whirlpools
13. genes

Glossary

• bleaching /ˈbliːtʃɪŋ/ n. 白化
• symbiont /ˈsɪmbaɪɒnt/ n. 共生体
• cilia /ˈsɪliə/ n. 纤毛
• larvae /ˈlɑːviː/ n. 幼虫
• metabolism /məˈtæbəlɪzəm/ n. 新陈代谢
• resilient /rɪˈzɪliənt/ adj. 有恢复力的
• thermotolerant /ˌθɜːməʊˈtɒlərənt/ adj. 耐热的
• hoard /hɔːd/ v. 囤积
• conservation /ˌkɒnsəˈveɪʃn/ n. 保护