IELTS Reading 練習 珊瑚礁生態系（含題目+詳解）

Coral Reef Ecosystems

Although coral reefs cover less than one per cent of the ocean floor, they shelter roughly a quarter of all known marine species. This extraordinary density of life is built on a partnership between two very different organisms. The corals themselves are tiny animals, related to jellyfish and sea anemones, that secrete the limestone cups in which they live. Inside the soft tissue of each coral are millions of single-celled algae called zooxanthellae. These algae photosynthesise like plants, converting sunlight into sugars; the corals absorb most of those sugars in return for providing the algae with shelter and a steady supply of carbon dioxide and waste nutrients.

The arrangement is so productive that reefs can flourish in the clear, nutrient-poor waters of the tropics where little else grows. Reef-building corals, however, have narrow tolerances. They thrive only in seawater between roughly 23 C and 29 C, in salinity close to that of the open ocean and at depths shallow enough for sunlight to reach the algae. When sea temperatures rise even one or two degrees above the local norm for several weeks, the corals expel their algae in a stress response known as bleaching. Without their photosynthetic partners, the corals turn ghostly white and, if the heat persists, eventually die.

Mass bleaching events have become dramatically more frequent since the 1980s. The Great Barrier Reef, which lies along the north-east coast of Australia and is the largest reef system on Earth, suffered five separate mass bleachings between 2016 and 2024. Other pressures compound the problem: agricultural runoff alters water chemistry, overfishing removes the herbivorous fish that keep algae in check, and ocean acidification — caused by the same carbon emissions that warm the water — dissolves the limestone faster than corals can replace it.

Restoration efforts include the cultivation of heat-tolerant coral varieties in coastal nurseries and their replanting onto damaged reef sections. Such projects have shown encouraging local results, but they cannot keep pace with the global scale of the threat. Most marine biologists now agree that the long-term survival of coral reefs as they currently exist depends on rapid reductions in greenhouse-gas emissions worldwide.

1. Around 25 per cent of marine species live on coral reefs.
2. Zooxanthellae receive shelter and waste nutrients from the corals.
3. Bleaching occurs when corals consume their algae.
4. The Great Barrier Reef is the world's oldest reef system.

Choose the correct letter, A, B, C or D.

1. According to the passage, reef-building corals require water that is:

- A. very deep and cold - B. shallow, warm and as salty as the open ocean - C. rich in nutrients from rivers - D. completely free of any algae

1. The writer suggests that local restoration projects:

- A. have failed to produce any positive results - B. cannot match the global scale of the problem on their own - C. are now the main hope for reef survival - D. have eliminated the need to reduce emissions

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

Reef-building corals live with single-celled algae called (7) ______ that produce sugars through photosynthesis. When sea temperatures stay too high for too long, the corals expel the algae and undergo (8) ______. Beyond warming, reefs are also damaged by agricultural runoff, overfishing and ocean (9) ______.

1. TRUE Supporting sentence: "they shelter roughly a quarter of all known marine species". "A quarter" = 25 per cent.

2. TRUE Supporting sentence: "providing the algae with shelter and a steady supply of carbon dioxide and waste nutrients". The corals provide both, the statement is direct paraphrase.

3. FALSE Supporting sentence: "the corals expel their algae in a stress response known as bleaching". The corals expel the algae; they do not consume them. The statement contradicts the passage.

4. NOT GIVEN The passage says the Great Barrier Reef is "the largest reef system on Earth" but says nothing about its age. Don't confuse largest with oldest. NOT GIVEN.

5. B — shallow, warm and as salty as the open ocean Supporting sentence: "They thrive only in seawater between roughly 23 C and 29 C, in salinity close to that of the open ocean and at depths shallow enough for sunlight to reach the algae". All three conditions appear.

6. B — cannot match the global scale of the problem on their own Supporting sentence: "Such projects have shown encouraging local results, but they cannot keep pace with the global scale of the threat". Direct paraphrase.

7. zooxanthellae Supporting sentence: "single-celled algae called zooxanthellae". Single-word answer, exact match.

8. bleaching Supporting sentence: "the corals expel their algae in a stress response known as bleaching". Single-word answer.

9. acidification Supporting sentence: "ocean acidification ... dissolves the limestone faster than corals can replace it". Single-word answer; the question supplies "ocean".