IELTS Reading 117: The Corals That Refuse to Die

Adapted from Science Advances / SevenSeas Media (2024-2025). IELTS Reading Section 3 difficulty, approximately 1,050 words. Source: https://sevenseasmedia.org/egypt-red-sea-super-corals-record-recovery-2024-bleaching/

🎧 Audio

🎧 Listen to the audio

🎧 Listen to the audio

🎧 Listen to the audio

🎧 Listen to the audio

Reading Passage

A. When a marine heat wave strikes a reef, the usual picture is one of universal disaster: corals bleach white, their tissues thin, and the reef that once teemed with fish turns grey and silent. But step back from that image and a more complicated truth appears. Place three corals of the same species, side by side on the same patch of seabed, and one may bleach and die, another may look perfectly untouched, and a third may pale and then recover. The heat does not act evenly. Some colonies, somehow, endure temperatures that would wipe out their neighbours, and when researchers tested northern Red Sea corals they found animals that could withstand rises of more than five degrees and survive conditions that kill related populations elsewhere. These hardy animals have become known, half-jokingly, as "super corals" — and understanding why they survive has become one of the most urgent questions in reef science. If the trick can be learned, copied or bred into more vulnerable populations, it might buy reefs the decades they need to outlast a warming ocean. If it cannot, the recovery of reefs will depend entirely on slowing the warming itself.

B. The first thing to grasp is that a coral is not one organism but a committee. The soft animal itself builds the stony skeleton, but it lives in intimate partnership with microscopic algae called zooxanthellae, which photosynthesise and pass food to the coral. Around them swims a community of bacteria and other microbes, and the whole living system — animal, algae, bacteria — functions as a single ecological unit that biologists call the holobiont. When the water gets too hot, the partnership breaks down: the stressed coral expels its coloured algae, turns white, and, if the heat persists, starves. A heat-tolerant coral, then, is not necessarily one whose animal alone is tougher. It may be one whose algae are more robust, whose bacterial community is better balanced, or whose internal chemical signals manage stress more gracefully. The Red Sea corals' resistance, studies show, runs through the whole holobiont rather than through the animal tissue in isolation. Pull the coral apart — animal, alga, bacteria — and the magic disappears; it lives in the combination, not in any one partner.

C. At the molecular level, the difference can be surprisingly concrete. Researchers examining a heat-loving algal partner called Durusdinium trenchii discovered that it had duplicated its entire genome at some point in its evolutionary past — a rare event known as whole-genome duplication. Carrying two copies of every gene gives an organism spare genetic material: one copy keeps doing its old job, while the other can drift, mutate and evolve new functions. The team concluded that this duplication helped equip the alga to tolerate heat better, and therefore to shield its coral host. Another line of work has measured heat-shock proteins — the cell's emergency repair crews that refold damaged proteins when temperatures spike. Across the Great Barrier Reef, researchers stress-tested more than seven hundred colonies and found that thermal thresholds varied by about six degrees between different reefs and three degrees even within the same reef. Tolerance, in other words, is not a yes-or-no switch but a spectrum hidden inside a single species. That spectrum matters enormously for management: if a reef already harbors a few tough individuals, natural selection may act faster than anyone assumed.

D. That variation has opened a tempting and controversial prospect: assisted evolution. Why wait for natural selection, researchers ask, when humans can speed it up? In one approach, the hardiest adult corals are chosen as parents and their heat-tolerant offspring bred and outplanted onto damaged reefs. In another, algal symbionts are "heat-evolved" in the laboratory, allowed to adapt to warm water over many generations, and then reintroduced. Early results were encouraging — heat-evolved symbionts did improve survival at stressful temperatures. But the same experiments delivered a warning. At comfortable temperatures of twenty-eight degrees, the very symbionts that protected corals at thirty-two slowed the animals' growth. Worse, when breeding and symbiont treatments were combined, the outcomes were mixed: sometimes they helped additively, sometimes one effect cancelled the other. Heat tolerance, it turned out, comes at a price that must be paid somewhere else. The trade-off is not a reason to abandon assisted evolution, but it is a reason to expect it to look less like a miracle cure and more like a complicated balancing act.

E. The most natural experiment of all is the Red Sea itself. After the record global bleaching of 2024, reefs in the northern Red Sea recovered by roughly eighty-five per cent — a recovery rate that stunned biologists watching other oceans. Here, the hardy corals may be the product of a long history: populations that colonised the narrow, hot basin thousands of years ago were already filtered for tolerance, and their descendants carry that legacy. The Red Sea, in this sense, is not an anomaly but an experiment written on the seabed — a place where corals have already been tested by heat and have, mostly, passed. The lesson for conservationists is double-edged. On the one hand, the existence of super corals proves that reefs are not helpless; some populations can survive what kills others. On the other, transplanting or breeding them will not, by itself, buy reefs the future. A coral engineered to withstand five extra degrees is still a coral that needs clean water, enough oxygen and, above all, a world that stops heating. The hardiest animal on Earth, researchers note, cannot outlive a climate that keeps breaking its own records. Super corals may buy time; they do not cancel the need to cut emissions. Transplanting a Red Sea strain onto a Great Barrier Reef that keeps hitting record temperatures is, in the end, a bandage on a wound that the atmosphere keeps reopening.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. A coral as a community — the holobiont ii. Whole genomes, spare genes and hidden variation iii. Assisted evolution — and its hidden cost iv. The Red Sea as a natural lesson v. How marine heat waves are measured vi. The commercial value of coral jewellery vii. Why all corals bleach at the same temperature

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

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

5. What is the "holobiont"? A. A single species of reef fish. B. The coral animal together with its algae and bacterial community acting as one unit. C. The stony skeleton alone. D. A type of marine heat wave.

6. Why might whole-genome duplication help an alga tolerate heat? A. It makes the alga larger. B. A spare gene copy can evolve new functions while another keeps the old job. C. It removes all the alga's genes. D. It allows the alga to leave the coral.

7. What did the assisted-evolution experiments reveal? A. Heat-evolved symbionts always improved growth at every temperature. B. Heat-evolved symbionts improved survival under heat but reduced growth at normal temperatures. C. Breeding had no effect whatsoever. D. Combining treatments never worked.

8. What is the main conclusion about super corals? A. They eliminate the need to cut carbon emissions. B. They prove reefs are not helpless, but cannot by themselves replace emission cuts. C. They will die within a decade. D. They exist only in aquariums.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Northern Red Sea corals can withstand temperature rises exceeding five degrees.
10. Thermal tolerance is identical across all colonies of the same species.
11. Heat-shock proteins act as emergency repair crews for damaged proteins.
12. After the 2024 bleaching, northern Red Sea reefs recovered by about eighty-five per cent.
13. Super corals have been shown to grow faster than ordinary corals at all temperatures.

Questions 14-15

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

When water becomes too hot, the stressed coral expels its coloured (14) __________ and turns white; a heat-tolerant holobiont may instead keep this partnership working and so avoid (15) __________.

Answers & Explanations

← Previous | Back to IELTS Focus | Next →