Current Affairs · · GS3 · Environment

Lab studies show bleached corals can regain algae from healthy neighbours

Two laboratory studies show bleached corals can recover by taking up algae from healthy corals. In one, bleached fragments touching healthy ones of the same genetic stock recovered within weeks. In the other, fused coral tissue may pass on a heat-tolerant algal strain called SS8. Scaling this up to whole reefs remains unproven.

Event date:

REq1
  1. Note1 / 5

    WHY IN NEWS?

    • Two lab studies, published in 2026, suggest new ways to help bleached corals recover.
    • A Stanford University study, in Science Advances, found bleached corals recovered after direct contact with healthy corals.
    • An Australian-led study tested whether fused coral tissue could pass a heat-tolerant algal strain, SS8, to bleached corals.
    • The work comes after the fourth global bleaching event, the most widespread on record.
  2. Note2 / 5

    WHAT THE STUDIES FOUND

    StudyMethodFindingLimitation
    Contact study (Stanford)Bleached fragments touched healthy ones3 days to start recovery; 5 days for full recovery; complete recovery within 8 weeksWorked only between corals of the same genetic stock
    Fusion study (Australia)Healthy fragments hosting SS8 fused to bleached onesTissues fused in 3–9 days, and SS8 may pass acrossEarly test in two species

    SS8 is a lab-evolved strain kept at 31°C for about ten years to become heat tolerant.

  3. Note3 / 5

    KEY CONCEPTS

    1. What is the coral–algae partnership?

    Corals host tiny algae called zooxanthellae in their tissue. The algae make food by photosynthesis and give corals colour; corals give them shelter.

    1. What is coral bleaching?

    Under heat stress, corals expel their algae and turn white. If heat persists, they can starve and die.

    1. Why do heat-tolerant algae matter?

    Corals with tougher algae can withstand warmer water before bleaching. Lab-evolved strains are one form of "assisted evolution".

    1. What was the fourth global bleaching event?

    From January 2023 to 2025, about 84% of the world's reef area faced bleaching-level heat stress, worsened by the 2023 El Niño. Earlier events reached 21%, 37% and 68%.

  4. Note4 / 5

    LIMITS AND RISKS

    • Lab, not reef: Both studies were done in tanks; reefs span thousands of square kilometres.
    • Genetic mismatch: In the contact study, corals of different stock attacked each other.
    • Unknown side effects: Introduced algae may behave differently in the wild.
    • Root cause: Restoration cannot replace cutting greenhouse gas emissions that warm the oceans.
  5. Note5 / 5

    WHY IT MATTERS FOR INDIA

    • India's main reefs are in the Gulf of Mannar, Gulf of Kachchh, Lakshadweep and the Andaman and Nicobar Islands.
    • Reefs protect coasts from waves and storm surges, support fisheries and draw tourists.
    • Indian reefs also bleached during recent heatwaves, so restoration science is directly relevant.

Sources

Syllabus

PaperSubjectSub-topic
GS3EnvironmentEnvironment, biodiversity conservation, climate change
GS1GeographyOcean ecosystems
PrelimsEnvironmentCoral bleaching, zooxanthellae, global bleaching events, coral regions

Topics

BiodiversityProtected Areas and ConservationSpecies

Related previous-year questions

Asked in earlier UPSC Prelims papers on this topic. Answer, then check.

  1. UPSC Prelims 2012 · Environment and Ecology · Climate Change

    The acidification of oceans is increasing. Why is this phenomenon a cause of concern? 1. The growth and survival of calcareous phytoplankton will be adversely affected. 2. The growth and survival of coral reefs will be adversely affected. 3. The survival of some animals that have phytoplanktonic larvae will be adversely affected. 4. The cloud seeding and formation of clouds will be adversely affected. Which of the statements given above is/are correct?

    1. 1, 2 and 3 only
    2. 2 only
    3. 1 and 3 only
    4. 1, 2, 3 and 4
    Show answer

    Answer: A. Correct Answer: 1, 2 and 3 only Ocean acidification occurs when oceans absorb excess atmospheric carbon dioxide, leading to a fall in seawater pH. It mainly affects marine organisms that depend on calcium carbonate shells or skeletons. Statement-wise Explanation 1. The growth and survival of calcareous phytoplankton will be adversely affected — Correct Calcareous phytoplankton, such as coccolithophores, need carbonate ions to build calcium carbonate shells. Ocean acidification reduces carbonate ion availability, making shell formation difficult. 2. The growth and survival of coral reefs will be adversely affected — Correct Corals build calcium carbonate skeletons. Acidification reduces calcification, weakens coral skeletons and makes reefs more vulnerable to erosion and damage. 3. The survival of some animals that have phytoplanktonic larvae will be adversely affected — Correct Many marine animals have larvae that are sensitive to changes in seawater chemistry. Ocean acidification can affect larval growth, development and survival. 4. The cloud seeding and formation of clouds will be adversely affected — Incorrect This is not a direct or well-established primary impact of ocean acidification for this question. Do not confuse indirect research links with standard biological impacts of ocean acidification. Extra UPSC Info * Ocean acidification is often called the “other CO2 problem”. * Oceans absorb a large share of atmospheric carbon dioxide. * Dissolved CO2 forms carbonic acid, which increases hydrogen ion concentration. * Increased acidity reduces carbonate ions needed by corals, molluscs and plankton. * Acidification can disturb marine food chains because plankton form the base of many marine ecosystems. * Coral reefs are also threatened by ocean warming, bleaching, pollution and destructive fishing. Final Takeaway Ocean acidification mainly threatens calcifying plankton, coral reefs and sensitive marine larvae, but cloud formation is not a standard direct concern in this context.

    Difficulty: hard · statement

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  2. UPSC Prelims 2014 · Environment and Ecology · Biodiversity

    Which of the following have coral reefs? 1. Andaman and Nicobar Islands 2. Gulf of Kachchh 3. Gulf of Mannar 4. Sundarbans Select the correct answer using the code given below.

    1. 1, 2 and 3 only
    2. 2 and 4 only
    3. 1 and 3 only
    4. 1, 2, 3 and 4
    Show answer

    Answer: A. VERDICT: The answer is 1, 2 and 3 only. The Andaman and Nicobar Islands, the Gulf of Kachchh and the Gulf of Mannar have coral reefs; the Sundarbans does not. ANALYSIS: Statements 1, 2 and 3 are CORRECT. These are three of India's four major reef areas, the fourth being Lakshadweep. Statement 4 is INCORRECT. The Sundarbans is a deltaic mangrove ecosystem where the Ganga and Brahmaputra discharge enormous volumes of fresh water and silt. Corals need clear, warm, saline, sediment free water with light reaching the zooxanthellae in their tissues, and a turbid delta provides the opposite of every one of those conditions. SOURCE: India Year Book and reporting on threats to Indian reef ecosystems. Source type EM. HOW TO CRACK IT: Rule out reef locations by physical conditions rather than by recall. Corals fail where rivers deliver fresh water and silt, so any delta or major river mouth is automatically out. That one test removes the Sundarbans without knowing anything else, and it also explains why India's reefs cluster in island groups and in shallow gulfs away from big river discharge. Learn the four reef areas as a set and the physical reason behind them, and the whole family of questions becomes deducible.

    Difficulty: medium · statement

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  3. UPSC Prelims 2014 · Environment and Ecology · Climate Change

    The scientific view is that the increase in global temperature should not exceed 2°C above pre-industrial level. If the global temperature increases beyond 3°C above the pre-industrial level, what can be its possible impact/impacts on the world? 1. Terrestrial biosphere tends toward a net carbon source. 2. Widespread coral mortality will occur. 3. All the global wetlands will permanently disappear. 4. Cultivation of cereals will not be possible anywhere in the world. Select the correct answer using the code given below.

    1. 1 only
    2. 1 and 2 only
    3. 2, 3 and 4 only
    4. 1, 2, 3 and 4
    Show answer

    Answer: B. VERDICT: The answer is 1 and 2 only. Beyond three degrees of warming the terrestrial biosphere may turn into a net carbon source and widespread coral mortality is expected, but the wetland and cereal statements are far too extreme. ANALYSIS: Statement 1 is CORRECT. Warming increases respiration and disturbance losses and can push forests and soils from absorbing more carbon than they release to releasing more than they absorb. Statement 2 is CORRECT. Corals bleach and die at sustained elevated sea temperatures, and widespread mortality is a standard projection at this level of warming. Statement 3 is INCORRECT because of the word all and the word permanently. Statement 4 is INCORRECT because it claims cereal cultivation would be impossible anywhere in the world, whereas higher latitudes may even gain. SOURCE: Assessment literature on the effects of global warming. Source type EN. HOW TO CRACK IT: This is applied reasoning, not recall. Scientific projections are stated in terms of risk, degree and region, so any statement written in absolutes, all, permanently, anywhere, nowhere, is almost certainly a distractor. Grade each statement for the strength of its claim before checking the science, and the two extreme statements fall immediately, leaving the two moderate ones. The same calibration test works across the entire climate change question set.

    Difficulty: medium · statement

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  4. UPSC Prelims 2018 · Environment and Ecology · Biodiversity

    Consider the following statements: 1. Most of the world's coral reefs are in tropical waters. 2. More than one-third of the world's coral reefs are located in the territories of Australia, Indonesia and Philippines. 3. Coral reefs host far more number of animal phyla than those hosted by tropical rainforests. Which of the statements given above is/are correct?

    1. 1 and 2 only
    2. 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Show answer

    Answer: D. All three statements are correct, so the answer is (d) 1, 2 and 3. Statement 1 is CORRECT. Most coral reefs are located in tropical shallow waters less than 50 metres deep, in the western Indian Ocean, the Pacific and the Greater Caribbean. Statement 2 is CORRECT. In the global distribution of corals, Australia holds about 17 percent, Indonesia about 16 percent and the Philippines about 9 percent, so together they host more than one-third of the world's corals. Statement 3 is CORRECT. Coral reefs host 32 of the 34 recognised animal phyla, compared with only 9 phyla in tropical rainforests. SOURCE: Coral Reef Alliance resource on coral reef geography. HOW TO CRACK IT: Statement 3 is the one that feels wrong, because rainforests are the standard emblem of biodiversity. The unlock is that phyla count body plans rather than species, and marine environments hold far more distinct body plans than terrestrial ones, even though rainforests hold more species. Distinguish the measure of diversity being used before judging such a claim, since species richness and phylum diversity give opposite answers here.

    Difficulty: medium · statement

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Practice questions

  1. Coral bleaching occurs when corals:

    1. Absorb excess calcium
    2. Expel their symbiotic algae under stress
    3. Are covered by sediment
    4. Grow too fast.
    Show answer

    Answer: B. Heat stress is the main trigger.

    Difficulty: medium · statement

  2. Consider the following statements about the fourth global coral bleaching event: 1. About 84% of the world's reef area experienced bleaching-level heat stress. 2. This means 84% of the world's reefs died. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both
    4. Neither.
    Show answer

    Answer: A. Heat stress can cause bleaching, but not all stressed reefs bleached or died.

    Difficulty: medium · statement

  3. Which of the following is NOT a major coral reef area of India?

    1. Gulf of Mannar
    2. Gulf of Kachchh
    3. Lakshadweep
    4. Chilika Lake.
    Show answer

    Answer: D. Chilika is a brackish-water lagoon.

    Difficulty: medium · statement

Mains practice

Answer-writing practice on this article. Attempt it first, then open the hints.

  1. GS3 · 150 words

    Discuss the causes of mass coral bleaching and evaluate emerging scientific approaches to reef restoration.

    Show hints
    1. Ocean warming, El Niño
    2. symbiosis
    3. assisted evolution, symbiont transfer
    4. lab vs field
    5. emissions cuts.