Maarg Manthan · Topic 12.3

Coral Bleaching and Zooxanthellae

Geography › Coral Reefs, Ocean Resources and Law of the Sea · Topic 12.3

Coral bleaching is the loss of colour of corals when they expel the microscopic algae called zooxanthellae that live in their tissues, usually because of heat stress. The algae give corals their colour and most of their food, so bleached corals are starving and may die if the stress continues. Mass bleaching has become more frequent with ocean warming, and a fourth global event began in 2023.

6 min read · UPSC Prelims · Geography · Coral Reefs, Ocean Resources and Law of the Sea

At a Glance

Algae Zooxanthellae, now Symbiodiniaceae
Trigger Water about 1 degree Celsius above the usual summer maximum for weeks
Food share Algae supply up to about 90 per cent of coral energy
Latest event Fourth global bleaching event, from 2023
Reefs affected About 84 per cent by April 2025
Exam link UPSC Prelims, Geography: Coral Reefs, Ocean Resources and Law of the Sea

Where Does It Fit?

This is the third topic of the category on coral reefs, ocean resources and the law of the sea. The previous topic gave the conditions for coral growth, and this one explains what happens when one of them, temperature, goes beyond the limit. The next topic covers copper, cobalt Regulator International Seabed Authority,…">ocean deposits and mineral resources. Previous topic: Conditions for Coral Growth and Distribution of Coral Reefs. Next topic: Ocean Deposits and Mineral Resources.

What Is the Relationship Between Corals and Zooxanthellae?

In short: corals are animals that farm algae in their bodies, and when the farm fails, they lose both colour and food.

Reef-building corals live in a symbiosis, a partnership beneficial to both sides, with tiny single-celled algae called zooxanthellae, now classified as the family Symbiodiniaceae. The algae live inside the coral tissues. They photosynthesise, using sunlight, and pass sugars and other nutrients to the coral, which can supply up to about 90 per cent of the coral’s energy. In return the coral gives the algae shelter, carbon dioxide and nutrients from its waste. The algae also give the coral its brown, green or golden colour. This partnership is what allows corals to build large reefs in the clear, nutrient-poor tropical water, and also why they need shallow, sunlit water, as in Conditions for Coral Growth and Distribution of Coral Reefs.

What Causes Coral Bleaching?

Heat stress: the main cause. When the sea temperature rises about 1 degree Celsius above the normal summer maximum for several weeks, the algae begin to produce toxic reactive oxygen compounds. The coral expels them, and its transparent tissue shows the white skeleton underneath. NOAA measures the stress as degree heating weeks, which sum the accumulated heat above the threshold: about 4 degree Celsius-weeks is enough for significant bleaching and about 8 for severe bleaching with widespread death.
Other stresses: an influx of fresh water after floods, pollution, very low tides that expose the coral, or sudden cold.
Ocean acidification: as the sea absorbs carbon dioxide from the air, its pH falls, and corals find it harder to build their calcium carbonate skeletons. It is not the direct cause of bleaching, but it weakens the reefs.

Bleached corals are not dead. If the temperature falls within a few weeks, the algae can return and the coral can recover. If the stress goes on, the coral starves and dies, and algae and seaweed overgrow the skeleton. Repeated events do not allow time for recovery, which is the reason why the frequency is more worrying than a single event.

How Serious Is Global Coral Bleaching?

Event	Years	Reefs affected
First global bleaching event	1998, a strong El Niño year	About 20 per cent
Second	2010	About 35 per cent
Third	2014 to 2017	About 56 to 68 per cent depending on the estimate
Fourth	From 2023; confirmed in April 2024 by NOAA and the International Coral Reef Initiative	About 84 per cent of the world’s reef area had experienced bleaching-level heat stress by April 2025, across more than 80 countries and territories, the largest on record

The Great Barrier Reef suffered mass bleaching in 2016, 2017, 2020, 2022 and 2024, and in India, bleaching has been recorded in the Lakshadweep, the Gulf of Mannar and the Andaman and Nicobar Islands in warm El Niño years, as in El Niño and Its Effects. Whether the fourth event has ended is not clear; some scientists warn that background heat stress may never again drop below the level that triggers global events. The IPCC estimated that 70 to 90 per cent of warm-water corals would be lost at 1.5 degrees Celsius of warming, and over 99 per cent at 2 degrees. A strong El Niño such as the one forecast for 2026 and 2027 raises the risk again, as in Temperature Distribution of Oceans.

The loss of reefs reduces fish stocks, tourism and coastal protection, as in Energy and Biotic Resources of the Oceans. The responses include cutting greenhouse gas emissions, which is the only solution at scale, reducing local pressures such as pollution and overfishing, creating marine protected areas, restoring reefs by growing and transplanting coral fragments, and research into heat-tolerant corals. As an analytical point, coral bleaching is a direct visible indicator of climate change, and there is no local fix for global heating: restoration can buy time for reefs but cannot replace emission cuts.

Why Is This Topic Important?

Concepts Zooxanthellae and bleaching asked
Current affairs Global bleaching event
Policy Climate and conservation

What Came Next?

The next topic begins the study of the resources of the ocean with ocean deposits and mineral resources. Next topic: Ocean Deposits and Mineral Resources.

Key People and Terms

Zooxanthellae Symbiotic algae in corals
Symbiosis Mutually beneficial partnership
Bleaching Loss of algae and colour
Degree heating weeks Measure of accumulated heat stress
Ocean acidification Fall in the pH of sea water
Restoration Replanting of coral fragments

Exam Corner

Points to Remember

Zooxanthellae give corals colour and food.
Bleached does not mean dead.
Trigger: about 1 degree Celsius above normal for weeks.
The fourth global event began in 2023.
At 1.5 degrees of warming, 70 to 90 per cent of warm-water corals may be lost.

Do Not Confuse With

Bleaching and death Stress response versus mortality
Acidification and warming Chemistry versus temperature
Zooxanthellae and plankton Algae inside corals versus drifting organisms

Memory Hook

Heat makes the algae toxic, the coral throws them out, the white skeleton shows; cool it fast or it starves.

Mains Angle

Questions link bleaching to climate change. These points add depth.

Mechanism Symbiosis breakdown
Scale Fourth global event
Response Emissions and protection

A Question You May Face

An original practice question, not a past paper question.

“What is coral bleaching? Discuss its causes, consequences and the measures to protect coral reefs.”

How to Answer

Symbiosis Corals and algae.
Cause Heat stress.
Effects Fish, tourism, coasts.
Measures Emissions and protection.

GS Relevance

Prelims: cause of bleaching, zooxanthellae, global bleaching events, remedies. GS Paper 3: environment, marine biodiversity, climate change.

Frequently Asked Questions

What are zooxanthellae?

Zooxanthellae are microscopic algae that live inside the tissues of reef-building corals. They photosynthesise and supply the coral with sugars, which can be up to about 90 per cent of its energy, and give it colour. In return, the coral provides them with shelter and nutrients.

What is coral bleaching?

Coral bleaching is the loss of colour that occurs when stressed corals expel their zooxanthellae, mostly due to unusually warm water. The tissue turns transparent, and the white skeleton shows. A bleached coral is starving, but it can recover if the stress ends soon enough.

What causes coral bleaching?

The main cause of coral bleaching is heat stress, when the sea temperature stays about one degree Celsius above the usual summer maximum for several weeks. Other causes are fresh water from floods, pollution, very low tides and cold snaps, and ocean acidification makes the reefs more vulnerable.

What is the fourth global coral bleaching event?

The fourth global coral bleaching event began in 2023 and was confirmed by NOAA and the International Coral Reef Initiative in April 2024. By April 2025, about 84 per cent of the world's coral reef area had experienced bleaching-level heat stress, making it the largest such event recorded.

How can coral reefs be protected?

Coral reefs can be protected by reducing greenhouse gas emissions to limit ocean warming and acidification, by controlling local pollution, runoff and overfishing, by creating marine protected areas, and by restoring damaged reefs through coral gardening and the study of heat-tolerant corals.

PYQ Practice — Statement Analysis

1 Zooxanthellae live in a symbiotic relationship with corals.

True

They supply food through photosynthesis.

2 Bleached corals are always dead.

False

They can recover if the stress ends.

3 Ocean warming is the main cause of mass coral bleaching.