What is coral bleaching?

What you need to know about coral bleaching:

White branches reaching out, stark against the blue. Where there was colour, now only ghostly white. This haunting transformation isn’t just a visual tragedy – it’s the silent SOS of some our Ocean’s most spectacular ecosystems. This is coral bleaching.

Coral reefs aren’t just beautiful — they’re nurseries for fish, protect coasts from storms, and feed millions of people. When coral reefs bleach, their whole ecosystem is at risk. But what is coral bleaching? What causes it, and why does it damage reefs? 

Are corals animals, plants or rocks?  

Corals are animals. Some may have stone skeletons and live with plants. But all corals are animals.

Corals are tiny animals called polyps. Each polyp has a soft body and a mouth surrounded by tentacles, like a little sea anemone or an upside-down jellyfish. All these animals are related – they are cnidarians (silent c), named after their cnidocytes – special cells that can sting.  

Where does coral’s colour come from? 

Corals are incredible animals. They build immense structures that provide homes for marine species, protect the coast and create oases in the ‘desert’ of tropical seas (there are very few nutrients in the waters of the tropical Ocean).  

To be able to do all this, they need some help. Corals have symbiotic algae called zooxanthellae living in their skin cells. Think of zooxanthellae as tiny solar-powered chefs living inside coral homes. 

They catch sunlight, cook up energy, and share over 80% of the meal with their coral landlords. The coral provides protection and prime real estate with an Ocean view. It’s a win-win (this is what symbiotic means) – until climate change cranks up the thermostat.  

It’s zooxanthellae that gives coral its colour. The magical, vivid world of coral reefs is painted by these little algae. Without them, corals are translucent, and the white of their calcium carbonate skeleton shines through.  

Why do corals bleach?  

The happy relationship between coral and zooxanthellae can be disrupted. When it is, this can lead to the expulsion of the algae from coral tissues, leaving the coral gleaming white (it is a spectrum, coral can partially bleach).  

The most common cause of coral bleaching is thermal stress AKA temperature. If conditions aren’t right, the systems that make photosynthesis (plants turning sunlight into food) can break.  

When these systems break, they can produce reactive oxygen species (ROS). ROS are produced in normal function, but too many ROS harm the coral. When the coral detects this build up, it acts in self-defence and throws the algae out. 

Usually, this is from it being too hot, but the system can be broken when it is too cold, or in too much sunlight, or exposed to harmful pollutants.

That’s a bit abstract. Let’s make an analogy.  

Imagine the coral as a battery, and the algae as a solar panel. Normally, the algae are providing energy to the battery from the sunlight. But if the solar panel gets too hot or is exposed to too much sunlight under a magnifying glass, it might start to malfunction. It starts to spark, so to protect itself the battery disconnects. Without its solar panels, our coral battery can only run on emergency power for so long before it’s completely drained. 

History of coral bleaching – how long has bleaching been about? 

We’ve known about coral bleaching for nearly a century. In 1929, scientists first described it during extreme low tides. But it wasn’t until 1984 that a mass bleaching was documented, linked to unusually warm waters.  

Then came 1998 — the first global mass bleaching event, when around 16% of the world’s coral reefs were lost.  

Places like the Maldives, Seychelles, and reefs in the Indian Ocean lost nearly half their coral cover. 2023 saw the start of the fourth global coral bleaching event, that over the next two years saw an estimated 84% of the worlds coral reef areas bleached. 

Sounds bad, but this isn’t the end. 

Does bleaching mean coral is dead? 

No. A bleached coral is still alive, it just doesn’t have its friend feeding it. This leaves the coral more vulnerable to disease, but also to starvation. Unless our battery reconnects to its solar panel, it will eventually run flat.  

Having repeated bleaching events reduces corals’ ability to recover. It’s like punching them while they are down.

When the coral eventually dies, it loses its white look and will begin to get covered with other algae and seaweed.

However, corals have shown us again and again they have an amazing ability to recover when given the chance.  

Different species of coral are more tolerant, and different species of zooxanthellae can take more heat too.  

Some species of coral bounce back faster than others; the marine equivalent of those friends who somehow recover from a night out while you’re still nursing a headache. The massive boulder corals? They’re the slow-but-steady marathon runners. The branching corals? More like sprinters – quick to bleach, but sometimes quicker to recover. 

After bleaching, it is possible that coral acquire more heat-tolerant photosynthesising friends, chefs that can take the heat in the kitchen. Corals aren’t going down without a fight.  

How can we help the corals? 

There is a lot of work going into understanding corals, and reef restoration methods continue to be tested and implemented (read here for more.)

Corals are the poster child of Ocean health. They are impacted by all our Ocean threats, which means you can help wherever you are.

Every time you switch off an unnecessary light, choose a reef-safe sunscreen (free from oxybenzone, octocrylene or octinoxate), or select a sustainably caught fish dinner, you’re casting a vote for coral survival.

The future of coral reefs could be written in bleached white, or in vibrant technicolour. The pen, rather excitingly, is in your hands.  

Post by

Will Steen