When corals are under stress, they expel the microscopic algae that live in their tissues. Without these algae, corals' tissues become transparent, exposing their white skeleton. This is called coral bleaching. Bleached corals are not dead, but are more at risk of starvation and disease. Rising ocean temperatures caused by climate change is the primary cause of coral bleaching. A temperature increase of just one degree Celsius for only four weeks can trigger bleaching.
Changes in water quality, increased sun exposure and extreme low tides can also cause corals to bleach. A relative of jellyfish and sea anemones, corals are invertebrates that belong to a large group of animals called Cnidaria. They are known as colonial organisms, which means many individuals live and grow while connected to one another.
Each individual coral is made of a polyp — a clear, tube-shaped structure attached to a reef at one end and open at the other.
The open end has a mouth that is surrounded by a ring of tentacles. Hard corals extract calcium from seawater to make limestone outer skeletons, which form coral reefs. Soft corals tend to be feathery in appearance such as sea fans and sea feathers. This algae, called zooxanthellae pronounced zo-UH-zan-thuh-lay , helps corals by removing waste and using it to produce food in a process known as photosynthesis. The mining and burning of coal releases carbon pollution into the air, which is heating our planet and warming our oceans.
If we continue to pollute the air and the ocean with carbon emissions at our high rate, coral reefs around the world will face a catastrophic future in coming decades — in our lifetime. The stunning colours in corals come from a marine algae called zooxanthellae, which live inside their tissues. This algae provides the corals with an easy food supply thanks to photosynthesis, which gives the corals energy, allowing them to grow and reproduce.
When corals get stressed, from things such as heat or pollution, they react by expelling this algae, leaving a ghostly, transparent skeleton behind. Some corals can feed themselves, but without the zooxanthellae most corals starve. In some instances corals can recover from bleaching. If conditions return to normal, and stay that way corals can regain their algae, return to their bright colours and survive.
However prolonged warmer temperatures and other stressors, like poor water quality, can leave the living coral in a weakened state. It can struggle to regrow, reproduce and resist disease — so is very vulnerable to coral diseases and mortality. It can take decades for coral reefs to fully recover from a bleaching event, so it is vital that these events do not occur frequently.
If we continue burning fossil fuels at our current rate, severe bleaching events are likely to hit reefs annually by the middle of the century.
This would be devastating for coral reefs as they would have no chance to recover. If the algae loss is prolonged and the stress continues, coral eventually dies.
Download this infographic: In English In Spanish. Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae zooxanthellae living in their tissues causing the coral to turn completely white.
This is called coral bleaching. Should MPs be allowed to do another job? How Christmas can still sparkle with plastic-free glitter.
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