#1498 Why does coral bleaching occur?

Why does coral bleaching occur? Coral bleaching primarily occurs because heat stress causes the coral to expel the algae that give them their beautiful colors.

If you go diving in a tropical region, you will probably see enormous numbers of coral. They should be a whole range of beautiful colors, but these days, enormous numbers of them are turning white. Coral do produce their own color pigments, but most of them are naturally white. They get their beautiful colors from the algae that live on them, and that they share a symbiotic relationship with.

It is easy to think that coral are just a form of rock, but they are actually living organisms. They are a collection of individual animals called polyps, and they are related to sea anemones and jellyfish. The individual polyps live together in groups to increase their ability to get food. They are very simple organisms, and they have a mouth at one end and a stomach in the middle. They have tentacles to pull in food, and often they have stinging cells called nematocysts, similar to jellyfish. They cannot move, so they are reliant on food swimming to them.

A type of algae called zooxanthellae has evolved to live on the coral. Rather, in fact, they live inside the coral. These algae live inside the tissue of the coral. Algae produce their energy by photosynthesizing, and their position inside the coral gives them a good angle to the sun. It also protects them from being eaten by fish or other predators. They can also use the waste products that the coral produce, such as ammonium, carbon dioxide, nitrates, and phosphates, which the algae can consume. The coral receive a lot of nutrients from the algae, which gives them energy and food. They can get as much of 90% of their energy requirements from algae. Coral that are inhabited by algae grow can lay down their skeleton ten times faster than coral that have no algae. The cells of the coral are transparent so without the algae, the skeleton of the coral is white. It is the algae that give coral their color. The pigments that the algae use to be able to photosynthesize show through the transparent coral skeleton.

Color bleaching occurs when coral expel the algae and go back to being transparent. This happens when they are stressed, and they get stressed when the water is too warm, there is too much sunlight, or there is too much pollution. There is a lot of ongoing research into this area, and no one actually knows exactly how the algae are expelled or why they are expelled. Here are some of the more common theories.

How they are expelled is probably the easier of the two things. They are either forcibly ejected from the cell, the host cell is ejected, taking the algae with it, the host cell dies, and the algae is trapped inside, or the algae is digested by the coral. Any of these are possible. Why it happens is far more difficult. Rising water temperature stresses the coral, and it gets rid of the algae is obvious, but what is the trigger is not obvious. One idea is that it is oxidative stress. When algae photosynthesizes it produces oxygen as a byproduct, and oxygen is a particularly harmful molecule. To deal with it, coral produces antioxidants. As the temperature of the water increases, the algae photosynthesizes more and produces more oxygen. The level of oxygen could start to overwhelm the corals antioxidant defensives, and it triggers the ejection of the algae. It could be that the nutrition sharing between the coral and the algae breaks down when the temperature of the water rises. If the coral is no longer getting nutrients from the algae, there is no reason to keep them. There may be other triggers as well. It is also not known if it is the coral that kicks out the algae, or the algae that abandons the coral. That is also a possibility.

When a coral loses its algae, it isn’t automatically dead, but it won’t be able to last very long. Coral don’t just lose their color, they lose their primary energy source. Once the algae have gone, they start to starve to death, and, unless there is a reversal, they will not last. This bleaching can be reversed, but the only way to do that is to get rid of whatever was stressing the coral in the first place. If that was a warming up ocean, then the only solution is to stop the oceans warming, which is not the easiest of tasks. And this is what I learned today.

Sources

https://oceanservice.noaa.gov/facts/coral_bleach.html

https://www.barrierreef.org/the-reef/threats/coral-bleaching

https://oceanservice.noaa.gov/education/tutorial_corals/coral01_intro.html

https://en.wikipedia.org/wiki/Zooxanthellae