Have you ever wondered what is happening when corals bleach? I for one, have learnt this so many times throughout my degree, but I am not sure how many people actually understand the science behind coral bleaching. I think it is really important to understand the mechanisms that is causing corals to turn white, all over the globe. This is only going to be a brief part of what actually happens, because it is such a complex topic, but I am going to try my best to synthesis the important parts in an easy to understand way.
So first, what you need to know is that coral in the same class of marine organisms that jellyfish are part of. This means that they only hold the basic elements of an animal, and only have two tissue layers. Believe it or not, the coral structure you see below, is actually a group of colonial polyps. A polyp is the juvenile or baby stage in the lifecycle, and coral have eliminated the adult stage, resulting in the formation of groups of genetically identical polyps, that you know as coral. These corals then grow simultaneously in the same area to form amazing structures coral reefs. The polyps of the coral are really tiny, so what you see when you look at a coral is the polyps waste. They produce waste in the form of limestone (CaCO3), which forms the skeleton that the polyps the sit around.
To help the corals obtain and digest their food, they have evolved a symbiotic relationship with a microscopic photosynthetic algae, called zooxanthellae. This symbiotic relationship means that both the coral and the algae benefit from their relationship; the coral gets important nutrients and the algae benefits by living in a protected environment and get the compounds it needs for photosynthesis.
Many people believe that the only way coral can loose their photosynthetic algae. But this is not true, there are actually two ways that this can happen, causing bleaching.
1. They loose or there is a reduction in algae in a region, or over an entire coral
This is the one that most people know about. When the water stays too warm for a long period of time, the coral can loose their algae. They can return if the temperature returns to more bearable conditions.
2. They loose the chlorophyll pigment per dinoflagellate cell
This is the one that many people don’t know about, because it is difficult to explain and often understand. This is mainly more of a response to light than heat, but they generally come together. If there is more light reaching the coral, then the algae with reduce the amount of chlorophyll, which will reduce the amount of light intercepted. If the zooxanthellae are processing too much light, it creates too much oxygen, which can be toxic to the coral. This can also run the other way, if there isn’t enough light, the algae will increase the chlorophyll, to harvest more light.
So when coral bleach, whats next?
Well, if the temperature returns to a bareable temperature for the coral, the algae can return, and the coral can recover. It will take a while, but if it remains undisturbed for enough time (at least 2 years) it can recover. In a recent documentary called Chasing Coral (which I would recommend you all watch!), they observed coral that exhibited fluorescent colouring. Like the sun can burn us if we stay in it for too long, so can it too for corals. As protection from the sun, they have fluorescent pigments. They protect the coral, but creating a barrier around the algae, and changing the wavelengths of the UV rays, therefore brighter corals tend to survive better during a bleaching event. The glowing that you see in the photo below, is the coral giving it everything its got to survive. It is after it bleaches, the algae is expelled, and the coral is attempting to protect itself just with the fluorescent pigments, causing them to glow. This protection is an evolutionary adaptation, that not all species possess.
I am sure that a lot of you know that coral reefs are important, but do you actually know why? Coral reefs are important habitats for so many reasons. They protect us from storms by diffusing wave energy, making them smaller. They are also important because they provide a home for so many animals, and increase biodiversity in that area. They do this by providing structure, which gives fish places to hide when there is a predator around. This gives juvenile fish a safe place to grow up, safe from predators. So with this in mind, if coral reefs bleach, and they don’t recover, they will lose their structure, so all the fish that live there will be more vulnerable to predators. So without these reefs, many things will be affected including our tourism and fisheries industry.
I hope this post has helped you understand a little more about coral and the processes that cause them to bleach, and the affect that bleaching coral As always, if you have any questions about coral don’t hesitate to leave a comment or find me on Instagram @the_bluescientist.