Colourful bleaching: solving a coral conundrum

A new study has finally solved a puzzle amongst coral scientists as to why some reefs end up glowing in multi-colour after bleaching, instead of turning the usual ghostly white. It turns out that this strange anomaly may also be a good thing for coral reefs.

colourful bleaching
These colourful corals may look healthy, but they have actually bleached and are slowly dying

There are few sights as distressing for marine biologists and ocean lovers as the ghostly white fields of dead coral reefs. It is the result of coral bleaching, a process in which corals expel their phytoplankton symbionts as a stress response to warming sea surface temperatures. However for a while now scientists have been stumped by the fact that some corals can instead turn a variety of different neon colours when their microscopic inhabitants abandon them. The surprising phenomenon, known as ‘colourful bleaching’, was first brought to people’s attention in the 2017 documentary Chasing Coral and although it is a vibrant warning that reefs are in danger, it may actually be a promising sign that these corals can also recover.

Turning ghostly white

Coral bleaching is the main problem facing corals and the thousands of other reef species that depend on them. It is caused by a variety of factors that induce stress in the corals, including ocean acidification, microplastic ingestion, disease and most importantly rising water temperatures. It is normally very easy to tell when bleaching has occurred because corals lose their spectacular colours and turn a translucent white, hence the name bleaching. The reason for this is because when they are stressed corals expel their phytoplankton inhabitants, known as zooxanthellae, which they rely on for energy provided through a symbiotic relationship. The loss of their zooxanthellae inhabitants not only results in a loss of vital energy, but also colour because they are the only species in this relationship to actually have any. The actual coral skeleton is translucent, because it needs to be able to let light inside its tissues for photosynthesis in their symbionts to occur, so when it is empty it appears a ghostly white.

bleached coral GBR
The classic ghostly white colour that is more commonly associated with coral bleaching

Too bright in here

Although coral bleaching is a devastating phenomenon that has killed large portions of coral reefs across the globe, it is not actually an immediate death sentence. That is because although the zooxanthellae have been kicked out of their coral hosts, they can still actually come back and resume their crucial symbiotic duties before the corals die. The problem is that this has to happen rather quickly and empty corals don’t make this very easy. That is because when they lose their colour and turn white, they actually become too bright for zooxanthellae as light gets rebounded within translucent tissues. This makes the dying corals an inhospitable place for the zooxanthellae to return to, and unfortunately means most corals that bleach will die soon after.

A colourful solution

This is where colourful bleaching comes into effect. New research by coral scientists from the University of Southampton in the UK have discovered that the neon colours produced by some dying corals are actually designed to protect them. In their new study, published in Current Biology, the team show that the high light levels within the coral tissue can trigger the production of colourful pigments as dying coral cells continue to try and perform vital functions. The now colourful bleached corals are able to absorb more of the light that hits them, like a form of colourful neon sunscreen, meaning that the interior of their cells becomes darker and therefore more hospitable for zooxanthellae to return. This means that colourful bleaching could actually increase the chances of corals surviving bleaching events.

colour comparisons
A colour comparison between healthy coral (brown), bleached coral (white) and colourful bleaching (purple)

Why don’t all corals do this?

One of the biggest questions raised by this new explanation for colourful bleaching is – if it increases the chances of a coral surviving its bleaching, why don’t all corals do this? The main reason is that there is huge variation between both the species of corals that make up a reef and also their positions in it. Colourful bleaching is thought to only be possible for a select few species that can produce these protective colourful pigments, most other species seem incapable of producing these pigments at all. Genetically speaking this method of defence is only likely to be effective at shallow depths, where light intensity is at its strongest. Deeper coral species on the reef are protected from having brightly lit bleached skeletons, because less light is penetrating their translucent tissues. Therefore only shallow species have evolved this behaviour. Nutrient levels are also thought to play a role, as colourful bleaching can also be triggered by a lack of phosphorus or an abundance of nitrogen.

Searching for hope

Unfortunately 2020 has been one of the worst years yet for coral bleaching, with vast parts of reefs (most of which are already significantly affected) being hit hard. The Great Barrier Reef in particular suffered after record heat waves and may have had its biggest bleaching event yet. Ultimately this new research isn’t going to solve the core problems surrounding coral bleaching, which need to be addressed on a global political level (i.e. properly tackling climate change). However, research like this is still important because it helps us better understand the problem, and maybe even identify areas where we should focus our efforts. For example some sections of the GBR did exhibit colourful bleaching this year and so their chances of survival may be higher. It is an example that coral reef research, one of the fastest growing and important areas of marine biology research, is still capable of providing some hope towards one of the most daunting issues facing our oceans.

To see more examples of colourful bleaching then check out this amazing clip from the award-winning Chasing Coral documentary below.

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