Ocean heatwaves cause extensive coral bleaching events almost every year due to climate change, threatening reefs worldwide. High temperature water stress reef building corals, causing them to remove photosynthetic algae that live in their tissues.
The absence of brownish-colored plant cells allows the white limestone coral to shine through, turning the reefs to whitewash.
But when some corals bleach, they undergo a mysterious change that confuses scientists. Instead of being white, these corals emit a variety of neon colors.
Colorful bleaching, as it is known, is covered in the documentary Coral weaving, which showed a full fluorescent fluorescence. The underwater photographer who documented the event said:
It was as if the corals were screaming for attention in vivid colors, trying to protect themselves from the heatwaves of the ocean. We witnessed the final warning that the ocean was in trouble.
With new research, we finally found out why corals do this.
Resolving coral conundrum
We know that the appearance of unusual fluorescent corals is linked to bleaching. But why don’t all the corals suddenly become more colorful? And why do they seem to only appear at certain bleaching events?
Things get even stranger when we try to expose laboratory corals to experimental heat stress.
In our first tests, instead of being more colorful, they only white. But after conducting more lab experiments with the help of our students Elena Bollati and Rachel Alderdice, we found an answer.
In healthy corals, most sunlight is absorbed by photosynthetic pigments of algae. When the corals lose their algae due to stress, excess light travels back and forth within the coral tissue, illuminated by the white outline. Algae within the coral may recover after bleaching, when conditions return to normal.
But when the coral interior is lit up like this, it can be very upsetting for algae, potentially delaying or even preventing their return.
If coral cells can still perform at least some of their normal functions during bleaching, increasing indoor light levels promotes the production of vibrant pigments that protect the skin. coral from lightweight damage, which forms a type of sunscreen layer that allows algae to return.
As the recovering algae begins to absorb the light for photosynthesis again, the light levels within the coral drop, and so the corals stop producing many of these colorful pigments.
But it is not just heat stress that can cause colorful bleaching. Corals and their algae are very sensitive to changes in nutrition levels in their environment. If there is too little phosphorous or too much nitrogen in the water – something that can happen when fertilizer runs from the farmland to the ocean – strong bleaching can occur.
A brighter future for reefs
Using satellite data, we reconstruct the temperature profiles for known colorful bleaching events. We find that there is a tendency to occur after short or mild phases of heat stress.
When corals are exposed to severe or prolonged obesity temperatures, they are likely to turn white. That’s why we only see tiny neon colors at particular stages of bleaching, when conditions are right.
Different members of the coral community may display different colors at these events, while some species do not produce colorful protective pigments. But even within coral species, there may be different color variants resulting from variations in their genetic makeup.
These variants have evolved to provide different species strategies to deal with light, depending on where they grow on reefs. For corals in heavy water it is useful to invest a lot of energy in making vibrant sunscreen.
In deeper depths or in shaded areas where light stress is lower, corals that produce less pigment protection are better than saving their energy for other uses, useful purpose. Although, these different variants often occur side by side, with some corals turning white as their neighbors turn white.
The good news is that colorful bleaching reefs seem to be more likely to recover than white corals, as they tend to appear when heat stress is less severe and the colorful pigments themselves offer of protection.
Reports suggest that colorful bleaching occurred in some parts of the Great Barrier Reef in March and April 2020, so some patches of the world’s largest reef system may have better prospects for recovery following the recent bleaching.
Now that we know that nutrition levels can affect colorful bleaching too, we can more easily identify cases where heat stress can be aggravated by poor water quality. It can be managed locally, while ocean heat waves caused by climate change will require global leadership. Together, these actions could save a future for coral reefs.
Jörg Wiedenmann, chair of Coral Reef Laboratory, University of Southampton and Cecilia D’Angelo, Senior Research Fellow, Coral Reef Laboratory, University of Southampton.
This article was published from The Conversation under a Creative Commons license. Read the original article.