Ocean swells cause major coral bleaching events almost every year due to climate change and threaten reefs around the world. The high water temperatures stress the build up of corals, which causes them to eject the photosynthetic algae found in their tissue.
Losing these brown-colored plant cells allows the coral’s white limestone skeleton to shine through and turn reef ghostly white.
But when some corals fade, they undergo a mysterious transformation that has confused scientists. Instead of turning white, these corals emit a variety of neon colors.
Colorful bleaching, as is known, was covered in the documentary Chasing Coral, which showed a whole tear reversal of fluorescent lamps. The underwater photographer who documented the incident said:
It was as if the corals were screaming for attention in vivid color, trying to protect themselves from the heat waves of the sea. We had witnessed the ultimate warning that the sea is in trouble.
In new research we have finally revealed why corals do this.
Dissolve a coral bandage
We knew that the appearance of unusually fluorescent corals was linked to bleaching. But why didn’t all the corals suddenly become more colorful? And why did they just appear during certain bleaching events?
It became even more strange when we tried to expose corals in the laboratory to experimental heat stress.
In our first attempts they bleached only white, instead of becoming more colorful. But after conducting more lab experiments with the help of our students Elena Bollati and Rachel Alderdice, we found an answer.
In healthy corals much of the sunlight is absorbed by the photosynthetic pigments in the algae. When corals lose the algae due to stress, the excess light travels back and forth into the coral tissue, which is reflected by the white skeleton. The algae inside the coral can recover after bleaching when conditions return to normal.
But when the coral interior lights up like this, it can be very stressful for the algae, possibly delaying or even preventing them from returning.
If the coral cells can still perform at least some of their normal functions during bleaching, the increased internal light levels increase the production of colorful pigments that protect the corals from light damage and form a kind of sunscreen layer that allows algae to return.
When the recovering algae begin to absorb light for photosynthesis again, the level of light inside the coral drops, and the coral therefore stops producing so much of these colorful pigments.
But it’s not just heat stress that can cause colorful bleaches. Corals and their algae are very sensitive to changes in nutrient levels in their environment. When there is too little phosphorus or too much nitrogen in the water – which can happen when fertilizers run off farmland into the sea – strong colorful fading can occur.
A brighter future for reefs
Using satellite data, we reconstructed the temperature profiles for known colorful bleaching events. We saw that they tend to occur after short or mild episodes of heat stress.
When corals are exposed to severe or prolonged temperatures, they tend to pale white. That’s why we only see bright neon colors in special bleaching episodes when conditions are just right.
Different members of the coral community may show different colors during these events, while some species do not produce these colorful protective pigments at all. But even within coral species, there may be different color variants that are the result of differences in their genetic makeup.
These variants have been developed to give species different strategies for managing light, depending on where they grow on the reef. For corals in shallow water, it is advantageous to invest a lot of energy in producing the colorful sunscreen.
At greater depths or in shaded areas where the light voltage is lower, corals that produce less of the protective pigment are better because they can save their energy for other useful purposes. Despite this, these different varieties often occur side by side, which is why some corals turn colorful while their neighbors turn white.
The good news is that colorful bleached reefs seem to be more likely to recover than corals that pale white, as they tend to show up when the heat stress is not so severe and the colorful pigments themselves offer protection.
Reports suggest that colorful bleaching occurred on 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 of recovery following the latest bleaching.
Now that we know that nutrient levels can affect colorful bleaching, we can more easily pinpoint cases where heat stress may have been exacerbated by poor water quality. This can be managed locally, while the ocean’s heat waves caused by climate change will need global leadership. Together, these measures can secure a future for coral reefs.
Jörg Wiedenmann, Head of Coral Reef Laboratory, University of Southampton and Cecilia D’Angelo, Senior Research Fellow, Coral Reef Laboratory, University of Southampton.
This article is published from The Conversation under a Creative Commons license. Read the original article.