New research from the Okinawa Institute of Science and Technology Graduate University is providing hope for the future of coral reefs. Researchers have discovered that different color variations of coral are more likely to cope with heat stress and therefore be less likely to bleach. Read more about this fascinating research below. 

What Is Coral Bleaching?

Coral reefs around the world are under threat from coral bleaching. This is a phenomenon that occurs when corals are stressed by changes in conditions such as temperature, light, or nutrients. This causes them to expel the symbiotic, colorful algae (zooxanthellae) living in their tissues, causing them to turn completely white. The algae help provide up to 90% of the coral’s energy, so is crucial for the coral’s survival. When a coral bleaches, it does not die instantly. The stress and lack of energy kill the coral over time. The leading cause of coral bleaching is rising water temperatures, brought about by climate change.

Why Does Coral Bleaching Matter?

Coral bleaching matters because, after these corals die, the reefs rarely come back. With few remaining corals, reproduction becomes a struggle, and whole reef habitats, on which humans and wildlife rely, deteriorate. Bleaching matters, too, because it is not an uncommon occurrence. According to the National Oceanic and Atmospheric Organization, between 2014 and 2017, about 75% of the world’s tropical coral reefs had enough heat stress to cause bleaching. The heat-stress was enough to destroy coral for 30% of the world’s reefs.

Colorful Corals’ Ability To Survive Higher Temperatures

Coral within the family Acropora are fast growers, making them an important group of corals for reef growth, island formation, and coastal protection. However, due to global environmental pressures, these corals are in sharp decline. The species Acropora tenuis is of particular interest in this case as it has three different color variations: purple, brown and yellow-green. Interestingly, researchers have found that each of the different color morphs respond differently to high water temperatures.

The marine biologists were curious as to whether the different proteins expressed by the different colored colors of the same species were related to their resilience to warmer water temperatures. It was found that the green colored coral was able to maintain high levels of green fluorescent proteins during local summer heatwaves and was therefore less likely to bleach than the other two color morphs. The results suggest this resilience to heat stress is highly influenced by the coral’s underlying genetics. Which makes sense as genes are responsible for producing the different color morphs.

Coral Reefs Under Threat

Everyone who experiences the Great Barrier Reef in Australia, the coral triangle of South-East Asia, or the Central American reefs will certainly talk about how beautiful and colorful these ecosystems are. In fact, coral reefs are known to provide more biodiversity than any other habitat on the planet, with coral offering protection and shelter to hundreds of species of fish and crustaceans.

However, these habitats are at risk. Global pressures, such as increasing ocean temperatures, are inducing the coral to become ghostly white, a process called coral bleaching, and to die. Acropora corals tends to be especially vulnerable and its numbers are predicted to fall in the future. This is particularly important because these corals are fast growers and thus structurally important for reef ecosystems.

How The Discovery Was Made

Researchers took a close look at Acropora tenuis, a species within the family of Acropora, which is known to have three color morphs – brown, purple, and yellow-green. This new study is entitled ‘Thermal tolerance of the hermatypic coral Acropora tenuis elucidated by RGB analysis and expression of heat shock proteins in coral and symbiotic dinoflagellates and was published in January 2021 in Elsevier’s Marine Pollution Bulletin.

It’s obvious that coral reefs are very beautiful and have a whole variety of different colors. But when researchers began to look a little closer at the different color morphs of Acropora tenuis they noticed that some morphs bleach more readily and die more frequently than others. Through observing the corals during a fieldwork expedition in the summer of 2017, marine biologists recognized a pattern: the brown and purple morphs bleached, with the brown morph dying at a higher rate, but the yellow-green color morph seemed to bleach less, highlighting that it was somewhat more resilient to the summer temperatures.

The Marine Genomics Unit at the Okinawa Institute of Science and Technology Graduate University collaborated with several individuals from the Okinawan community, including with Sea Seed, a private aquarium that has been cultivating and growing different color morphs of various coral species for around 20 years. This aquarium was indeed instrumental for researchers to study coral over the past two decades and to assess how responsive this ecosystem is to climate change and the underlying causes.

Decoding The Genome

A genome is all genetic material of an organism. The genome consists of DNA, which contains both the genes (the coding regions) and the non-coding DNA, as well as other types of DNA. Decoding the genome helps researchers unravel the secrets stored within a species DNA, such as which proteins they use when under stress or whether they don’t ‘turn on’ protein making genes at all and instead slowly begin to die.

In 2020, the researchers and their collaborators decided to decode the genome of Acropora tenuis, to unlock what happens to the DNA of the different color morphs under stress, aka whether they turn on different genes or make different proteins. This provided a roadmap that allowed them to look at the genetic foundations that cause the different variations within the same species.

At first, they assumed that the difference in heat stress tolerance may be attributed to corals having different kinds of symbiotic algae, which are photosynthesized for coral and thus supply nutrition to the coral. Previous research has shown that some symbiotic algae are more resilient than others to climate change. But when they looked at the three color morphs, they realized that they all had very similar algae.

With this in mind, the research group instead turned their attention to the expression levels of the proteins that are thought to be responsible for the coral variation in color. There are four different groups of these proteins: green fluorescent proteins, red fluorescent proteins, cyan fluorescent proteins, and non-fluorescent blue/purple chromoproteins. Through laboratory experiments they discovered that the green morph expressed high quantities of the green fluorescent proteins.

What was found to be even more surprising was that these green fluorescent proteins were expressed at even higher levels during summer, which indicates that they help the coral to withstand the warmer water temperatures. Specifically, these proteins seemed to protect the symbiotic algae, which meant that this color morph experienced very little bleaching in comparison to the other color morphs.

The brown color coral variation was found to express much lower quantities of the green fluorescent protein and this color morph bleached by around 50% over July and August 2017. The purple morph was different again. It expressed very little of any of the fluorescent proteins, but much higher levels of non-fluorescent blue/purple chromoproteins. The purple corals bleached less than the brown morph but more than the green variation, so it was in the middle.

The Impact

Coral reefs are so important to biodiversity.  Learning more and delving deeper is going to help us conserve them. For now, we can’t help too much with the coral reef crisis unless greenhouse gas emissions slow down, but collecting this simple information, learning how corals function, is really important for their long-term survival.

This study has shown that the color morphology of coral is very much involved in its sensitivity to high temperatures. The fundamental explanations for this, such as specifically how the green fluorescent protein supports the symbiosis, will certainly be the focus of future study