Some coral reefs can really take the heat

Changes in gene activity may be helping these animals survive in the Red Sea

Chad Scott

Corals, known for their bright rich colors, are quickly dulling away into black and white as oceans warm and greenhouse gases alter ocean chemistry. And yes, it has become more common and severe over the last three decades.

But sometime early in the pandemic, Christian Voolstra of the King Abdullah University of Science and Technology in Saudi Arabia, along with his team, put out to sea to study a system they had developed to study the effect of warming oceans on coral. They took samples from the north Red Sea near Eilat, a tourist destination that is also Israel’s southernmost port, and at two reefs in the central Red Sea, near the university. They found that, unlike in other areas of the world, the corals of the Red Sea could handle more heat.

What is coral bleaching?

To recap, corals are colonial animals closely related to sea anemones and jellyfish. They have very thin skin and clear body tissues, within which live millions of unicellular algae cells called zooxanthellae. These algae live symbiotically with the coral and produce about 80-95% of the coral’s energy, allowing corals to produce the limestone skeletons that build the reef and create the ecosystem. These algae also give the animal their beautiful and vibrant colors that inspire so much awe and wonder.

These algae are similar to the leaves of a tree, using the sun’s energy to produce sugars and carbohydrates through photosynthesis, which they share with the animal. However, when water temperatures increase by only a small amount (1-2 degrees Celsius, or 1.8 to 3.6 degrees Fahrenheit) for a few days this symbiotic relationship breaks down, the algae are expelled from the coral tissue, and the corals lose their color (and their energy production).

When this occurs, we see white corals, and refer to it as coral bleaching. Today, every respectable scientist agrees that this is one of the top threats to the survival of reefs, and is a direct result of climate change. We have already lost 50% of the Great Barrier Reef and 90% of the Caribbean corals. If action is not taken soon, reefs could be functionally extinct by 2050.

According to a paper published in Global Change Biology  Voolstra team detailed how reefs in the Red Sea can survive in temperatures as high as 36 degrees Celsius whereas those in the Coral Triangle will turn white after only a few days at 32 degrees Celsius.

The Coral Triangle is located in the western Pacific Ocean. It includes the waters of Indonesia, Malaysia, the Philippines, Papua New Guinea, Timor Leste and Solomon Islands. Over 120 million people living there rely on its reefs for food, income, and protection from storms.

How these Red Sea corals can tolerate such high temperatures? Is it possible for this adapted heat tolerance to be somehow transferred to other species?

Resilience

Scientists studying corals and bleaching often use the term coral resilience to refer to the animal’ ability to resist, survive and recover from bleaching during to the warming of the oceans. For this, they have to study the effects of warmth on the entire reef (animal, algae and bacteria) not in the oceans but in simulated conditions in the laboratory.

Armed with the new setup, dubbed the Coral Bleaching Automated Stress System (CBASS), Voolstra subjected fragments of the smooth cauliflower coral they collected to a series of stresses. Basically, some of the animals were tested for their responses to short bursts of heat stress, while others were tested for longer exposures.

The system consisted of a series of tanks. For the short-term stresses, the baseline (control) tank was maintained at 30 degrees Celsius throughout the experiment. The other three tanks were heated to 33 degrees Celsius, 36 degrees Celsius, and 39 degrees Celsius over three hours, maintained there for three hours, then brought back to 30 degrees Celsius in an hour. They assumed that when the heat reduced photosynthesis in the bacteria associated with the coral to 50%, the reef was at risk of bleaching.

Voolstra and his team found that the Red Sea corals’ remarkable resilience to thermal stress saw them holding up even 5 degrees Celsius above the summer maximum, while most coral elsewhere metaphorically wilted, to begin bleaching at only 1-2 degrees Celsius above the maximum.

The team found that some Red Sea corals increased or decreased the activity of certain genes to help them adapt to new conditions. This allowed the animals to continue without major signs of stress in both the short- and long-term experiments for all but the largest temperature change, of 7.5 degrees Celsius.

The results of this study indicate that we may be able to help corals develop resilience to increasing temperatures, but the methodology still eludes scientists. Voolstra has converted the research that his team has conducted into an online repository so that other researchers can study the ability of other reefs around the world to withstand thermal stress as the ocean temperatures rise due to the global warming.

What next?

Truly Curious asked Voolstra if his findings – which suggest that it is the corals and not the bacteria and algae they live it that makes them resilient – imply that trying to genetically engineer heat-resistant zooxanthellae is a dead end. Voolstra said research is needed to help the coral, the zooxanthellae algae, and the bacteria all handle take the heat. He said that though his team’s study showed Red Sea corals can survive in warmer waters, that did not mean corals elsewhere might be similarly resilient.

To address corals elsewhere, he wants the CBASS system to be used in other parts of the world, to rapidly evaluate the ability of the animal to undergo genetic change. To speed things up, he has created a repository for other researchers to use to build and effectively run the unit. The plan is to not just identify the more resilient coral populations, but also use them to restore reef communities.

Chad Scott is program director for the New Heaven Reef Conservation Program, and the project coordinator for the Save Koh Tao Marine Branch in Thailand. 

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