Saudi Green Initiative

What lies beneath: Can Red Sea coral show us how to save the world’s reefs?

Feathery orange moss, bouquets of tightly bound lime green and mustard yellow buds, gently swaying blood red branches, and huge pebbles adding a shock of blue and purple – to the uninitiated, this dazzling display might resemble an underwater garden, albeit an improbably colourful one.

And in a way it is. Except when you get up close, really close, and you see the thousands of tiny creatures, often no bigger than half an inch wide, packed tightly together in colonies to create this ethereal Red Sea landscape. We are of course talking about coral polyps, the soft-bodied organisms that together can form reefs the size of islands.

They only cover a tiny area of the earth – less than one per cent – but they are the vital building blocks of a healthy marine ecosystem. By providing food, and habitats for breeding and nursery grounds, they support around 25 per cent of all marine life, including over 4,000 species of fish.

Humans benefit too. Reefs provide food and livelihoods for millions of people around the world; they protect shorelines from intense wave impact and are the source of many medicines. A UN report from 2006 estimated that just one square kilometre of reef could be worth up to US$600,000 a year.

But corals are facing the very real threat of extinction due to the pressures of climate change. Globally, reefs have already suffered a decline of over 50 per cent in the last 50 years and research suggests warming and acidifying oceans could wipe out up to 90 per cent of the world’s corals by 2050 if nothing is done to save them.

A reef’s vibrant colours actually come from tiny algae called zooxanthellae that live inside the coral tissue. Their relationship is symbiotic: in return for a home within the coral, the zooxanthellae provide most of the coral’s food and nutrients.

The zooxanthellae are incredibly sensitive to changes in the ocean. If temperatures become unsuitable – usually too high – the thermal stress will cause them to be expelled from their host. The corals, in turn, lose their main source of food, becoming ghostly white in a phenomenon known as coral bleaching.

At this stage, the coral is still alive – but it can’t survive for long without the zooxanthellae. If temperatures remain high for prolonged periods of time, and the zooxanthellae don’t return to their host, the coral will eventually die, withdrawing life support for other sea life.

The devastation will echo across the entire marine ecosystem and beyond – the ocean will lose enormous biodiversity, fisheries will suffer and millions of people will lose their source of food and income.

In 2020, researchers discovered that some corals in the Gulf of Aqaba – a trench at the meeting point of Egypt, Israel, Jordan and Saudi Arabia – could withstand extreme temperatures. One degree above an area’s summertime maximum monthly mean sea surface temperature for a period of a week (generally given as Degree Heating Week, or DHW) can be enough for bleaching to occur but these corals have been known to survive a seven-degree rise.

The reason these super-corals exist in the Gulf of Aqaba is because “the Red Sea has a strong latitudinal temperature gradient,” according to Dr Sebastian Schmidt-Roach from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. And because the Red Sea’s coral population is relatively young.

During the last ice age – two and a half million years ago – sea levels fell and the Red Sea was cut off from the Indian Ocean, becoming intensely dry and salty. Then when the ice caps melted, the region reflooded. Over generations, as corals migrated into the Red Sea, passing a bottleneck in the south where temperatures reach average highs of 34°C every summer to the very north where temperatures drop to 16°C in winter, they adapted to the extremes in temperature and transported their newly acquired heat resilience with them.

It’s hard to underestimate the importance of this discovery given how much hotter our oceans are getting. The Red Sea, which already has warmer waters than elsewhere in the world, experienced 23 DHW events in 2020. Just 40 years ago, there were next to none.

KAUST’s Professor Manual Aranda compares some of the current coral restoration efforts to the legend of Sisyphus, who rolled a boulder up a hill only to have it plummet to the bottom as soon as he reached the top. “Because you’re planting the same corals that were there to begin with, the next heat wave just kills them again, destroying all the work you’ve done,” he explained.

But the discovery of a heat-resilient super-coral is changing the game.

Coral in the process of spawning

Professor Aranda and Dr Schmidt-Roach, along with the rest of the research group at KAUST known as the Coral Hub, are focusing on enhancing the natural reproductive success of specific coral colonies that have shown increased thermal tolerance.

Once they identify the corals that can naturally survive in warmer environments, they cross-breed these with multiple others within their species. During breeding, the heat-resistant corals have a small advantage over the others and reproduce more as temperatures are slowly raised, meaning the entire population gradually becomes more heat-resilient.

In effect, this intervention speeds up the genetic exchange that would normally happen over thousands of years, giving corals a fighting chance to keep up with the rapidly changing climate.

And by encouraging the thermo-resistant corals to interbreed in this way, rather than growing multiple cultures from the same coral in a petri dish, genetic diversity is maintained; it protects the population from things like disease that could wipe out entire colonies in one go.

Researchers exploring the Red Sea

Researchers collecting coral to study in labs

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