Northern Red Sea reefs resist bleaching in warming seas

Super corals in the northern Red Sea may hold some secrets to reef survival in warming seas.

Coral reefs form the basis of the livelihoods of nearly one billion people worldwide, from providing food to attracting tourism and protecting coastlines from storms. But these ecosystems are increasingly threatened by more frequent and severe marine heat waves.

Prolonged sea temperature rises of just 1 degree Celsius above the summer average can cause coral bleaching and mass death, and yet some reefs appear to be immune to this thermal threshold. “In the northern Red Sea, corals did not experience any bleaching and remained healthy despite high heat stress,” says Eslam Osman of KAUST, “raising questions about how long these corals can withstand global warming.”

Red Sea corals are thought to have acquired a high temperature tolerance about 7,000 years ago after crossing a 32 degree Celsius thermal barrier at the sea’s southern entrance. In contrast, corals that could not stand the heat were filtered out.

Thanks to their “evolutionary memory,” Red Sea corals now survive heat stress up to 32 degrees Celsius, but this is difficult to confirm without consistent bleaching data. “The lack of observations of bleaching along the Red Sea limits a broader understanding of how reefs in the region are responding to climate change,” adds Mike Fox, a marine ecologist who worked on this study.

Using the citizen science platform Reef Check, the researchers collected coral bleaching data from the last 20 years off the coast of Egypt. They compared this to remotely sensed sea surface temperatures over the past 40 years, confirming that bleaching in the Red Sea typically occurs at 32 degrees Celsius.

Corals in the northern Red Sea live in temperatures between 25 and 28 degrees Celsius and do not experience mass bleaching, whereas corals in the central and southern Red Sea, which are currently 30-32 degrees Celsius, do.

The team used global climate models to predict future Red Sea temperatures under different greenhouse gas emission scenarios. In the most extreme scenario, the northern region will warm by 2.5 degrees Celsius by 2100. “We’ve already shown that corals in the Northern Red Sea have a 4-7 degree Celsius buffer against climate change,” says Osman, “so suggesting they’ll be among the last to survive by the end of this century.”

Conservation efforts must be prioritized to preserve these “reefs of hope” for the future, the authors say. “Our work will contribute to the coral reef monitoring program in the Saudi Arabian Red Sea and will also accelerate broader regional coordination in data sharing,” says Fox.

The research was published in the journal Frontiers in Marine Science.

Then, Osman hopes they can uncover beneficial genetic and physiological traits that have existed for 7,000 years. “Our findings suggest that corals have the capacity to retain evolutionary traits that enable them to withstand heat stress,” he says. “The Red Sea offers an invaluable opportunity to unlock genetic secrets that could help us understand how other corals will adapt to warming oceans.”