Corals that grow faster in warm water could beat climate change

Parts of the Great Barrier Reef and many other coral reefs around the world are already dying because of global warming.

But if the fabulous ones in the northern Red Sea are protected from pollution, their unique evolutionary history means they might survive – and even thrive – late into this century despite the rising heat.

Corals normally expel the algae living within them – a process called bleaching ­– if stressed by water 1°C warmer than the usual summer maximum for several weeks.

However, a coral common in the northern part of the Red Sea can thrive even at temperatures 2°C higher than the present maximum in the area. When the coral (Stylophora pistillata) was subjected for six weeks to conditions expected from 2050 to 2100, it grew even faster than it does now.

“The coral did not bleach,” says team member Thomas Krueger at the Swiss Federal Institute of Technology in Lausanne. “The health of the algal partner actually improved.”

And there is reason to think other coral species in the northern Red Sea will also tolerate high temperatures. During the last ice age, the global sea level dropped 120 metres, largely shutting off the Red Sea from other oceans. It became highly saline, killing off corals.

Then, around 6000 years ago, after sea levels rose again, corals recolonised the Red Sea from the south, where conditions were much hotter. The corals growing in the northern part, where maximum sea temperatures are now around 27 °C in summer, are descendants of corals that lived in waters that can reach more than 30 °C.

This suggests that corals in the northern Red Sea might be naturally heat-resistant and able to survive summer maximum temperatures several degrees higher than normal.

But only a few small experiments have been done until now. The test by Krueger and his colleagues is the most rigorous so far, using new outdoor aquaria known as the “Red Sea simulator” at the Interuniversity Institute for Marine Sciences in Eilat, Israel.

Basking in the warmth

“The experimental temperatures were quite extreme, yet the corals survived,” says Terry Hughes at James Cook University in Townsville, Australia, who has been documenting the bleaching and even die-off of large areas of the Great Barrier Reef.

Surprisingly, the coral was also unaffected by the level of ocean acidification expected in the second half of this century. Why that would be the case isn’t clear, says Krueger.

The results are cause for optimism about the reefs in this one small part of the world. However, the study involved only one coral species, and did not look at reproductive success or the effect on coral ecosystems as a whole.

So the study does not necessarily mean that the reefs there will be fine – not least because those in the Red Sea are just as sensitive to pollution as others. Sewage and industrial waste could kill them off even if the corals are heat-tolerant.

In theory, if northern Red Sea reefs do survive much longer than others, they could provide a source of material for repopulating devastated reefs. But it’s not clear if corals from the Red Sea, which is more saline than other oceans, will thrive elsewhere.

Even if they do, transplanting corals on the scale required is unlikely to be feasible. “The required effort is unbelievable,” says Gareth Williams at Bangor University in the UK.

If the planet was warming gradually, all the world’s reefs would have a chance of adapting – but the pace is just too fast. “We are asking them to adapt and evolve at an unrealistic rate,” says Williams.