Certain species of coral reefs have been able to adapt or acclimatise to heat stress from unusually warm seas, according to a  study published in the open-access online journal PLoS One.

Corals live in partnership with algae that provide the coral with food and give them their bright colours. When faced with long periods of environmental stress like unusually warm seawater or slight changes to ocean acidity, the algae are expelled from the coral tissue, causing “coral bleaching”.

It’s possible for coral reefs to recover from bleaching, but frequent, extensive and widespread bleaching can cause large swathes of reef to die-off. This has led scientists to voice concern over the future of coral reef systems, because of the warmer seas and increased ocean acidification projected to accompany man-made climate change over the coming century. There have even been suggestions that as many as a third of reef building corals face extinction.

Previous research suggests that large, fast-growing reef-building corals – Acropora corals – are particularly vulnerable to coral-bleaching events, leading scientists to suggest that reef ecosystems could see significant changes in the future with Acropora corals are replaced by hardier varieties.

But this research looked at corals in three sites that unexpectedly survived unusually warm seawater temperatures in 2010. It found that in two cases in Singapore and Malaysia corals survived a bleaching event in 2010, and had also survived a major coral bleaching event in 1998. In the third site in Indonesia however the Acropora corals responded typically, dying off when faced with the unusual warmth of 2010. It turns out that the Indonesian site did not experience a coral bleaching back in 1998. So some coral reefs can withstand coral bleaching from warm seawater if they’ve previously survived a similar bleaching.

The finding has surprised the scientists conducting the research. Lead author Dr James Guest, joint research fellow at the UNSW Centre for Marine Bio-innovation and Singapore’s Nanyang Technological University, writes in the paper:

“This suggests that the thermal history of these sites may have played an important role in determining the bleaching severity in 2010.[…] The most parsimonious explanation, therefore, is that coral populations that bleached during the last major warming event in 1998 have adapted and/or acclimatised to thermal stress. This is controversial because many scientists believe that corals have exhausted their capacity to adapt to thermal stress.”

So how does this unexpected result fit with other pieces of scientific research on the subject? It agrees with a previous study which found that coral reefs that had been exposed to frequent warming events were less likely to experience coral bleaching. And this isn’t the only way that coral communities can adapt to climate change. Another study suggests that corals might be migrating (slowly) in response to climate change, a type of behaviour that has been also been documented in the fossil record.

A by-product of these adaptations is that new reef communities become established, dominated by hardier corals, which alters the coral reef ecosystem. The high population turnover associated with the new communities established in mass bleaching events can help enhance disease resistance.

The finding that some coral reefs might be able to adapt to climate change is encouraging, but it isn’t all good news. The ability of coral to adapt to climate change relies on the climate changing slowly enough to allow adaptation, and there are likely to be limits to the seawater temperatures to which corals can adapt. Scientists have warned that the heat stress of warmer seas and ocean acidification resulting from man-made climate change might interact with other threats – including pollution, overfishing and disease – to make coral reefs more vulnerable in the future.

Still, as Guest writes in the paper:

“The results of the present study do indicate, however, that the effects of coral bleaching will not be as uniform as previously thought and fast-growing branching taxa such as Acropora and Pocillopora are likely to persist in some locations despite increases in the frequency of thermal stress events.”