Can corals adapt their algae to stay cool?


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Recent research has shown that corals hosting a single species of zooxanthellae can exhibit differing levels of thermal tolerance – a significant discovery as this feature was only previously known for coral species that hosted a mix of zooxanthellae.

Zooxanthellae are the algae cells that are hosted by a coral and they provide the coral with energy; this relationship is essential for coral survival as without the zooxanthellae the coral will not only lose its tissue colour, but its primary source of energy often leading to the death of the coral.

Rising ocean temperatures are known to stress corals causing them to expel their zooxanthellae in a phenomenon known as "bleaching". Coral bleaching has led to a significant loss of corals around the world and is one of the major threats faced by today’s coral reefs.

Whilst it was previously known that corals hosting more than one species of zooxanthellae were better able to cope with temperature changes - by favouring those zooxanthellae with greater thermal tolerance – it was not known if corals hosting only a single type of the algae could tolerate differing levels of temperature.

Now research conducted at the Australian Institute of Marine Science (AIMS)  and published in the journal Nature Climate Change, demonstrate for the first time that corals hosting only a single type of zooxanthellae are able to differ in their thermal tolerance, an important finding given that many species of coral are dominated by only a single type of zooxanthellae.

The scientists collected two populations of the zooxanthellae, known as C1, from two locations on the Great Barrier Reef. The population collected from the area of Magnetic Island is subject to average ocean temperatures that are 2°C higher than the population collected from the Whitsunday Islands. Young corals were then treated with one or other of the two different zooxanthellae and exposed to elevated water temperatures.

The results were striking, with corals hosting the zooxanthellae from the warmer region coping well with elevated temperature – staying healthy and growing rapidly. The corals hosting the other zooxanthellae didn’t fare so well, suffering severe bleaching and partly dying off.

"Our research suggests that populations of a single type of zooxanthellae have adapted to local conditions, as can be seen from the remarkably different results of the two populations used in this study. If zooxanthellae populations are able to further adapt to increases in temperature at the pace at which oceans warm, they may assist corals to increase their thermal tolerance and survive into the future." says Emily Howells, a PhD student from the Australian Research Council Centre of Excellence for Coral Reef Studies (CoECRS) at James Cook University.

"However, we do not yet know how fast zooxanthellae can adapt, highlighting an important area of future research", says Bette Willis, Professor from the CoECRS.

Research at AIMS is therefore currently assessing whether zooxanthellae can continue to adapt to increasing temperatures and at what rate. This work in progress will provide insights into the capacity of zooxanthellae to adapt to future climate change.

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