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Heat resistant corals: It’s all in the preparation

By on January 17, 2013 in EDGE Updates

Following on from our recent blog on the effects of global warming on coral bleaching, there has been an exciting new discovery into just how some species of coral show enhanced resistance to thermal stress.

It is important to understand that coral species, and indeed populations, are not all equally susceptible to thermal stress. Until this week however, the underlying mechanisms governing this variation was poorly understood.

According to new research, heat resistant corals have prepared for thermal extremes by activating a collection of 60 genes which convey heat resistant properties to their host.

A typical low tide in the Ofu back reef. Notice the extra still surface waters, it's this stillness that allows the water to heat up so much in the midday sun, yet a diversity of corals appear to healthily withstand these extreme temperature exposures.


These findings, which have a number of implications for future coral research, were made by studying populations of the coral species Acropora hyacinthus residing in a segregated reef environment in American Samoa. Here, a number of isolated pools are known to differentially exhibit high natural variation in conditions such as temperature, pH and oxygen availability.

Through analysing the genes of various populations within these pools the authors found that corals residing in pools with extreme temperature variation exhibited a greater resistance to thermal stress.

A photo of the medium variability pool taken from the shore with the Ofu/Olosega mountains in the background. The still water is why it heats up so much during the low tide, there's not much mixing with the colder water of the open ocean and the "pool" heats up in the sun creating the extreme temperatures.
An experimental transplant setup where the authors moved corals from the moderate pool into the more extreme pool and vice versa to investigate whether all corals can acquire increased stress tolerance in the more extreme pool.










This is perhaps not surprising. Throughout animals, plants and fungi it is well documented that exposure to mild stress allows a species to increases its tolerance of that stress in the future.

Nonetheless, the identification of some 60 genes that have a higher level of expression in heat tolerant corals is significant. It means that scientists can begin to search for these genes in other coral populations and thus more accurately predict future bleaching scenarios worldwide.

The experimental stress tanks that we use to conduct our simulated bleaching experiments. All tanks are computer controlled for high precision temperature manipulations.

The genes identified through this study are known to code for, amongst other things, heat shock proteins and enzymes that can counteract the lethal toxins produced during coral bleaching.

A photo of researcher Stephen Palumbi taking a genetic sample of an experimental coral in the Ofu back reef. Each individual coral is tagged and has a temperature logger to record the exact temperature conditions experienced over multiple years.

Such toxins are primarily produced by the symbiotic algae living within the reef building coral. Under high temperatures the algae become damaged and as a result they begin to produce high levels of reactive oxidants, compounds which are particularly harmful to their coral host.



Oxidants trigger chemical processes within corals which lead to cell death in susceptible populations.  This, in turn, leads to algae expulsion and bleaching. Problematically, even after the alga is expelled enough damage may have been done for the coral to start producing its own reactive oxidants. These, in turn, will cause additional tissue damage and death.

Interestingly, the stress response that makes certain corals resistant to bleaching is present in all coral species. This means that all corals have the genes required to confer heat resistance and furthermore they all have the potential to activate them. The problem is that this activation takes time, and during this time, unprepared corals often die.

In comparison, bleaching resistant corals have these genes permanently activated. This is advantageous as activated genes can rapidly trigger the creation of resistance conferring compounds, thus preventing the onset of thermal stress and cell death in the first place.

Following this exciting study, further research is needed to identify specific genes involved in heat resistance. Such data will provide scientists with greater predictive power over future trends in coral biogeography, and also the ability to predict potentially catastrophic bleaching events in reefs worldwide.

With these tools scientists may be able to plan mitigation strategies or emergency procedures to protect or temporarily relocate the rarest and most vulnerable species.

What seems clear at present is that rising ocean temperatures will impose a selective pressure not only in favour of thermally resistant corals, but also thermally resistant algal symbionts. Both need to adapt to survive in our ever warming oceans.