Surprising Heat Tolerance In Some Corals May Provide Hope For The Future Of Reefs

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A new study has demonstrated that some corals are able to acclimatize rapidly to rising sea temperatures, acquiring the same level of heat tolerance in little over two years that could be expected to develop from natural selection over many generations. The results have been published in Science.

Coral reefs are one of the most biodiverse and economically valuable ecosystems on the planet. Despite only making up around 1% of the ocean floor, they are home to approximately 25% of all marine life. Not only do they serve as habitats for a diverse array of organisms, they also protect coastlines from storms and provide a source of income in terms of recreation and tourism.

Unfortunately coral reefs are under threat, for example from pollution and rising sea temperatures, and they have been experiencing decline worldwide. New research, however, has shown that it might not be all doom and gloom for corals, and some can acclimatize to increases in temperatures remarkably quickly.  

Coral reefs in certain areas of American Samoa can tolerate very high temperatures; some pools often experience temperatures above the bleaching temperature of 30oC, and can sometimes reach as high as 35oC. These are known as the Highly Variable (HV) pools in this study. Other cooler pools (Moderately Variable [MV]) rarely exceed temperatures of 32oC. Corals found in the HV pool demonstrate higher growth rates and better survival than those found in MV pools.

In this study, the researchers investigated acclimatization in the MV and HV corals by taking samples from each pool and swapping them over. They then tested these transplanted corals for heat resistance at particular time intervals for the next 27 months. They found that over time the transplanted MV corals became more heat tolerant, although they never achieved the resistance found in native HV corals. Furthermore, the resistance they developed over this short period was similar to what would be expected to develop over many generations through strong natural selection.

In order to gain an insight into the mechanisms behind this heat resistance, the researchers investigated gene expression in the transplanted corals. They found changes in expression of numerous genes between genetically identical samples from the two pools, demonstrating that environmental changes can trigger important genes in the corals to switch on or off in order to adapt.

Although these results demonstrate that some coral species can relatively rapidly acquire heat tolerance through acclimatization, only one species of coral was investigated in this study. The fate of other coral species in the face of changing ocean temperatures therefore remains unclear. It is also possible that when subjected to multiple stressors, such as heat and acidification, the ability to adapt will be reduced. However, they do demonstrate that particular corals may certainly be able to resist the harsh effects of rising sea temperatures for now at least. 

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