Different thermal tolerance mechanisms
In the current study, the research team used the testing system to evaluate the thermal tolerance of the smooth cauliflower coral in different regions of the Red Sea. The results show that corals from the Gulf of Aqaba, the most northeastern arm of the Red Sea, demonstrate a remarkable thermal tolerance – up to about 7°C above the respective maximum monthly average for the warmest summer month – just like their peers from the central part of the Red Sea. However, the absolute thermal tolerance of smooth cauliflower corals from the central part of the Red Sea is up to 3°C higher than for the same species in the Gulf of Aqaba, which could suggest that different tolerance mechanisms are at work.

In order to investigate this possibility, the research team conducted molecular analyses to elucidate mechanisms of thermal tolerance in corals from the different locations. Genetic examinations showed that smooth cauliflower corals from the Gulf of Aqaba respond to heat stress with a strongly altered gene expression – for example the increased production of certain proteins. Parallel to this, the composition of the coral-associated bacterial communities changed. By comparison, corals from the central part of the Red Sea did not exhibit any of these changes when exposed to heat stress.

The molecular results support the idea that smooth cauliflower corals have different thermal tolerance mechanisms. "We interpret the response of the corals from the Gulf of Aqaba as that of a "resilient" population that directly and proportionally reacts to increases in temperature. By contrast, the more static expression of genes of the corals from the central part of the Red Sea indicates a fixed reaction norm, irrespective of the heat stress applied, which provides "resistance" to high water temperatures, but at the cost of the ability to flexibly respond to further increases in temperatures", says Voolstra.

Toolbox of methods for protecting coral reefs worldwide
At the moment, it is unclear which of these tolerance mechanisms protects corals better from the global increase in ocean temperatures caused by climate change. The fact that "resilient" and "resistant" tolerance mechanisms can be distinguished using molecular methods could be of great importance for the conservation prioritization of existing coral reefs or for restoration approaches that could use heat-tolerant corals for sexual propagation.

For this reason, the research team is making plans to employ the methodological approach used successfully in the Red Sea study around the world. "Our study shows the tremendous value of an integrative, combined approach: using the CBASS system for the standardized identification of thermal tolerance in corals with subsequent follow-up molecular analyses to identify the underlying tolerance mechanisms and marker genes", concludes Voolstra.

Financial support for this project is provided by the Paul G. Allen Family Foundation, among others, that funds the CBASS project and the further development of the rapid test system with a total of 4 million dollars (3.4 million euros), as reported by the University of Konstanz in July. Funding for the current study was also provided by the German Research Foundation (DFG) and the US-Israeli Binational Science Foundation (BSF).