Australian scientists studying coral have identified heat-tolerant individuals at sites spanning the entire Great Barrier Reef. This good news suggests that some corals already have naturally occurring variation that could help them adapt to a warming climate.
The Great Barrier Reef consists of over 2,500 individual reefs stretching over 340,000 square kilometres off the northeast coast of Australia. The reef contains over 400 species of coral, which grow into three-dimensional colonies that create habitats for thousands of other marine species.
Healthy corals form symbiotic relationships with microscopic algae called zooxanthellae, which shelter in the coral’s tissues. In exchange, the zooxanthellae provide their coral host with nutrients, and they also give coral their colour.
This symbiotic relationship breaks down when the coral is stressed by heat.
When surface sea temperature rise above a certain threshold, corals eject the zooxanthellae from their tissues and become bleached. But not all corals have the same temperature threshold for bleaching.
In a study published in Communications Earth and Environment, Australian researchers measured the bleaching thresholds of more than 500 colonies of table coral at 17 different reefs across the Great Barrier Reef.
“We found heat-tolerant corals at almost all the reefs that we studied, highlighting how corals across the entire Great Barrier Reef may hold genetic resources that are important for protection and restoration,” said lead author Melissa Naugle of Southern Cross University in Lismore, Australia.
“This is important news for corals, which are experiencing the fourth global mass bleaching event and unprecedented summer sea temperatures on the Great Barrier Reef,” adds Naugle.
Global mass bleaching events happen when significant coral bleaching is reported in all the ocean regions where warm-water corals live, including the Great Barrier Reef. These events are becoming increasingly common, occurring in 1998, 2010, 2014-17 and 2024.
On the Great Barrier Reef alone, there have been five mass coral bleaching events since 2016. Recently researchers have calculated that the Great Barrier Reef has sustained the highest ocean heat in four centuries.
These patterns reflect anthropogenic climate change, which has increased the global average temperature by about 1 degree Celsius since 1880. This extra heat drives regional extremes, including extreme surface sea temperatures that impact corals.
“Naturally occurring heat tolerance variation is crucial for corals to adapt to climate warming and for the success of restoration initiatives,” says Naugle.
Naugle’s findings for widespread variation in heat tolerance in table coral (Acropora hyacinthus) were recently substantiated by Southern Cross University colleagues in a different species of coral (Acropora spathulata).
If the researchers determine that the variation in heat tolerance is genetic, then it could inform targeted reef preservation efforts.
“Heat tolerance variation can be useful for restoration programs such as selective breeding, which may accelerate adaptation to produce offspring better suited to warmer waters,” says Emily Howells, co-author and Project Lead in the Reef Restoration and Adaptation Program.
The most heat tolerant corals identified in this study are currently being used for a selective breeding trial through the Reef Restoration and Adaptation Program.
During the experiment, Naugle and colleagues tested for environmental influences on heat tolerance, including thermal history, nutrient concentrations and zooxanthellae present.
They found that substantial variation was not explained by these environmental sources, suggesting at least some heat tolerance was determined by genetics.
“Next, we’ll analyse DNA-sequencing data from these individuals to identify gene variants associated with heat tolerance. This can help us understand the adaptation potential of natural coral populations and inform selective breeding work,” says Naugle.
This research comes at a particularly vulnerable time for the Great Barrier Reef, which was inscribed as a World Heritage Site by UNESCO in 1981. In 2021, the same UN body drafted a decision to inscribe the Reef on the List of World Heritage in Danger.
The Ultramarine project – focussing on research and innovation in our marine environments – is supported by Minderoo Foundation.