Stress test for corals

Researchers at the ARC Centre of Excellence for Coral Reef Studies (CoECRS) are developing a test to measure the expression of haemoglobin by the resident microalgae that live within coral. Ideally, the test will provide a way of monitoring these reef-building creatures and lead to the improved management of the ocean environment in which corals live.

Most reef-building corals have a mutually beneficial relationship with microalgae, called zooxanthellae. The microalgae live within the cells of the coral, where they are protected and provided with the compounds needed for its photosynthesis. In turn, the algae produce oxygen, help remove wastes and supply the coral with the products of its photosynthesis. The coral uses these products (mostly carbohydrates) to make its calcium carbonate skeletons.

When these algal symbionts abandon the coral due to changes in the environment, such as high water temperatures or pollution, the corals are deprived of their main energy source and whiten or bleach and potentially die.

The researchers at CoECRS have been exploring the genetic make-up of corals and their resident microalgae. They found that microalgae contain haemoglobin genes, which encode for haemoglobin-like proteins that rapidly respond to temperature and nutrient stresses.

It is hoped these proteins will provide a readout on how stressed a particular coral is as well as provide an explanation for how they become stressed.

More than half of the Great Barrier Reef has undergone coral bleaching in recent years. In fact, the majority of coral reefs around the world are experiencing bleaching to smaller or larger extents and this is largely due to climate change and human activity.

“Despite the importance of coral reefs to hundreds of millions of people worldwide, we still do not clearly understand how well they can cope with changed conditions of climate and environment they now face,” said Professor Ove Hoegh-Guldberg, who works at CoECRS.

Unlike animals, who use haemoglobin to carry oxygen in the blood, plants and algae use haemoglobin to mop up spare oxygen and toxic gases before they cause harm. Haemoglobin may also form a vital part of the day-night energy storage system in corals and their algae.

“When the coral undergo temperature stress, the system goes into overdrive and haemoglobin genes are expressed at a higher level. Due to its sensitive nature, haemoglobin has potential to be used as a stress biomarker. This, for the first time, gives us a clear readout of stress levels in the corals and their symbiotic algae,” explained Dr Nela Rosic, from the University of Queensland.

These insights into the physiology of coral bleaching at the molecular level have the potential to be developed into a type of reef monitoring system.

Monitoring the stress levels of a coral would help determine whether they were more vulnerable to bleaching and death. And with this knowledge, strategies could be adopted to reduce the pressure on the coral.

The test could also be used to monitor the success of improved management of catchments and other steps taken to improve conditions surrounding coral reefs.

However, the scientists caution that the primary stress on corals is a result of high ocean temperatures due to global warming - and this can only be addressed by cutting carbon emissions.

This work was published in the journal Ecology and Evolution.