Heat-tolerant algae could save coral reefs from warming seas

Coral reefs under threat from rising ocean temperatures could be saved by training algae to become more tolerant of warmer waters, a study has found.

Corals turn white in a process known as bleaching when a rise in water temperature causes them to expel the colourful algae that live in their tissue and supply most of their nutrition.

The Great Barrier Reef off Australia lost about half its corals after heatwaves in 2016 and 2017 caused mass bleaching. Scientists reported last month that bleaching was taking place on several sections of the 2200km reef.

Scientists took algae from reefs and exposed them in a laboratory for four years to a temperature of 31C, which is 3-4C above what they normally experience and comparable to the water temperature during the Great Barrier Reef bleachings.

When the algae were reintroduced into coral larvae they were found to have adapted to become more tolerant of heat and were able to share this benefit with the coral.

‘‘We found that the heat tolerant microalgae are better at photosynthesis and improve the heat response of the coral animal,’’ Madeleine van Oppen, one of the authors, from the University of Melbourne, said. ‘‘These exciting findings show that the microalgae and the coral are in direct communication with each other.’’

The scientists believe that the process could be scaled up to produce large amounts of heat-tolerant algae, which could be introduced to reefs.

Patrick Buerger, another author of the study published in Science Advances, said: ‘‘We can grow these heat-evolved algae strains in very high quantities in aquaculture facilities. The benefit of this potential intervention is that even a few types of these heatevolved algae can form a symbiosis with many different coral species.’’

However, he said that more research was needed to check for potential sideeffects on adult coral and demonstrate that the symbiotic relationship between the adapted algae and the coral was maintained over time.

The Intergovernmental Panel on Climate Change warned last year that almost all coral reefs would decline even if the world met the objective of the Paris Agreement to keep the increase in global average temperature below 2C.