New Survival Strategy for Heat-Stressed Coral Discovered in Natural Phenomenon

Coral reefs support an astonishing one quarter of all marine species, protect shorelines from storms, and provide food and income for over 1 billion people worldwide. But coral are fragile. Fundamental to their survival is a relationship with microscopic algae that live inside their tissues. This relationship provides coral with ninety percent of their food and their iconic bright colors. Without the algae, colorful reefs turn ghostly and white. The delicate relationship between coral and algae, however, is threatened by climate change. Under high heat, coral expel the algae in a process called coral bleaching. If bleached for several months, coral will essentially starve to death. With the death of coral follows the loss of many of the benefits they provide.  

Conventional wisdom has held that coral could only recover from a warming event if water temperatures cooled. However, a new study in Nature Communications out of the University of Victoria in Canada brings new optimism to coral conservation. The researchers showed that coral may be able to recover from bleaching naturally by adopting heat-tolerant algae during a warming event when local stressors, including pollution, are absent. 

The study followed 103 coral colonies (of the species Favites pentagona and Platygyra ryukyuensis) off Kiritimati Island, south of Hawaii, during an abnormally long warming event from 2015 to 2016. A varying level of pollution allowed researchers to explore the role of human impacts on coral recovery from bleaching. Before the heat-induced bleaching event, coral in the pristine water hosted heat-sensitive algae. Under good conditions, coral prefer hosting the sensitive algae because they provide the coral with high energy. By contrast, coral in the polluted water tended to host a heat- and stress-tolerant algae.  

Several months after the coral in the pristine water bleached, the bleached coral were repopulated by heat-tolerant algae. This change occurred while the water temperature remained high, which had not been previously observed. Through this adoption of stress-tolerant algae, coral in the pristine waters then survived the warming event more successfully than, or as well as, the coral living in the polluted water. One explanation could be that the coral in the pristine water had benefited from high nutritional content their whole lives, so they were stronger overall than the coral from the polluted water. Even though the coral from the pristine water had to uptake the less productive, heat-tolerant algae for a few months to survive the bleaching, their baseline health was superior and could explain their higher survival rates during and after bleaching.  

This study helps researchers identify new pathways for coral recovery from bleaching. The link between low pollution and high survival rates also highlights the importance of local marine management. It is important to note that local land managers should not be held entirely responsible for maintaining reefs. Climate change, primarily induced by humans in the Global North, remains a main driver of coral stress. However, it is good news that land management efforts to reduce local pollution can buy more time for coral that are threatened by climate change.