Heat-Resistant Zooxanthellae Prevent Coral Bleaching

Rising temperatures due to climate change can cause an event called coral bleaching. When a reef undergoes bleaching, the coral expel the algae living inside their skeletons (known as zooxanthellae). These algae normally form a symbiotic relationship with the coral; the coral provides a safe place to live and the zooxanthellae provide the coral with energy through photosynthesis. When a coral becomes temperature stressed, they flush the algae out. The exact mechanisms and reasons aren’t known but a major factor seems to be that, in high temperatures, these normally symbiotic algae begin to produce toxic byproducts. However, a small percentage of coral reefs don’t bleach at all. A recent paper published in Molecular Biology and Evolution provides a possible explanation for why these reefs don’t experience bleaching.

A team of Australian researchers suspected that the specific species of symbiotic zooxanthellae might have some sort of trait that prevents bleaching. Different coral have slightly different species of zooxanthellae and these differences had never been studied in depth. The researchers found that the species present in bleach-resistant coral was tolerant of heat. Compared to the more common species, they did well at high temperatures. The algae had several stress resistance genes that seemed to allow them to cope with temperature changes. Not only did they adapt better overall but they also produced less of the poisonous byproducts, explaining why coral may not expel zooxanthellae of this species.

Coral bleaching events are devastating to reefs. The coral die rapidly and that causes a domino effect in which other reef inhabitants also perish. If some species of zooxanthellae are heat-resistant, it could provide clues as to how to save the reefs. Scientists can also now pinpoint the stress-resistance genes, allowing them to determine just how at-risk a specific reef is. Understanding how and why coral expel the symbiotic algae is the first step to finding out how to stop the process.


Rachel A. Levin et al, Sex, Scavengers, and Chaperones: Transcriptome Secrets of Divergent Symbiodinium Thermal Tolerances, Molecular Biology and Evolution (2016).

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