In the midst of the vast, relatively unpopulated ocean, coral reefs are hotspots of biological diversity. Reefs host thousands of species within their colourful, protective borders. They support many of the fisheries that coastal communities rely upon and sustain local economies by providing billions of dollars of income from tourism.

The vibrant colours that attract both fish and humans are the result of a symbiotic relationship between invertebrate coral polyps and colourful zooxanthellae algae. Corals provide algae with protection and nutrients. In return, the algae provide the corals with sugars produced via photosynthesis.

In 1979, incidents in which coral hosts cast out their resident algae began to be reported on an unprecedented scale. This occurrence, known as coral bleaching, leaves the coral a bleached white color because of the ejection of the colourful algae. Bleaching events are not necessarily lethal to corals. However, if bleaching occurs over a prolonged period of time, the coral host will eventually die. What is the cause of this sudden breakdown in the mutualistic relationship between corals and algae?