Late 2016 an obituary for the Great Barrier Reef posted on Outside Online went viral. Rising ocean temperatures and harmful human activities such as overfishing threaten the health of coral reefs across the world. But what are coral reefs and why do they matter?

Coral reefs are made up of colonies of corals, which are living organisms related to anemones and similar to both vegetable and animal. They have a simple structure like a can—one end is open and surrounded by tentacles. This structure, called a polyp, serves as the home for food intake, digestion and reproduction. Because the tentacles can sting, the polyp captures small organisms that come too close. Unlike sea anemones, corals have skeleton-like structures they build themselves. Coral skeletons are mineral—calcium carbonate or limestone, the same material the Egyptians used to build the pyramids. Coral secretes proteins that precipitate this calcium carbonate out of the water. These limestone skeletons make up the basis for the reefs themselves that house corals and other diverse sea life.

The bright color of corals doesn’t come from this limestone backbone, but rather from algae that live symbiotically with the coral. The algae, called zooxanthellae, live in the tissues of the coral polyps. The algae can photosynthesize, converting sunlight into energy for itself and the coral to use, and in turn the coral provides the algae with a safe place to live and nutrients such as carbon dioxide. When something is not right in the coral’s environment, the symbiotic relationship can be broken. The coral’s reaction to this environmental disruption is called a stress reaction. When the coral experiences stress, the coral’s cellular functions start to fail, and in an attempt to save itself expresses a different set of genes that it normally does. If its environmental conditions don’t get better, the coral will expel the algae living in its tissue: this is the process we call bleaching.

Like all living creatures, corals are sensitive to their environments and can only live under certain conditions.

What happened on the Great Barrier Reef was a mass bleaching event, where 30% of the coral died in a single heat wave. Like all living creatures, corals are sensitive to their environments and can only live under certain conditions. When these conditions change, it affects the health of the coral. Rising sea temperatures, for example in an El Niño year or because of altered ocean currents, can cause the coral to expel their algae. These rising sea temperatures also contribute to ocean acidification. When the ocean temperatures are warmer, they absorb more carbon dioxide. Carbon dioxide makes the water more acidic – the acidity in soda comes partially from the carbon dioxide bubbles. When the ocean is more acidic, the carbonate needed to make coral becomes bicarbonate, which cannot be precipitated into the limestone backbone. With the carbonate concentration decreasing, corals become increasingly fragile and at risk because they can’t produce their skeletons. Direct human activity, including overfishing, pollution, and sediment runoff from agriculture, also threaten coral health.

The destruction of coral reefs due to bleaching events and other coral destruction events leads to a loss of biodiversity in the oceans, because coral reefs are home to fish and other species. It is also devastating to coastal communities. These communities’ economies may be bolstered by coral reefs that draw in tourism or support fishing. They are not just economically important – coral reefs are deeply culturally and spiritually significant to native coastal communities. Coral’s symbiotic relationship does not begin and end with zooxanthellae; rather, it extends beyond the reach of the limestone skeleton to the wider ocean and to the people whose lives depend on it. 

Kelly Luis, a PhD student at UMass Boston, is developing a tool to remotely sense the health of coral reefs. As a native Hawaiian and a coral researcher, she had this to say about the importance of coral, ecologically and culturally:

Special thanks to Jose Luis Montalvo Proaño, MSc. (PhD Candidate at James Cook University & Australian Institute of Marine Science) and Kelly Luis (PhD student at University of Massachusetts Boston).