Coral reefs support more species per unit area than any other marine environment, provide over half a billion people worldwide with socio-economic benefits, and produce an estimated USD $30 billion annually.1 Many people do not realize that these diverse ecosystems are at risk of extinction as a result of human activity--the Caribbean has already lost 80% of its coral cover in the past few decades2 and some estimates report that at least 60% of all coral will be lost by 2030.1 One of the most predominant and direct threats to the health of these fragile ecosystems is the enormous amount of carbon dioxide and methane that have spilled into the atmosphere, warming the planet and its oceans on unprecedented levels.

Corals are Cnidarians, the phylum characterized by simple symmetrical structural anatomy. Corals reproduce either asexually or sexually and create stationary colonies made up of hundreds of genetically identical polyps.3 The major reef-building corals belong to a sub-order of corals, called Scleractinia. These corals contribute substantially to the reef. framework and are key species in building and maintaining the structural complexity of the reef.3 The survival of this group is of particular concern, since mass die- offs of these corals affect the integrity of the reef. Corals form a symbiosis with tiny single-celled algae of the genus Symbiodinium. This symbiotic relationship supports incredible levels of biodiversity and is a beautifully intricate relationship that is quite fragile to sudden environmental change.3

The oceans absorb nearly half of the carbon dioxide in the atmosphere through chemical processes that occur at its surface.4 Carbon dioxide combines with water molecules to create a mixture of bicarbonate, calcium carbonate, and carbonic acid. Calcium carbonate is an important molecule used by many marine organisms to secrete their calcareous shells or skeletons. The increase of carbon dioxide in the atmosphere shifts this chemical equilibrium, creating higher levels of carbonic acid and less calcium carbonate.4 Carbonic acid increases the acidity of the ocean and this phenomenon has been shown to affect the skeletal formation of juvenile corals.5 Acidification weakens the structural integrity of coral skeletons and contributes to heightened dissolution of carbonate reef structure.3

The massive influx of greenhouse gases into our atmosphere has also caused the planet to warm very quickly. Corals are in hot water, literally. Warmer ocean temperatures have deadly effects on corals and stress the symbiosis that corals have with the algae that live in their tissues. Though coral can procure food by snatching plankton and other organisms with protruding tentacles, they rely heavily on the photosynthesizing organism Symbiodinium for most of their energy supply.3 Symbiodinium provides fixed carbon compounds and sugars necessary for coral skeletal growth. The coral provides the algae with a fixed position in the water column, protection from predators, and supplementary carbon dioxide.3 Symbiodinium live under conditions that are 1 to 2° C below their maximum upper thermal limit. Under warmer conditions due to climate change, sea surface temperatures can rise a few degrees above their maximum thermal limit. This means that a sudden rise in sea temperatures can stress Symbiodinium by causing photosynthetic breakdown and the formation of reactive oxygen species that are toxic to corals.3 The algae leave or are expelled from the coral tissues as a mechanism for short-term survival in what is known as bleaching. Coral will die from starvation unless the stressor dissipates and the algae return to the coral’s tissues.3

Undoubtedly, the warming of the seas is one of the most widespread threats to coral reef ecosystems. However, other threats combined with global warming may have synergistic effects that heighten the vulnerability of coral to higher temperatures. These threats include coastal development that either destroys local reefs or displaces sediment to nearby reefs, smothering them. Large human populations near coasts expel high amounts of nitrogen and phosphorous into the ecosystem, which can increase the abundance of macroalgae and reduce hard coral cover. Increased nutrient loading has been shown to be a factor contributing to a higher prevalence of coral disease and coral bleaching.6 Recreational fishing and other activities can cause physical injury to coral making them more susceptible to disease. Additionally, fishing heavily reduces population numbers of many species of fish that keep the ecosystem in balance.

The first documented global bleaching event in 1998 killed off an estimated 16% of the world’s reefs; the world experienced the destruction of the third global bleaching event occurred only last year.1 Starting in mid-2015, an El Niño Southern Oscillation (ENSO) weather event spurred hot sea surface temperatures that decimated coral reefs across the Pacific, starting with Hawaii, then hitting places like American Samoa, Australia, and reefs in the Indian Ocean.7 The aftermath in the Great Barrier Reef is stunning; the north portion of the reef experienced an average of 67% mortality.8 Some of these reefs, such as the ones surrounding Lizard Island, have been reduced to coral skeletons draped in macroalgae. With climate change, it is expected that the occurrence of ENSO events will become more frequent, and reefs around the world will be exposed to greater thermal stress.1

Some scientists are hopeful that corals may be able to acclimatize in the short term and adapt in the long term to warming ocean temperatures. The key to this process lies in the genetic type of Symbiodinium that reside in the coral tissues. There are over 250 identified types of Symbiodinium, and genetically similar types are grouped into clades A-I. The different clades of these algae have the potential to affect the physiological performance of their coral host, including responses to thermotolerance, growth, and survival under more extreme light conditions.3 Clade D symbiont types are generally more thermotolerant than those in other clades. Studies have shown a low abundance of Clade D organisms living in healthy corals before a bleaching event, but after bleaching and subsequently recovering, the coral has a greater abundance of Clade D within its tissues.9,10 Many corals are generalists and have the ability to shuffle their symbiont type in response to stress.11

However, there is a catch. Though some algal members of Clade D are highly thermotolerant, they are also known as selfish opportunists. The reason healthy, stress-free corals generally do not have a symbiosis with this clade is that it tends to hoard the energy and organic compounds it creates from photosynthesis and shares fewer products with its coral host.3

Approaches that seemed too radical a decade ago are now widely considered as the only means to save coral reefs from the looming threat of extinction. Ruth Gates, a researcher at the Hawaii Institute of Marine Biology is exploring the idea of assisted evolution in corals. Her experiments include breeding individual corals in the lab, exposing them to an array of stressors, such as higher temperatures and lower pH, and picking the hardiest survivors to transplant to reefs.12 In other areas of the globe, scientists are breeding coral larvae in labs and then releasing them onto degraded reefs where they will hopefully settle and form colonies.

Governments and policy makers can create policies that have significant impact on the health of reefs. The creation of marine protected areas that heavily regulates or outlaws harvesting of marine species offers sanctuary to a stressed and threatened ecosystem.3 There is still a long way to go, and the discoveries being made so far about coral physiology and resilience are proving that the coral organism is incredibly complex.

The outlook on the future of healthy reefs is bleak; rising fossil fuel consumption rates mock the global goal of keeping rising temperatures below two degrees Celsius. Local stressors such as overfishing, pollution, and coastal development cause degradation of reefs worldwide. Direct human interference in the acclimatization and adaptation of corals may be instrumental to their survival. Rapid transitions to cleaner sources of energy, the creation of more marine protection areas, and rigid management of reef fish stocks may ensure coral reef survival. If humans fail in this endeavor, one of the most biodiverse and productive ecosystems on earth that has persisted for millions of years may come crashing to an end within our lifetime.


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