Bleaching of Coral Reefs

Coral bleaching is a phenomenon in which corals lose their normal pigments and appear bright white or dull and muted. Bleached corals are starved and weakened and often die if their conditions are not improved. Coral bleaching occurs when zooxanthellae, symbiotic algae that normally live in the tissues of the coral polyps, are expelled by the corals. Coral bleaching can have many causes, both natural and anthropogenic, but large-scale mass bleaching events are primarily linked with rising ocean temperatures as part of global climate change. The frequency of such events has increased dramatically during the twenty-first century.

Coral Biology and Coral-Algal Symbiosis

Corals, like hydras, jellyfish, and sea anemones, are cnidarians: living organisms that belong to the phylum Cnidaria. All cnidarians have a single body cavity, known as a coelenteron, through which they ingest food and release waste.

Members of this phylum also are characterized by radial symmetry and the presence of stinging cells on their surfaces known as nematocysts. These nematocysts are used to poison and capture prey. The term coral is most often applied to hermatypic corals, which are corals that build hard skeletons around their soft bodies by secreting calcium carbonate (limestone); they also grow and reproduce in large colonies, forming impressively intricate branching structures called coral reefs. Reefs typically comprise thousands of individual coral polyps, each of which is a single invertebrate marine animal.

Most hermatypic corals share another critical attribute: Some of their cells contain vast numbers of a particular type of single-celled organism. These organisms are tiny algae known as zooxanthellae. A single square centimeter of tissue from the gastrodermis (the lining of the digestive cavity) of a coral may contain millions of zooxanthellae. The relationship between the corals and the zooxanthellae is generally described as being symbiotic, or mutually beneficial. The coral provides the algae that inhabit its cells with a protected environment, and the waste products of its digestion also provide the algae with beneficial inorganic nutrients, such as nitrates and phosphates.

In return, the zooxanthellae remove waste products from the coral’s tissues, serve as a crucial source of oxygen, and produce carbohydrates that the coral can use to manufacture fats and its protective calcium carbonate skeleton. As a result of their photosynthesis, the zooxanthellae also provide the coral with many useful organic products, including nitrogen compounds, vitamins, amino acids, and nutrients that are essential for supporting and speeding the coral’s growth.

Color is another by-product of the symbiotic relationship between corals and zooxanthellae. Although the skeletons of hard tropical corals normally appear to be brightly colored, they are actually white. Their apparent colors come from vast quantities of fluorescent pigments produced by the algae; these striking pigments can be seen through the clear tissues that make up each coral polyp’s soft body.

Causes of Coral Bleaching

Coral bleaching is the phenomenon in which a coral expels most or all of the zooxanthellae that normally inhabit its tissue. When this happens, the coral loses its normal coloration, appearing anywhere from a dull shade of its original hue to bright white, depending on the severity of the bleaching. This is a generalized reaction that occurs when corals, and therefore the algae they contain, are subject to stress.

Stressed zooxanthellae produce a form of oxygen that is toxic to the corals. Rather than suffer the cellular damage that would result from this toxic compound, corals are forced to expel the zooxanthellae. Most coral colonies go through natural, temporary periods of mild bleaching, often in response to seasonal alterations in water temperature, but they recover when conditions return to normal.

Other natural environmental stresses that can lead to bleaching include increased predation, changes in the salinity or oxygen levels of ocean water, or an outbreak of a coral disease, such as a bacterial infection. Two pathogens that have been shown to cause bleaching in corals are the bacteria Vibro shiloi and V. patagonica.

Normally, coral reefs display a high level of resilience to such localized environmental stresses. Mass bleaching events (when bleaching occurs to vast swathes of reef at the same time) rarely occur in nature, and in time corals that have expelled their zooxanthellae will typically regain them and recover their health. By the late twentieth century, however, human activities had reduced the resilience of many coral reefs and introduced new and much more powerful causes of bleaching. These harmful activities include pollution, oil spills, overfishing, and stress caused by reef tourism.

The most significant cause of mass coral bleaching, however, has been increased sea surface temperatures (SST). Elevations in SST of as little as 1 to 2 degrees Celsius (33.8 to 35.6 degrees Fahrenheit) above the long-term average, if they persist for between one to two months, can lead to bleaching. Widespread increases in SST were observed in the early twenty-first century as a result of global climate change. According to the 2007 Intergovernmental Panel on Climate Change (IPCC), there is unequivocal evidence that both the atmosphere and oceans are warming, primarily as a result of anthropogenic greenhouse gas emissions, especially carbon dioxide.

Notable Instances and Trends

In 2014, scientists announced the occurrence of a prodigious, widespread instance of coral bleaching. It was only the third such global event recorded at that point, and it lasted longer and affected more areas than previous instances. Researchers suggested it was exacerbated by a strong El Niño Southern Oscillation (ENSO) event in 2015–16, and the bleaching trend continued until May 2017. It also affected some reefs that were not been known to have undergone bleaching before, such as the northernmost part of the famous Great Barrier Reef off the coast of Australia. Scientists suggested that it would take ten to fifteen years for the affected reefs to recover, but expressed concern that the reefs would not have that much time before waters warmed again.

The unprecedented mass bleaching event drew significant media attention and increased public awareness of the phenomenon. There was also further research connecting escalated coral bleaching with climate change. In early 2018, a study was published in the journal Science in which researchers analyzed one hundred tropical reefs worldwide and reported that the average amount of time between severe bleaching events had decreased from 27 years to 5.9 years.

As global warming patterns continued, and back-to-back bleaching events became the norm, scientists grew increasingly concerned that more coral reefs would be unable to recover. Those concerns grew even more dire in 2020, when yet another mass bleaching event occurred, during which approximately one-quarter of the Great Barrier Reef showed signs of severe bleaching. In 2022, the Australian Government's Great Barrier Reef Marine Park Authority reported another mass bleaching event, with 654 reefs, or 91 percent of those surveyed, impacted. This incident was especially notable as it came despite cooler La Niña conditions in the ENSO cycle.

Scientists studying the Great Barrier Reef mass bleaching events of the early 2020s did report some optimism around findings that the coral showed an unexpected level of resilience in its ability to survive and recover. Some researchers suggested that certain coral species might be evolving to be more adaptable in response to climate change.

Effects of Coral Bleaching on Ocean Ecosystems and Humans

In addition to losing their color, corals that lack zooxanthellae are deprived of up to 90 percent of the nutrients and oxygen they require for their survival, reproduction, and growth. The stress caused by zooxanthellae expulsion also leaves corals susceptible to disease and predation. Corals that have lost all their algae and do not recover them can typically survive a few months only before starving.

Research has shown that branching corals and plate corals, which grow quickly, are more susceptible to bleaching and less likely to survive it. Massive corals, which grow more slowly, are more resilient. Thus, even when coral reefs do eventually recover from bleaching events, their overall genetic and species diversity is likely to be reduced.

Bleaching also has a profound negative impact on the numerous other organisms that live in and around the coral ecosystem. For example, coral reef degradation has been shown to cause declines in fish communities. Various studies of coral reefs around the world indicate that bleaching events result in marked declines in the numbers of species that depend heavily on coral reefs for food and shelter. Some coral species also have been shown to disappear completely from particular areas (a phenomenon known as a local extinction) when reefs undergo mass bleaching. These types of negative effects often persist for many years after the bleaching event itself.

Finally, bleaching has a negative impact on human communities. Colorless reefs are less likely to attract dive tourism, which may make it more difficult for local communities to sustain their economic activities. In addition, weakened reefs provide less protection from waves, which can lead to faster and more damaging coastal erosion near coral reefs.

Principal Terms

anthropogenic: caused by or resulting from human activities; mainly used to describe environmental pollution and pollutants

cnidarian: any organism, including corals, hydras, jellyfish, and sea anemones, which belongs to the phylum Cnidaria; all cnidarians are radially symmetrical and have stinging cells known as nematocysts

coral bleaching: a pale or whitened appearance that arises from the loss of zooxanthellae, the symbiotic algae that are present in healthy reef-building corals

coral polyp: the basic unit of a coral reef; a single invertebrate marine animal with a protective skeleton made up of limestone, or calcium carbonate, around a soft inner body

coral reef: a living ecosystem in the shape of a ridge or mound, composed of colonies of coral polyps and the organisms that live in, on, and around them

fluorescent pigment: a coloring matter that absorbs light at a particular wavelength and emits it at a longer wavelength; these pigments, found in zooxanthellae, give hard corals their bright colors

hermatypic: of or relating to corals with stony skeletons that are formed from limestone and that grow in colonies to form coral reefs

resilience: in ecological terms, the ability of an ecosystem as a whole to recover from natural or anthropogenic disturbances and stresses and to continue to thrive and reproduce; the opposite of resilience is vulnerability

SST: commonly used abbreviation for sea surface temperature, or the temperature of water measured within the top few meters of the surface; a critical factor affecting ocean ecosystems, including coral reefs

zooxanthellae: single-celled organisms that inhabit the living cells of hard corals and provide them with energy in the form of oxygen and nutrients; a type of tiny algae

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