Coral bleaching

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Bleached corals
Healthy corals

Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) through either expulsion or loss of algal pigmentation.[1] The corals that form the structure of the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like unicellular flagellate protozoa that are photosynthetic and live within their tissues. Zooxanthellae give coral its coloration, with the specific color depending on the particular clade. Some scientists consider bleaching a poorly-understood type of "stress" related to high irradiance; environmental factors like sediments, harmful chemicals and freshwater; and high or low water temperatures.[1] This "stress" causes corals to expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term "bleached".[2] Bleaching has been attributed to a defense mechanism in corals; this is called the "adaptive bleaching hypothesis," from a 1993 paper by Robert Buddemeier and Daphne Fautin.[3] Bleached corals continue to live, but growth is limited until the protozoa return.


Bleaching occurs when the conditions necessary to sustain the coral's zooxanthellae cannot be maintained.[4] Any environmental trigger that affects the coral's ability to supply the zooxanthellae with nutrients for photosynthesis (carbon dioxide, ammonium) will lead to expulsion.[4] This process is a "downward spiral", whereby the coral's failure to prevent the division of zooxanthellae leads to ever-greater amounts of the photosynthesis-derived carbon to be diverted into the algae rather than the coral. This makes the energy balance required for the coral to continue sustaining its algae more fragile, and hence the coral loses the ability to maintain its parasitic control on its zooxanthellae.[4]

Physiologically the lipid composition of the symbiont thylakoid membrane affects their structural integrity when there is a change in temperature, which combined with increased nitric acid results in damage to photosystem II. As a result of accumulated oxidative stress and the damage to the thylakoid of chloroplasts there is an increase in degradation of the symbiosis and the symbionts will eventually abandon their host. Not only does the change in temperature in the water increase the chances of bleaching, but there are other factors that play a role. Other factors include an increase in solar radiation (UV and visible light), regional weather conditions, and for intertidal corals, exposure to cold winds.[5]


Coral bleaching is theorized to be a generalized stress response of corals that may be caused by a number of biotic and abiotic factors, including:

While most of these triggers may result in localized bleaching events (tens to hundreds of kilometers), mass coral bleaching events occur at a regional or global scale and are triggered by periods of elevated thermal stress resulting from increased sea surface temperatures.[13] The coral reefs that are more subject to continued bleaching threats are the ones located in warm and shallow water with low water flow. Physical factors that can prevent or reduce the severity of bleaching are available for the reefs located under conditions that include low light, cloud cover, high water flow and higher nutrient availability.[5]


Healthy coral at left and bleached, but still living, coral to right

The color of a coral depends largely on the species of symbiont. A reduction in concentration of zooxanthellae causes paling and an increase results in deepening of color. Stony corals have calcium carbonate skeletons and most have transparent tissues, so expulsion of the zooxanthellae causes them to lose their color and become white. Although the coral polyps feed on zooplankton and other food particles, the majority of reef-forming corals rely for a large proportion of their nutritional requirements on their zooxanthellae. This means that without them they are liable to starve. Coral growth and reproduction are reduced and the coral becomes increasingly susceptible to disease. If stress factors reduce and the zooxanthellae return, the coral can recover, but prolonged bleaching causes death of the coral.[26]

Ejection increases the polyp's chance of surviving short-term stress[why?][how?]. It can regain symbionts, possibly of a different species, at a later time. If the stressful conditions persist, the polyp eventually dies.[27]

Mass bleaching events[edit]

Bleached Acropora coral (foreground) and normal colony (background), Keppel Islands, Great Barrier Reef

Most evidence indicates that elevated temperature is the cause of mass bleaching events. Sixty major episodes of coral bleaching have occurred between 1979 and 1990,[28][29] with the associated coral mortality affecting reefs in every part of the world. Correlative field studies have pointed to warmer-than normal conditions as being responsible for triggering mass bleaching events. These studies show a tight association between warmer-than-normal conditions (at least 1 °C higher than the summer maximum) and the incidence of coral bleaching.[30]

Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence.[31] Coral and zooxanthellae health and genetics also influence bleaching.[31]

Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such Acropora are far more susceptible to stress following a temperature change.[32] Corals consistently exposed to low stress levels may be more resistant to bleaching.[33][34]

Monitoring reef sea surface temperature[edit]

The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots", areas where sea surface temperature rises 1 °C or more above the long-term monthly average. This system detected the worldwide 1998 bleaching event,[35][36] that corresponded to an El Niño. NOAA also uses a nighttime-only satellite; these observations are taken at night to avoid the increase in temperature due to daily warming caused by solar heating at the sea surface during the day. This is also a precaution to avoid glare from the sun.[37]

Changes in ocean chemistry[edit]

Increasing ocean acidification due to rises in carbon dioxide levels exacerbates the bleaching effects of thermal stress. Acidification affects the corals' ability to create calcareous skeletons, essential to their survival.[38] A recent study from the Atkinson Center for a Sustainable Future found that with the combination of acidification and temperature rises, the levels of CO2 could become too high for coral to survive in as little as 50 years.[38]

Infectious disease[edit]

Infectious bacteria of the species Vibrio shiloi are the bleaching agent of Oculina patagonica in the Mediterranean Sea, causing this effect by attacking the zooxanthellae.[39][40][41] V. shiloi is infectious only during warm periods. Elevated temperature increases the virulence of V. shiloi, which then become able to adhere to a beta-galactoside-containing receptor in the surface mucus of the host coral.[40][42] V. shiloi then penetrates the coral's epidermis, multiplies, and produces both heat-stable and heat-sensitive toxins, which affect zooxanthellae by inhibiting photosynthesis and causing lysis.

During the summer of 2003, coral reefs in the Mediterranean Sea appeared to gain resistance to the pathogen, and further infection was not observed.[43] The main hypothesis for the emerged resistance is the presence of symbiotic communities of protective bacteria living in the corals. The bacterial species capable of lysing V. shiloi had not been identified as of 2011.


Two images of the Great Barrier Reef showing that the warmest water (top picture) coincides with the coral reefs (lower picture), setting up conditions that can cause coral bleaching

In the 2012–2040 period, coral reefs are expected to experience more frequent bleaching events. The Intergovernmental Panel on Climate Change (IPCC) sees this as the greatest threat to the world's reef systems.[44][45][46][47]

Great Barrier Reef[edit]

The Great Barrier Reef along the coast of Australia experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, and 2006.[47] Some locations suffered severe damage, with up to 90% mortality.[12] The most widespread and intense events occurred in the summers of 1998 and 2002, with 42% and 54% respectively of reefs bleached to some extent, and 18% strongly bleached.[48][49] However coral losses on the reef between 1995 and 2009 were largely offset by growth of new corals.[50] An overall analysis of coral loss found that coral populations on the Great Barrier Reef had declined by 50.7% from 1985 to 2012, but with only about 10% of that decline attributable to bleaching, and the remaining 90% caused about equally by tropical cyclones and by predation by crown-of-thorns starfishes.[51]

The IPCC's moderate warming scenarios (B1 to A1T, 2 °C by 2100, IPCC, 2007, Table SPM.3, p. 13[52]) forecast that corals on the Great Barrier Reef are very likely to regularly experience summer temperatures high enough to induce bleaching.[48]

Other areas[edit]

Other coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles.[53]

Coral in the south Red Sea does not bleach despite summer water temperatures up to 34 °C.[33][54]


Evidence of thermal tolerance in Hawaiian corals and of oceanic warming from research in the 1970s led researchers in 1990 to predict mass occurrences of coral bleaching throughout Hawaii. Major bleaching occurred in 1996 and in 2002.[55] Biologists from the University of Queensland observed the first mass bleaching event for Hawaiian coral reefs in 2014, and attributed it to The Blob.[56]

Economic and political impact[edit]

According to Brian Skoloff of The Christian Science Monitor, "If the reefs vanished, experts say, hunger, poverty and political instability could ensue."[57] Since countless sea life depends on the reefs for shelter and protection from predators, the extinction of the reefs would ultimately create a domino effect that would trickle down to the many human societies that depend on those fish for food and livelihood. There has been a 44% decline over the last 20 years in the Florida Keys, and up to 80% in the Caribbean alone.[58]

Coral adaptation[edit]

In 2010, researchers at Penn State discovered corals that were thriving while utilizing an unusual species of symbiotic algae in the warm waters of the Andaman Sea located in the Indian Ocean. Normal zooxanthellae cannot withstand temperatures as high as in that location, so this finding was unexpected. This gives researchers hope that with rising temperatures due to global warming, coral reefs will develop tolerance for different species of symbiotic algae that are resistant to high temperature, and can live within the reefs.[59][60]

Recovery and macroalgal regime shifts[edit]

After corals experience a bleaching event to increased temperature stress some reefs are able to return to their original, pre-bleaching state.[61][62] Reefs either recover from bleaching, where they are recolonized by zooxanthellae, or they experience a regime shift, where previously flourishing coral reefs are taken over by thick layers of macroalgae.[63] Discovering what causes reefs to be resilient or recover from bleaching events is of primary importance because it helps inform conservation efforts and protect coral more effectively.

Corals have shown to be resilient to short-term disturbances. Recovery has been shown in after storm disturbance and crown of thorns starfish invasions.[61] Fish species tend to fare better following reef disturbance than coral species as corals show limited recovery and reef fish assemblages have shown little change as a result of short term disturbances.[61] In contrast, fish assemblages in reefs that experience bleaching exhibit potentially damaging changes. One study by Bellwood et al. notes that while species richness, diversity, and abundance did not change, fish assemblages contained more generalist species and less coral dependent species.[61] Studies note that better methods are needed to measure the effects of disturbance on the resilience of corals.[61][64]

lemon damselfish
The lemon damselfish (Pomacentrus moluccensis) is a coral associated species that has been shown to decline dramatically following coral bleaching.[65]

Until recently, the factors mediating the recovery of coral reefs from bleaching were not well studied. Research by Graham et al. (2005) studied 21 reefs around Seychelles in the Indo-Pacific in order to document the long-term effects of coral bleaching.[62] After the loss of more than 90% of corals due to bleaching in 1998 around 50% of the reefs recovered and roughly 40% of the reefs experienced regime shifts to macroalgae dominated compositions.[62] After an assessment of factors influencing the probability of recovery, the study identified five major factors: density of juvenile corals, initial structural complexity, water depth, biomass of herbivorous fishes, and nutrient conditions on the reef.[62] Overall, resilience was seen most in coral reef systems that were structurally complex and in deeper water.[62]

The ecological roles and functional groups of species also play a role in the recovery of regime shifting potential in reef systems. Coral reefs are affected by bioeroding, scraping, and grazing fish species. Bioeroding species remove dead corals, scraping species remove algae and sediment to further future growth, grazing species remove algae.[66] The presence of each type of species can influence the ability for normal levels of coral recruitment which is an important part of coral recovery.[66] Lowered numbers of grazing species after coral bleaching in the Caribbean has been liked to sea urchin dominated systems which do not undergo regime shifts to fleshy macroalgae dominated conditions.[63]

There is always the possibility of unobservable changes, or cryptic losses or resilience, in a coral community's ability to perform ecological processes.[61][66] These cryptic losses can result in unforeseen regime changes or ecological flips.[61] More detailed methods for determining the health of coral reefs that take into account long term changes to the coral ecosystems and better informed conservation policies are necessary to protect coral reefs in the years to come.[61][62][64][66]

Coral damage by sunscreens[edit]

Marine and coastal tourism is the fastest growing sector of the global tourism industry. Between 1992 and 2004, the amount of tourists grew from 463 to 763 million. In 2020 the expected number of tourists is estimated to increase to 1.56 billion.[67] As a result, more and more sunscreen is being released into marine environments and contributing to coral bleaching. The amount of sunscreen estimated to be released into coral reefs each year is between 6,000 and 14,000 tons. 10% of the world's coral reefs are said to be threatened by coral bleaching induced by sunscreen alone.[24]

The main components in sunscreens are known as UV filters. UV filters are chemicals that have been developed to absorb the sun's UV radiation to decrease the negative effect the sun has on the skin.[68] However, it has been seen that some UV filters break down in aquatic environments and produce by-products that contribute to coral bleaching. Many UV filters break down by photo-degradation and produce reactive oxygen species (ROS) as their by-products.[69] ROS are known to cause stress to coral reef systems that results in coral bleaching.[70]

One popular UV filter that is looked for in sunscreens is Titanium dioxide (TiO2). TiO2 has been seen to easily photo-degrade into ROS in aquatic environments.[71]

This occurs when TiO2 particles absorb UV light (hv) from the sun. Conduction band electrons (e-) and valence band holes (h+) form in the compounds' structure. The electrons then reduce oxygen in seawater to a superoxide (O2-) ROS and the h+ will react with water to produce a hydroxyl radical (OH•).

TiO2 + hv → h+ + e-

h+ + H2O → OH•

e- + O2 → O2-

The superoxides (O2-) can further protonate to form hydrogen peroxide (H2O2).

O2- + H+ → HO2•

HO2• + H+ → H2O2

The hydroxyl radicals (OH•) may combine in a radical polymerization reaction to also form H2O2.[72][73]

OH• + OH• → H2O2

Superoxides, hydroxyl radicals, and hydrogen peroxide are all types of ROS that contribute to coral bleaching. They do this by disrupting the coral zooxanthallae's photosynthesis, which then causes stress to the coral system.[70] Titanium dioxide isn't the only UV filter that contributes to coral bleaching. However, the chemical break down of other harmful UV filters needs to be studied further.[74] Some steps have already been made to try to reverse coral bleaching. In many marine-managed areas in Mexico, certain sunscreens are banned that are known to contain harmful UV filters.[75] With more information, we will be able to change the composition of sunscreen so that it's not harmful to the environment.

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Watson, Megan E. "Coral Reefs." Encyclopedia of Environmental Issues. Rev. ed. Vol. 1. Pasadena: Salem Press, 2011. pp. 317–318. ISBN 978-1-58765-736-8

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