The Effects of Anthropogenic Climate Change on Tropical Corals and their Symbionts

While there are a great number of ecological and social benefits of the tropical reef environment, coral reefs are starting to disappear due to a myriad of anthropogenic causes. These pristine, shallow tropical waters of the Earth’s oceans include millions of individual coral colonies, each consisting of hundreds, sometimes thousands, of minute animals called corals. As these corals grow and develop, they secrete calcium carbonate skeletons that become the building blocks of the reef environment. These areas of the ocean act as carbon sinks; in absorbing carbon dioxide from the environment, they serve as areas of great abundance that sustain millions with seafood, and they have the capacity for many potential pharmaceutical compounds. Increasing ocean temperatures, the primary anthropogenic threat facing coral reefs today, cause coral bleaching and death as conditions overwhelm corals’ ability to adapt to changing conditions.

Contrary to the popular belief that corals are plants, corals are actually transparent animals that harbor colorful symbiotic zooxanthellae in the vacuoles of their tissue.1 In this relationship, the coral provides an optimal location for the zooxanthellae with ample lighting, water flow, nutrients, and protection from zooxanthellae’s natural predators.1 In return, the zooxanthellae export a large percentage of the energy they produce through photosynthesis to the coral, thereby sustaining the life of the coral.1 Because the zooxanthellae and the coral live so intimately with one another, a series of complex communications between the host and the symbiont must be maintained to preserve the symbiosis.1 This symbiosis has thrived for millennia, but is now threatened by anthropogenic carbon dioxide and the associated rising temperatures and alterations in weather patterns.

When water parameters begin to differ from the norm, corals and their symbionts begin to respond to this stress. For example, rapid or long-term changes in the pH or temperature of the water, both attributed to climate change, can lead to the eventual death of the coral and the zooxanthellae. While both symbionts are somewhat resistant to change and are able to adapt to an altering environment, certain conditions such as increasing temperature can begin to threaten the symbiosis.1 The breakdown of this symbiosis causes the coral and the colorful zooxanthellae to separate, revealing the coral’s calcified skeleton.1 Because calcium carbonate is white and the coral’s tissue is colored in the presence of the algae, this process is called coral bleaching—the transition of the coral from a colored state to an uncolored state. As a whole, coral bleaching is shown to correlate to increased rates of coral disease and more often than not, death of the coral.1 Many factors can create stress in the coral; however, increasing ocean temperatures has generally been considered the key factor in coral bleaching.

One study, conducted by Virginia M. Weis, examines the role of reactive oxygen species in the symbiosis.1 According to Weis, coral reef conditions favor extremely high rates of photosynthesis.1 Oxygen, a byproduct of photosynthesis can combine with electrons to form reactive oxygen compounds that can lead to severe tissue damage in both the host and its symbionts.1 Enzymes in both of the symbionts are generally able to break down these reactive oxygen species to less toxic oxygen and water.1 Essentially, this means that under normal conditions, both the coral and the zooxanthellae are able to cope with the reactive oxygen compounds that form in the coral tissue. As a result, corals are able to adapt to minor stresses in the dynamic tropical ocean environment, which has allowed them to survive throughout history.

However, Weis has shown that increasing temperatures make corals more susceptible to further increases in temperature, thereby threatening their survival: rising temperatures, in combination with increased light, break down certain components of the zooxanthellae’s chloroplast.1 Essentially, the chloroplast continues to produce electrons but fails to use these electrons in the production of food energy molecules. As a result, these electrons combine with the excess produced oxygen to form reactive oxygen species.1 These reactive oxygen species further deteriorate the chloroplast in a positive feedback loop that continues to produce even more reactive oxygen species.1 As levels of these reactive oxygen compounds increase they continue to degrade both the host coral and the symbiont zooxanthellae.1 Because of this positive feedback, the initial deterioration will lead to further deterioration, which then leads to the death of either the host or the symbiont. Constantly polluting our oceans and our atmosphere thus creates an environment that is uninhabitable for corals, which may lead to the eventual extinction of a keystone species of the coral reef environment. In other words, this positive feedback loop resulting from anthropogenic climactic changes will eventually destroy tropical coral reef ecosystems and negatively impact the people and the animals dependent on them.

Jeremie Vidal-Dupiol et al. researched other factors involved in the stress imposed via rising seawater temperatures. Studies of the coral Pocillopora damicornispaired with zooxanthellae Symbiodinium spp. showed that corals in the experimental group began to bleach after exposure to temperatures of 32˚C for 15 days with only 21.1% of the zooxanthellae remaining 3 days later.2 This rate of bleaching is incredibly rapid. Within just three days, corals were almost completely bleached, making them more susceptible to diseases and death as their symbiont dies off or is expelled. Increased temperatures to this degree could potentially be triggered in shallower waters during the more frequent El Niño events resulting from climate change. Research suggested that two proteins, Pdcyst-rich and PdC-Lectin, play a major role in coral-zooxanthellae interactions.2 The first, Pdcyst-rich, was hypothesized to function in the calcification of the coral’s skeleton.2 While the corals became increasingly stressed, less of this protein was produced, which could be related to lower rates of calcification.2 Therefore, it is likely that increasing temperatures cause coral growth rates to decrease. The second protein, PdC-Lectin, was found to play a crucial role in the interactions between the coral and the zooxanthellae.2 As temperatures increased, levels of this protein recorded in coral tissue decreased before and during the bleaching process; as a result, Vidal-Dupiol et al. determined that bleaching may be caused in part due to the coral’s inability to host current symbionts and take in new zooxanthellae.2 Aside from this, both of these proteins are relatively toxic to the coral causing harm to its general, overall health. If the coral loses its ability to maintain symbiosis, then the host and symbiont separate in a bleaching event. While different aspects were studied, Vidal-Dupiol et al. supplemented Weis’s research in that reactive oxygen species were also considered to be the primary cause of coral bleaching.2

Other scientific research has been conducted to determine why corals bleach. According to David O. Obura, limited periodic bleaching in corals can be perceived as a method by which the coral regulates homeostasis to allow the coral and the zooxanthellae to adapt to stress.3 Therefore, by adjusting to external factors, bleaching in this way could actually preserve both symbionts so that the symbiosis could be restored after a return to normalcy.3 Research has shown zooxanthellae to be incredibly diverse organisms; therefore, evolutionary processes of these organisms may lead to future coral-symbiont resiliency.3 Because corals and zooxanthellae are not able to resist stress concurrently while maintaining symbiosis, research suggests that the symbiosis breaks down to reduce stress, which often allows both symbionts to survive in the long run.3 In other words, bleaching is a method by which corals and zooxanthellae temporarily adapt to changing environmental conditions. While this does show that corals are resilient to external forces and changing conditions, the constantly increasing temperatures and shifting conditions caused by global climate change has already been shown to create situations in which corals are unable to adapt. The mass bleaching events that result from these scenarios create a graveyard of white coral skeletons that serve as a eerie reminder of the previously teeming coral reef environment and the effects we are having on it.

In researching the various methods by which the corals bleach, Weis supplemented Obura’s bleaching hypothesis by outlining how the coral and zooxanthellae separate under stressful conditions. One hypothesis suggests that reactive oxygen species simply break down the zooxanthellae within the coral tissue leaving the coral to either consume or expel the zooxanthellae.1 The coral may also simply expel the defunct zooxanthellae from its tissue via transport of the symbiont outside of the cell membrane through vesicles.1 Alternatively, the coral may simply detach an entire damaged coral cell, including zooxanthellae, from the colony.1 Controlled cell death of the coral is another method by which corals can respond to minor stress and remove the potentially reactive oxygen compounds before the rest of the cells can be affected.1 Finally, uncontrolled cell death occurs in corals under severe stress.1 Thus, corals respond to stress and the breakdown of symbiosis in a variety of methods with equally varying effects depending on the given method.

Weis determined that corals adapt to thermal stress via a variety of methods, despite being harmed in the process. Zooxanthellae released early in the bleaching process may have some ability to survive while zooxanthellae released at the peak of bleaching are generally already severely damaged.1 Despite this apparent ability to adapt, the resulting bleaching negatively impacts the health of both the zooxanthellae and the host coral. In many cases, the thermal stress that creates these situations leads to the death of either the coral or the zooxanthellae. Increasing temperatures and other human impacts have already been linked to mass bleaching events and the deaths of many corals. As our population continues to grow and develop, these human impacts are projected to increase, making the future of corals and coral reefs uncertain.

Further studies have shown that past bleaching events may make corals less susceptible to further bleaching events. J. A. Maynard et al. conducted an experiment in which corals were studied along the Great Barrier Reef.4 Comparison of the bleaching rates of the 1998 El Niño event and the more severe 2002 event showed that bleaching rates were significantly fewer in number during the 2002 event even though overall thermal stress was greater.4 As a result of this discovery, Maynard et al. concluded that a combination of long-term evolution in combination with short-term acclimatization of corals was responsible for the increase in thermal resistance on the Great Barrier Reef.4 Additionally, Maynard et al. determined that this adaptation could play a crucial role in allowing corals to survive future temperature increases.4 Another study, conducted by Thompson and Woesik, determined that “thermal acclimatization ‘memory’ may be as long as seven years” .5 They also concluded that coral survival is most likely explained by the natural selection of symbiotic pairs that were more resistant to higher temperatures.5 Because lower rates were determined by both research groups, it is highly likely that corals exposed to previous thermal stress may be more able to adapt to future scenarios of increasing temperatures. Despite this ability to adapt, the increasing frequency and intensity of bleaching conditions make it unlikely that corals will be able to continue to adapt to these constantly changing conditions in the future.

In conclusion, corals generally respond to stress resulting from increasing ocean temperatures by bleaching. These studies have suggested that, while corals are able to adapt to minor fluctuations and gradual changes, current predictions for global climate change indicate that temperatures will increase too much causing more drastic changes on coral reefs.4 However, many of these predictions are based solely on the effects of increasing seawater temperatures and neglect a variety of other anthropogenic factors currently threatening reef environments. Increasing levels of carbon dioxide resulting in ocean acidification, shifting global weather patterns due to climate change, continued runoff and pollution, as well as other natural threats compound this danger of thermal stress. While this research has shown that corals are able to gradually adapt to thermal stress, rapid temperature increases combined with these factors limit the corals’ abilities to respond and adapt to a rapidly changing environment. As a result, the countless benefits that coral reefs provide for both the global earth system and the human population may be lost as we continue to drive coral reefs towards extinction.

While there are a great number of ecological and social benefits of the tropical reef environment, coral reefs are starting to disappear due to a myriad of anthropogenic causes. These pristine, shallow tropical waters of the Earth’s oceans include millions of individual coral colonies, each consisting of hundreds, sometimes thousands, of minute animals called corals. As these corals grow and develop, they secrete calcium carbonate skeletons that become the building blocks of the reef environment. These areas of the ocean act as carbon sinks; in absorbing carbon dioxide from the environment, they serve as areas of great abundance that sustain millions with seafood, and they have the capacity for many potential pharmaceutical compounds. Increasing ocean temperatures, the primary anthropogenic threat facing coral reefs today, cause coral bleaching and death as conditions overwhelm corals’ ability to adapt to changing conditions.

Contrary to the popular belief that corals are plants, corals are actually transparent animals that harbor colorful symbiotic zooxanthellae in the vacuoles of their tissue.1 In this relationship, the coral provides an optimal location for the zooxanthellae with ample lighting, water flow, nutrients, and protection from zooxanthellae’s natural predators.1 In return, the zooxanthellae export a large percentage of the energy they produce through photosynthesis to the coral, thereby sustaining the life of the coral.1 Because the zooxanthellae and the coral live so intimately with one another, a series of complex communications between the host and the symbiont must be maintained to preserve the symbiosis.1 This symbiosis has thrived for millennia, but is now threatened by anthropogenic carbon dioxide and the associated rising temperatures and alterations in weather patterns.

When water parameters begin to differ from the norm, corals and their symbionts begin to respond to this stress. For example, rapid or long-term changes in the pH or temperature of the water, both attributed to climate change, can lead to the eventual death of the coral and the zooxanthellae. While both symbionts are somewhat resistant to change and are able to adapt to an altering environment, certain conditions such as increasing temperature can begin to threaten the symbiosis.1 The breakdown of this symbiosis causes the coral and the colorful zooxanthellae to separate, revealing the coral’s calcified skeleton.1 Because calcium carbonate is white and the coral’s tissue is colored in the presence of the algae, this process is called coral bleaching—the transition of the coral from a colored state to an uncolored state. As a whole, coral bleaching is shown to correlate to increased rates of coral disease and more often than not, death of the coral.1 Many factors can create stress in the coral; however, increasing ocean temperatures has generally been considered the key factor in coral bleaching.

One study, conducted by Virginia M. Weis, examines the role of reactive oxygen species in the symbiosis.1 According to Weis, coral reef conditions favor extremely high rates of photosynthesis.1 Oxygen, a byproduct of photosynthesis can combine with electrons to form reactive oxygen compounds that can lead to severe tissue damage in both the host and its symbionts.1 Enzymes in both of the symbionts are generally able to break down these reactive oxygen species to less toxic oxygen and water.1 Essentially, this means that under normal conditions, both the coral and the zooxanthellae are able to cope with the reactive oxygen compounds that form in the coral tissue. As a result, corals are able to adapt to minor stresses in the dynamic tropical ocean environment, which has allowed them to survive throughout history.

However, Weis has shown that increasing temperatures make corals more susceptible to further increases in temperature, thereby threatening their survival: rising temperatures, in combination with increased light, break down certain components of the zooxanthellae’s chloroplast.1 Essentially, the chloroplast continues to produce electrons but fails to use these electrons in the production of food energy molecules. As a result, these electrons combine with the excess produced oxygen to form reactive oxygen species.1 These reactive oxygen species further deteriorate the chloroplast in a positive feedback loop that continues to produce even more reactive oxygen species.1 As levels of these reactive oxygen compounds increase they continue to degrade both the host coral and the symbiont zooxanthellae.1 Because of this positive feedback, the initial deterioration will lead to further deterioration, which then leads to the death of either the host or the symbiont. Constantly polluting our oceans and our atmosphere thus creates an environment that is uninhabitable for corals, which may lead to the eventual extinction of a keystone species of the coral reef environment. In other words, this positive feedback loop resulting from anthropogenic climactic changes will eventually destroy tropical coral reef ecosystems and negatively impact the people and the animals dependent on them.

Jeremie Vidal-Dupiol et al. researched other factors involved in the stress imposed via rising seawater temperatures. Studies of the coral Pocillopora damicornispaired with zooxanthellae Symbiodinium spp. showed that corals in the experimental group began to bleach after exposure to temperatures of 32˚C for 15 days with only 21.1% of the zooxanthellae remaining 3 days later.2 This rate of bleaching is incredibly rapid. Within just three days, corals were almost completely bleached, making them more susceptible to diseases and death as their symbiont dies off or is expelled. Increased temperatures to this degree could potentially be triggered in shallower waters during the more frequent El Niño events resulting from climate change. Research suggested that two proteins, Pdcyst-rich and PdC-Lectin, play a major role in coral-zooxanthellae interactions.2 The first, Pdcyst-rich, was hypothesized to function in the calcification of the coral’s skeleton.2 While the corals became increasingly stressed, less of this protein was produced, which could be related to lower rates of calcification.2 Therefore, it is likely that increasing temperatures cause coral growth rates to decrease. The second protein, PdC-Lectin, was found to play a crucial role in the interactions between the coral and the zooxanthellae.2 As temperatures increased, levels of this protein recorded in coral tissue decreased before and during the bleaching process; as a result, Vidal-Dupiol et al. determined that bleaching may be caused in part due to the coral’s inability to host current symbionts and take in new zooxanthellae.2 Aside from this, both of these proteins are relatively toxic to the coral causing harm to its general, overall health. If the coral loses its ability to maintain symbiosis, then the host and symbiont separate in a bleaching event. While different aspects were studied, Vidal-Dupiol et al. supplemented Weis’s research in that reactive oxygen species were also considered to be the primary cause of coral bleaching.2

Other scientific research has been conducted to determine why corals bleach. According to David O. Obura, limited periodic bleaching in corals can be perceived as a method by which the coral regulates homeostasis to allow the coral and the zooxanthellae to adapt to stress.3 Therefore, by adjusting to external factors, bleaching in this way could actually preserve both symbionts so that the symbiosis could be restored after a return to normalcy.3 Research has shown zooxanthellae to be incredibly diverse organisms; therefore, evolutionary processes of these organisms may lead to future coral-symbiont resiliency.3 Because corals and zooxanthellae are not able to resist stress concurrently while maintaining symbiosis, research suggests that the symbiosis breaks down to reduce stress, which often allows both symbionts to survive in the long run.3 In other words, bleaching is a method by which corals and zooxanthellae temporarily adapt to changing environmental conditions. While this does show that corals are resilient to external forces and changing conditions, the constantly increasing temperatures and shifting conditions caused by global climate change has already been shown to create situations in which corals are unable to adapt. The mass bleaching events that result from these scenarios create a graveyard of white coral skeletons that serve as a eerie reminder of the previously teeming coral reef environment and the effects we are having on it.

In researching the various methods by which the corals bleach, Weis supplemented Obura’s bleaching hypothesis by outlining how the coral and zooxanthellae separate under stressful conditions. One hypothesis suggests that reactive oxygen species simply break down the zooxanthellae within the coral tissue leaving the coral to either consume or expel the zooxanthellae.1 The coral may also simply expel the defunct zooxanthellae from its tissue via transport of the symbiont outside of the cell membrane through vesicles.1 Alternatively, the coral may simply detach an entire damaged coral cell, including zooxanthellae, from the colony.1 Controlled cell death of the coral is another method by which corals can respond to minor stress and remove the potentially reactive oxygen compounds before the rest of the cells can be affected.1 Finally, uncontrolled cell death occurs in corals under severe stress.1 Thus, corals respond to stress and the breakdown of symbiosis in a variety of methods with equally varying effects depending on the given method.

Weis determined that corals adapt to thermal stress via a variety of methods, despite being harmed in the process. Zooxanthellae released early in the bleaching process may have some ability to survive while zooxanthellae released at the peak of bleaching are generally already severely damaged.1 Despite this apparent ability to adapt, the resulting bleaching negatively impacts the health of both the zooxanthellae and the host coral. In many cases, the thermal stress that creates these situations leads to the death of either the coral or the zooxanthellae. Increasing temperatures and other human impacts have already been linked to mass bleaching events and the deaths of many corals. As our population continues to grow and develop, these human impacts are projected to increase, making the future of corals and coral reefs uncertain.

Further studies have shown that past bleaching events may make corals less susceptible to further bleaching events. J. A. Maynard et al. conducted an experiment in which corals were studied along the Great Barrier Reef.4 Comparison of the bleaching rates of the 1998 El Niño event and the more severe 2002 event showed that bleaching rates were significantly fewer in number during the 2002 event even though overall thermal stress was greater.4 As a result of this discovery, Maynard et al. concluded that a combination of long-term evolution in combination with short-term acclimatization of corals was responsible for the increase in thermal resistance on the Great Barrier Reef.4 Additionally, Maynard et al. determined that this adaptation could play a crucial role in allowing corals to survive future temperature increases.4 Another study, conducted by Thompson and Woesik, determined that “thermal acclimatization ‘memory’ may be as long as seven years” .5 They also concluded that coral survival is most likely explained by the natural selection of symbiotic pairs that were more resistant to higher temperatures.5 Because lower rates were determined by both research groups, it is highly likely that corals exposed to previous thermal stress may be more able to adapt to future scenarios of increasing temperatures. Despite this ability to adapt, the increasing frequency and intensity of bleaching conditions make it unlikely that corals will be able to continue to adapt to these constantly changing conditions in the future.

In conclusion, corals generally respond to stress resulting from increasing ocean temperatures by bleaching. These studies have suggested that, while corals are able to adapt to minor fluctuations and gradual changes, current predictions for global climate change indicate that temperatures will increase too much causing more drastic changes on coral reefs.4 However, many of these predictions are based solely on the effects of increasing seawater temperatures and neglect a variety of other anthropogenic factors currently threatening reef environments. Increasing levels of carbon dioxide resulting in ocean acidification, shifting global weather patterns due to climate change, continued runoff and pollution, as well as other natural threats compound this danger of thermal stress. While this research has shown that corals are able to gradually adapt to thermal stress, rapid temperature increases combined with these factors limit the corals’ abilities to respond and adapt to a rapidly changing environment. As a result, the countless benefits that coral reefs provide for both the global earth system and the human population may be lost as we continue to drive coral reefs towards extinction.