Coral-Zooxanthellae Symbiosis – Environment: Light & Water Depth

The productivity of the zooxanthellae is also largely dependent on environmental factors including, significantly, the amount of light penetrating the corals and reaching the dinoflagellates. Productivity is proportional to increasing light intensity, although it peaks when productivity becomes independent of light intensity. This explains why corals only inhabit relatively shallow coastal regions – light intensity in deep water is not great enough to permit photosynthesis within the zooxanthellae and in turn the corals themselves cannot be supported. However the depth that corals can survive is also dependent on the clarity of the water. Corals are found living at a depth of 100m in reefs in Jamaica for example, but only 12m in certain reefs in Puerto Rico. Different species of zooxanthellae are however adapted differently to cope with light intensities in varying habitats, for example zooxanthellae that specialise in slightly deeper water have more photosynthetic pigments in order to absorb more light for photosynthesis.

Coral-Zooxanthellae Symbiosis: Conditions for Optimal Productivity

Temperature is also a key environmental factor influencing the productivity of zooxanthellae, as fluctuating temperatures affect their metabolic rate. This is mainly due to the effects temperature has on both the rate of photosynthesis and respiration. On studies of A. palmata corals in Jamaica, it was found that growth rates were proportional to the increase in sea surface temperature (SST). Therefore temperature also affects the corals and temperatures outside the tolerant range of a species of zooxanthellae may cause loss of the algae from the coral, leading to possible coral death. As with light, different species may be physiologically adapted to cope with varying temperature ranges, depending on their habitats.

In order to achieve a stable symbiosis between coral and zooxanthellae, the density of zooxanthellae within the coral must remain fairly constant. Usually this process is self-regulating as zooxanthellae are expelled if their reproduction rate exceeds that of the coral, and vice versa. Environmental cues are central in determining a stable symbiosis.

Threats to Coral Reefs - Human vs Natural Causes?

There are many threats to coral health, both natural and anthropogenic. But it is thought that human activity poses a much greater threat than natural causes. Factors such as fishing, sedimentation and pollution all have an impact on coral health. However here I will look in detail at coral bleaching, perhaps the biggest threat to coral health, a process generally regarded as being influenced by human behaviour. I will also discuss in less detail the other main human threats to corals.

Coral Reef Bleaching - What is Coral Reef Bleaching?

Coral bleaching is the term given to the event where the zooxanthellae residing within the corals die or leave the cells, either by expulsion or by their own accord. The coral appears bleached because of the loss of pigmented zooxanthellae. The corals themselves have no colour as the underlying calcium carbonate framework beneath and surrounding the polyps is a white colour, and the polyps on the surface layer are translucent. Bleaching can also occur when the zooxanthellae simply lose their pigment but remain in the host cells. Figure 4 shows a coral in the process of bleaching; the central polyps have lost their zooxanthellae but some outer polyps are still healthy, with a yellow-green colouration. The phenomenon occurs at times when the corals are under unsustainable environmental stress. Environmental factors include extreme temperatures, UV radiation, aerial exposure, nutrient imbalance, sedimentation and chemical contamination.

Coral Bleaching - A Major Threat to Coral Reef Health

Coral bleaching is often regarded as one of he greatest threats to the health of coral reefs. Due to the relatively fragile nature of the coral-zooxanthellae symbiosis, a small change in environmental conditions can cause the symbiosis to collapse and the zooxanthellae to be expelled, leading to bleaching. Although corals can usually recover over a period of years, by re-forming a symbiosis with zooxanthellae, in extreme cases the coral polyps will die which can have more serious knock-on effects for the whole ecosystem. Coral bleaching events have been occurring since records began, but what is worrying is that bleaching events seem to be occurring at a more frequent rate than in the past. The highest SSTs ever recorded were in 1998, and with this the most widespread coral bleaching events recorded. The rising SSTs over the last fifty years correlate with the amount of bleaching events being seen around the world, and in turn subsequent death to the coral. Figure 5 shows recent bleaching events, the majority took place in the West Atlantic and East Pacific.

Coral Bleaching - El Nino Southern Oscillation

Coral bleaching seems to be influenced most strongly by the El Nino Southern Oscillation (ENSO). This is a cyclic phenomenon of sustained SST of more than 0.5˚C, occurring across the central Pacific Ocean, caused by complex interactions between atmospheric pressure systems (Huppert & Stone, 1998). It occurs irregularly but recent patterns have been more regular and stronger than in the past. Mass coral bleaching events have been closely correlated with the occurrence of ENSO, as it can push SSTs above the threshold for the symbiosis between coral and zooxanthellae. The symbiosis can usually resist short term stress but as the oscillations typically last for months, the sustained stress leads to expulsion of zooxanthellae (Douglas, 2003).

An unusually strong ENSO back in 1982-1983 led to severe mass coral bleaching. Air temperature rose by 2-3˚C leading to a significant rise in SST (Glynn & Colgan, 1992). It affected mainly the Eastern Pacific, from Northern Costa Rica to Southern Ecuador. Mortality was seen to correlate with the increase in sea surface temperature. Many areas have been analysed since, with 70-95% mortality being seen due to bleaching in reef communities around Panama, Columbia and Costa Rica (Glynn & Colgan, 1992). The worst coral mortality was recorded at 97% in the reefs of the Galapagos Islands, Ecuador. Rapid recovery via reproduction and immigration of new species took place but the reefs have not fully recovered.

Coral Bleaching - Most Severe Bleaching Event Recorded

The most severe mass bleaching event recorded took place in 1998. This coincided with a strong ENSO at the same time, which caused SSTs around the Pacific Ocean to rise by up to 0.9˚C (Goreau et al., 2000). For example, studies conducted on the effects in the West Pacific found that some areas of the inner and southern Great Barrier Reef were hit by 80-90% mortality rates as a result of bleaching (Goreau et al., 2000), far higher than any levels previously recorded. Again research shows that recovery is taking place however the timescale for a full recovery, or whether or not a full recovery is possible, is not known. However due to the extent of the bleaching, recruitment of new corals was limited in the short term as most source regions of corals were also badly damaged in the incident.

In order to gain a better biological understanding of bleaching, the process can be split into three subdivisions: triggers, mechanisms and symptoms. These will be considered below in more detail.

Triggers of Coral Reef Bleaching

There are several environmental conditions that can initiate the process of coral bleaching. These include:

• Temperature. This is generally seen as the principle trigger of bleaching, and is the main consequence of ENSO (Douglas, 2003). Temperatures that are sustained above, or below, the threshold of the coral or zooxanthellae, can lead to bleaching.

• High exposure to solar irradiance. This is another major trigger which is also linked to ENSO. Both the photosynthetically active wavelengths of the spectrum and the ultraviolet band are both linked to bleaching.

• A rapid fall in the nutrient levels of sea water. This can occur after storms and flooding, the ENSO can again be involved in causing such events.

• Subaerial exposure. This occurs when sea levels fall below the corals exposing them to air. Exposure to the atmosphere leads to other triggers in the form of increased irradiance, and increased or decreased temperature.

• High levels of inorganic nutrients. Pollution is the primary cause of this, such as run-off from inland and oil spills. Substances such as copper, oil and cadmium are all potential triggers.

• Salinity. This can affect coral growth but can also trigger bleaching, via both hypo and hyper salinity.

• Falling ocean pH. This makes the process of calcification more difficult within the corals, due to the increasing acidity. This in turn reduces the availability of carbonate ions which are incorporated into their framework. This may not directly lead to bleaching but a weakened framework may leave corals vulnerable to other environmental conditions.

These triggers are not independent of one another, bleaching may occur due to a combination of two or more of these triggers (Porter et al., 1999; Smith & Buddemeier, 1992).

Mechanisms of coral reef bleaching

To date, the mechanisms involved in coral bleaching are not fully understood, with many possible theories to explain the phenomenon.

One involves damage to Photosystem II (PSII) in the chloroplasts in algae from the Symbiodinium species (Warner et al., 1999). Without an efficiently functioning PSII, photosynthesis cannot occur and as a result will not be able to supply their symbiotic corals with nutrients. This may lead to expulsion by the corals. Warner et al. (1999) studied a number of species of zooxanthellae that had been naturally affected by significant bleaching in Florida in 1999, after SST rose to above 30˚C for a prolonged period, as well as experimentally bleaching corals. They found that an increase in SST leads to heat stress within the zooxanthellae, damaging the D1 protein that is key to the PSII reaction. At 32˚C the rate of degradation of the D1 protein was higher than its resynthesis, leading to the loss of PSII function. As a result, a reduction in photosynthesis was recorded before algae densities fell within the corals. The events leading up to the loss of D1 are not known however.

Warner et al. (1999) also found that corals more resistant to bleaching have a greater ability for the maintenance of PSII, with the resynthesis of the D1 protein occurring at a rate that matches its destruction. Another study demonstrated that corals with high protein turnover rates are better adapted for acclimatisation via the synthesis of heat shock proteins and regulatory enzymes (Gates & Edmunds, 1999).

Mechanisms of coral reef bleaching (continued)

Photosystem II can also be damaged by sustained high temperature via a reduction in the efficiency of the Calvin cycle to fix carbon. This is thought to be due to the degradation of enzymes involved in the Calvin cycle, such as possibly the unstable nature of the Rubisco enzyme found in zooxanthellae, unlike that found in most plants (Jones et al., 1998). A reduction in the operating efficiency of the electron transport chain between PSII and PSI also appears to take place (Jones et al., 1998; Fitt & Warner, 1995). Jones et al. (1998) also found that zooxanthellae located in areas of coral with high levels of irradiance also had an impaired PSII with lower photochemical efficiency, or quantum yield. This in turn leads to a decrease in oxygen production. A combination of high temperature and high irradiance led to the greatest decrease in PSII quantum yield, but temperature was found to be the primary determinant. The results of Jones et al. (1998) differ slightly to those of Warner et al. (1999) in that Jones et al. (1998) suggest that damage to PSII is the secondary effect of environmental stress, as a result of damage to the Calvin cycle and a reduction in electron flow, not the primary effect. Further research into the molecular mechanisms involved in bleaching is certainly needed to fully understand the process.

Mechanisms of coral reef bleaching (continued)

The fundamental results of Jones et al. (1998) and Warner et al. (1999) are backed up by Takahashi et al. (2008). Their investigations found that photobleaching occurred in the CS-73 strain of Symbiodinium at temperatures over 31˚C, with inhibition of PSII taking place. They also found that loss of photosynthetic pigment can occur as a result of heat stress combined with irradiance, which is due to the loss of antennae proteins that contain large concentrations of photosynthetic pigments to harvest large quantities of light. The molecular mechanisms leading up to this loss of antennae are not fully known, but it is likely due to the degradation and inhibition of protein synthesis of structural proteins within the antennae, such as acpPC. Another strain of zooxanthellae studied, the OTcH-1 strain, showed no loss of pigment after exposure to the same temperatures, indicating differences in the resistance to environmental stress between species of zooxanthellae (Takahashi et al., 2008).

Symptoms of Coral Reef Bleaching

The symptoms of coral bleaching are either expulsion of zooxanthellae from the coral host, or loss of pigmentation from the zooxanthellae cells (Venn et al., 2006). Most past investigations have stated that the coral polyps initiate the expulsion of zooxanthellae, but recent studies provide evidence suggesting that the expulsion of zooxanthellae is initiated by the zooxanthellae themselves, rather than the corals (Trapido-Rosenthal, 2005). The studies found the zooxanthellae produced large quantities of nitric oxide synthase before bleaching occurs. This enzyme leads to nitric oxide production in host cells, a reactive molecule that can lead to expulsion of zooxanthellae.

Symptoms of Coral Reef Bleaching (cont)

As previously mentioned, different species of zooxanthellae possess differing resistances to environmental stress, and in turn the symptoms of different species of zooxanthellae to stress also differ. It is thought that the resistance of zooxanthellae is largely genetically programmed and is linked to the species physiology and morphology (McClanahan et al., 2004). The resistance of corals to bleaching and death has also been shown to be affected by the environmental conditions that the corals are exposed to. The threshold conditions that corals can tolerate are thought to be greater if the environmental conditions in that habitat are more variable (McClanahan & Maina, 2003). A coral living in a reef with very small variation in SST is more likely to be affected by a rare warm water event than a coral living in an area with slightly higher variation in SST. McClanahan and Maina (2003) hypothesise that due to climate change, and the likely future trend of an increasing annual temperature range, corals will become more resilient to more extreme rare environmental disturbances such as ENSO events. However they also predict that diversity will decrease as a result, with only the most resistant species of coral surviving, and consequently recolonising areas of dead reef previously occupied by less resilient species, a process generally known as the adaptive bleaching hypothesis (Kinzie III et al., 2001). It has also been shown via molecular studies that there is a certain degree of flexibility within the partnerships that exist between zooxanthellae and their hosts, and that following bleaching recolonisation can occur by more resistant corals (Baker, 2003).

Symptoms of Coral Reef Bleaching (Cont)

Another study provides evidence that corals can acclimatise to increasing temperature by altering the concentrations of different species of zooxanthellae within their tissues (Berkelmans & Van Oppen, 2006), so over time a coral can be populated by less resistant species and then more resistant species following a rise in SST. This ability is only possible in a minority of coral species that are capable of giving residence to more than one zooxanthellae species simultaneously. An increase in tolerance of 1-1.5°C was discovered in the coral species studied. Further research into these issues could help identify the threat posed by bleaching more accurately as if these results are backed up, long term recovery of bleached reefs may be more viable than currently thought.

Human threats to coral reefs and coral damage

In 2001, it was estimated that 58% of the world’s coral reefs were threatened by human behaviour (Spalding et al., 2001). While there is debate over whether coral bleaching is a naturally occurring event or largely caused by humans, there are many causes of coral destruction that are undoubtedly anthropogenic.

Firstly, overfishing is regarded as one of the main anthropogenic threats to reefs. This has a direct impact on the ecosystem as the balance of producers, consumers and predators is shifted, but the indirect consequences of this are also significant. As many reef fish are herbivores, feeding on algae, removal of these consumers leads to high rates of algal growth on the surface of corals. Unlike the algae living within the coral polyps, these algae are larger, fleshy algae and are incapable of forming a symbiosis with the coral. Instead, they can reproduce at rapid rates and completely cover large surface areas of corals at a very high rate. This can either suffocate the corals, because of the increased levels of algal respiration leading to decreased levels of oxygen, or can block sunlight from reaching the intracellular zooxanthellae (Roberts, 1995). The latter means the zooxanthellae cannot photosynthesise, leading to their death, which can also lead to coral death as the polyps will no longer be receiving nutrients from the zooxanthellae. In many countries, fishing within coral reefs has become such a large source of income that it is being carried out at unsustainable levels. In order to try and allow fishing to proceed at sustainable levels, regulations need to be put in place to prevent overfishing.

Human threats to coral reefs and coral damage (continued)

Dynamite or blast fishing is a method of fishing used extensively in South East Asia that is highly damaging to coral reefs (Spalding et al., 2001). It involves using dynamite or other explosives to create an explosion underwater, to stun or kill fish and bring them to the surface where they can be collected. Often a secondary explosion is carried out in order to kill predators that have been attracted to the sounds and the smells of the first explosion. This method of fishing is devastating to the ecosystem because it instantly destroys the corals and the limestone matrix, the habitat for countless organisms, as well as directly killing any plants or animals in close proximity. Evidence shows that most reefs are somehow affected by fishing, as a study of 315 reefs in 31 countries showed lower than expected levels of ‘indicator’ organisms, even in reefs that had not been affected by factors such as pollution and sedimentation (Hodgson, 1999).

Human threats to coral reefs and coral damage (continued)

Nutrient enrichment is another threat to coral reefs. Because corals require relatively low nutrient levels in the water to maintain a balanced system within the ecosystem, nutrient enrichment disturbs this balance and can have detrimental effects on the ecosystem. Nutrient enrichment is usually a result of either agricultural runoff, or human waste. In both examples, nutrient rich waste enters rivers from factories or farms inland, and flows downstream and enters the oceans. Due to rapidly increasing human populations in communities near to the coast, especially in less developed countries where reefs are most common, this problem is becoming increasingly significant with respect to coral damage. An excess of nutrients in the water encourages algae to grow over the corals. They utilise phosphorus and nitrogen in particular. Phosphorus is used to manufacture DNA and ATP, and nitrogen is used primarily for amino acid synthesis. The consequences of this lead to high rates of algal growth, and therefore zooxanthellae death or coral suffocation. In the Amazon rainforest, for example, due to deforestation, runoff has increased by around 30%, while nitrogen and phosphorus levels in the Mississippi River were around ten times higher in 1997 than they were in the 1960s (Birkeland,1997). This has detrimentally affected coastal reefs in South America. However, some evidence suggests that nutrient enrichment may not be as damaging as other anthropogenic impacts, and may only act at a local level. Nutrient enrichment occurs most often in coastal regions with low water flow, such as in bays, whereas the majority of coral reefs are located in more exposed areas with high water flow (Szmant, 2002).

Human Threats to Coral Reefs and Coral Damage (continued)

Sedimentation can also occur, which can have similar effects on corals to algae preventing sunlight from reaching the corals. There are a number of causes of sedimentation, such as soil erosion, dredging, deforestation and construction. Sediment as a by-product from activities such as these can suspend in the water and block light reaching zooxanthellae. Unfortunately, the majority of sediment is entering the oceans in developing countries with high coral reef densities (Birkeland, 1997), therefore the sedimentation is having much more of an effect on the ecosystem than it would otherwise have in regions with less coral reefs. The countries producing the highest levels of sedimentation include those in South East Asia and in the Western Pacific. In fact, almost 100% of all the sediment entering the oceans originates from these areas (Birkeland, 1997). Sediment can also form a thin film on the upper surface of coral, which can be removed by direct movement of the polyps, via secretion of mucus from the mucus glands within the polyps, or tissue swelling (Weber et al., 2006). However, these solutions use up resources, and therefore if these defensive mechanisms are sustained due to settling sediment over long periods the polyps may become damaged, and can die in extreme cases. Damage to the coral, and its recovery, is directly related to the sediment type as well as the quantity (Weber et al., 2006). High nutrient sediment has been found to lead to greater stress levels in corals than lower nutrient sediment. In Puerto Rico, reefs subjected to high levels of sediment influx had significant loss of coral species in comparison to other locations. Most of the species affected usually reside in other geographical locations in deeper water, from 25-30m, but due largely to the lack of light caused by the sediment these coral species cannot survive, leading to reefs occurring only in shallower waters (Acevedo et al., 1989).

Human Threats to Coral Reefs and Coral Damage (Continued)

The twentieth century introduced another threat to coral reefs, in the form of weapons testing. However, opinions differ as to whether testing does in fact have a long term detrimental effect to reefs. After World War 2, between 1946 and 1958, testing of the nuclear bomb was carried out in the atolls of the Pacific Ocean by the US (Reuther, 1997). Atolls are circular coral islands that enclose a central lagoon, often occurring on top of submerged mountains or volcanoes. The most notable nuclear testing was carried out on the Marshall Islands, or the ‘Pacific Proving Grounds’. It is thought 67 bombs were tested in this period, and clearly had direct effects on the surrounding reefs. One bomb, codenamed Castle Bravo, was detonated in 1954 at the Bikini Atoll, the first testing of the more powerful hydrogen bomb. Its yield was underestimated by the military by two and a half times and as a result had significantly high impacts on the environment, spreading nuclear fallout over thousands of kilometres, infringing on many other reefs and inhabitants (Reuther, 1997). Some data, however, shows that the ecosystem in these areas is in pristine condition, and that species that were affected at the time have quickly recovered. For example, in France, despite direct damage to coral caused by explosions, there is little evidence to suggest the balance of species has been affected long term. Data shows that loss of fish species was complete in the area most affected by the bomb. However, recovery of the populations, initially via immigration followed by reproduction, was very swift, lasting just 1-5 years (Planes et al., 2005). In this case the coral habitats for species were not badly damaged, so it seems as though as long as the habitats are not badly damaged, and niches remain free, coral communities may recover fairly quickly in the case of nuclear weapons. Another study on coral reef gastropods in French Polynesia gave similar results. Populations of gastropod fell immediately after nuclear testing, but they were quickly able to recolonise empty niches left as a result of the bombs (Lanctot et al., 1997).

Human Threats to Coral Reefs & Coral Damage (Continued)

In areas where standard military weapons testing takes place, studies on the reefs have shown them to be in pristine condition. This could be because as the military occupy these regions, detrimental human activities such as fishing, sedimentation and pollution cannot take place without civilian human presence and so, despite minor habitat damage from explosions, coral organisms can thrive (Spalding et al., 2005). Differing results from studies on weapons testing have lead to no definite conclusion as to the impact such actions have had, and are having, on coral reefs. Further investigations need to be carried out in order to increase our knowledge on the effects of weapons testing on coral reefs.

Summary and conclusion

It seems clear from numerous investigations that human influences are severely harming the health of coral reefs (Reaser et al., 2000; Obura, 2005; Barber et al., 2001). Fishing and sedimentation in particular appear to be having a large impact, with nutrient enrichment seemingly slightly less damaging. More studies into the effects of weapons testing need to be carried out in order to determine their impact, although the discontinuation of nuclear testing in the Pacific atolls could make the results of these studies of less value. However, climate change appears to offer the greatest threat to coral health. Steadily rising SSTs over time have led to stronger ENSO events, and in turn mass bleaching events have increased. Bleaching events brought about by the climate are much more widespread and directly affect far more corals than any of the other human factors mentioned, which apply to more local areas of coral. Although the timescale being studied is a small one, it seems as though the rising temperatures are a result of climate change and not one-off events. Whether or not global warming as a result of increasing greenhouse gases in the atmosphere is occurring is unproven and open to speculation, but it seems likely in my opinion that there is a significant human aspect to this phenomenon.

Conclusion (Continued)

Further studies to improve our awareness of coral reef ecosystems could include selecting a coral native to a particular region, and exposing it to different species of zooxanthellae in turn. There is current evidence, as stated earlier, suggesting that after bleaching, corals become recolonised by more resistant zooxanthellae (Kinzie III et al., 2001; McClanahan, 2000). Running experiments on this topic will lead to a better understanding of recolonisation and this will lead to a better insight into the changing diversity and adaptation of zooxanthellae. This could aid in the long term conservation of coral reefs. Also further experiments to gauge the natural ability of corals to acclimatise to increasing temperatures may be useful.

Although susceptibility models already exist, such as that developed by Maina et al. (2007), it would be beneficial to continue designing models to predict the effects of non-steady state processes such as climate change will have on coral reefs in the future (Crabbe, 2007).

Conclusion (Continued)

I also agree with the views of Nystrom & Folke (2001), who suggest more trans-continental studies into reef resilience, as the majority of studies conducted in the past have focused on collections of reefs in a small geographical range. These further studies will hopefully aid in the conservation of coral reefs, as well as help gain a further insight into the ways climate change may affect the interactions between reefs.

Genetic studies could also be carried out on resistant species of corals and zooxanthellae in order to determine which genes they possess that give them a greater resilience to environmental extremes.

Conclusion (Continued)

There are extensive management issues that must be addressed in order to reduce coral reef damage. Education and communication are perhaps the most important, as at present many people are ignorant to the effects of, for example, pollution, climate change and overfishing. Educating communities will allow changes to be made locally to the management of crucial issues such as those explained earlier.

In a recent survey amongst 286 scientists (2007), reducing atmospheric carbon dioxide emissions was ranked as the second most effective change to be made to help improve the health of coral reefs. This is because it seems probable human behaviour resulting in CO2 emissions is at least partly responsible for the slowly increasing global temperatures and extreme environmental conditions that impact upon reefs via bleaching. This is a global problem however and requires participation from many countries in order to have a significant impact.

Conclusion (continued)

Stricter regulations on fishing should be introduced in certain parts of the world, as currently it is being carried out at an unsustainable level that is damaging to the reefs. If corals are to continue to thrive fishing must be carried out at a sustainable level via improved management. Studies based on an existing bio-economic model, the Gordon-Schaefer model, show that in many regions where fishing exceeds recruitment a reduction in fishing of approximately 60% is needed in order for fish stocks to return to a steady equilibrium. A complete ban on blast fishing would also be beneficial due to its direct destructive effects on the coral reefs and their organisms.

Finally, advances in waste management should be made, especially in China, which, due to its rapid industrialisation, is the main contributor to the pollution of coral reefs. However, in reality, due to the political situation in the country, and the speed of industrialisation, major reductions in pollution in the near future seem unlikely.

Introduction to Coral Reefs

Coral reefs that exist today are generally less than 10,000 years old. However, they have been around for over 500 million years, acting as a backbone for many coastal ecosystems. They play a significant role in food production supporting countless other organisms that rely on their health for their own survival. Not only are reefs important for biological reasons, they also have huge socio-economic impacts on nearby communities. However, evidence shows the health of coral reefs is currently in decline, with fears that human impacts are facilitating the destruction of these extremely diverse ecosystems. This research will look at what corals are and how they function; their role in ecosystem health and human society; and then focus on the human-influenced reasons why reefs are in decline, specifically looking at a process known as coral bleaching. This research will conclude with a look at the future prospects of corals and the sustainability of their health.