Rising Temperatures And Impacts On Coral Bleaching Environmental Sciences Essay

The Earth’s surface temperature is predicted to rise over the next few decades and much of this warming can be attributed to the rapid increase in greenhouse gases produced through human activities. Elevated temperatures will cause weather extremities, thus posing a problem for many ecosystems. Out of the many ecosystems, coral reefs have shown to be greatly affected by the rising temperature. Having evolved in a thermally stable aquatic environment, corals do not have the necessary mechanisms or physical structures to combat temperature stress, thus making them vulnerable to temperature changes.

Coral reefs are often considered the “Tropical rainforest of the sea” as they contain a large diversity of life that is comparable to that of a rainforest (Connell, 1978). The reef is one of the most productive ecosystems (Odum and Odum, 1955) and it provides numerous ecological goods and services to mankind (Moberg and Folke, 1999). Most of the coral reefs in the world are found within 30° N and S latitudes throughout the tropical and sub-tropical areas of the Indo-Pacific and Western Atlantic oceans where conditions are warm and sunny with shallow, clear, well-oxygenated and nutrient poor waters (Spalding et al., 2001). These environmental conditions allow the establishment of stony corals (Phylum: Cnidaria, Class: Anthozoa) which greatly contribute to the structural foundation of a reef (Muscatine and Porter, 1977; Sorokin, 1993).

Stony corals are small marine animals that live together with other identical individuals in dense colonies. The colonies secrete calcium carbonate, forming a hard skeleton and providing a platform for colonization by other plants and animals (Sorokin, 1993). A single coral polyp contains numerous dinoflagellate symbiotic algae, known as Zooxanthellae (Genus: Symbiodinium), that live symbiotically within coral tissues. Through photosynthesis, the zooxanthellae provide their host with a supply fixed carbon, thus allowing the coral to allocate additional resources for growth, reproduction and calcification (Muller-Parker and D’Elia, 1997). In return, the coral protects the zooxanthellae against herbivores and damage from ultraviolet rays and even provides a supply of carbon dioxide and waste nutrients for the algae’s photosynthetic process (Muller-Parker and D’Elia, 1997).

Coral reefs are easily affected by the many man-made and natural disturbances occurring in the world (Glynn, 1993; Hughes, 1994; Hughes et al., 2003). Man-made disturbances that are responsible for the recent decline of coral reefs include overfishing, overexploitation for aquarium trade, increased nutrient and sedimentation run-off and anthropogenic forcing on climate change. Natural disturbances include fluctuations in temperatures, increase in solar radiation, storms, flooding, disease outbreak and El Nino Southern Oscillation (ENSO). In response to these stressors, corals have shown to undergo bleaching (expulsion of their symbiotic zooxanthellae) (Glynn, 1984) and the prevalence and distribution of bleaching events across the world have increased over the years (Brown, 1997).

Of the various disturbances above-mentioned, rising ocean temperatures seem to have a large impact on corals, affecting the coral-zooxanthellae symbiosis, thus causing polyps to bleach and eventually die when they are unable to recover from the stress (Glynn and D’Croz, 1990; Jokiel and Coles, 1990; Lesser et al., 1990). Moreover, majority of the world’s coral reefs are located in the tropics where ocean temperatures are high and consequently, many of the corals are living near their lethal limits (Jokiel and Coles, 1990). Any further rise in temperatures may cause temperatures to rise above the lethal limits, resulting in bleaching and subsequent death. Therefore, rising ocean temperatures is a key stressor on coral bleaching.

The rise in ocean temperatures can be attributed to various climatic drivers. Firstly, based on the Milankovitch theory, environmental conditions on earth have shown to be affected by the earth’s orbit around the sun (Hays et al., 1976). The orbit of the earth around the sun is elliptical, thus causing the earth’s distance from the sun to vary over the year (Berger, 1988). When the earth is closest to the sun at perihelion, it receives a greater amount of solar radiation as compared to when the earth is furthest at aphelion, thus raising the temperatures of the earth and its oceans (Berger, 1988).

Secondly, internal variations of the earth’s climate system will affect ocean temperatures. The presence of a thermocline in the ocean prevents the effective mixing of cold and warm waters. In a normal Walker Circulation cell, trade winds push ocean surface water from the Eastern Pacific to the Western Pacific, thus allowing the upwelling of cold water in the Eastern Pacific (Bjerknes, 1969). Consequently, this allows the mixing of cold and warm waters and the subsequent lowering of the overall ocean temperature. During the occurrence of an ENSO event, the change in the Walker Circulation cell causes trade winds to drive warm currents towards the upwelling of cold ocean water, thus preventing the mixing of cold and warm water (Wyrtki, 1975). The suppression of surface water cooling will eventually cause sea surface temperatures to rise (Jacobs et al., 1994).

Thirdly, there have been increases in the amount of greenhouse gases in the atmosphere due to human activities. Greenhouse gases in the atmosphere absorb and emit infra-red waves, trapping heat within the atmosphere (Schneider, 1989). A large amount of greenhouse gases in the atmosphere will prevent the lost of infra-red radiation to space, thus increasing the earth’s temperature (Schneider, 1989). The increase in carbon dioxide since the pre-industrial period is due to an increase in fossil fuels burning and deforestation driven by land use change (Denman et al., 2007). Methane in the atmosphere has gradually increase due to an increase in rice agriculture, cattle ranching and land use change of wetlands (Denman et al., 2007). There is also a significant rise in nitrous oxide due to the increase in fertilizer application in croplands. In aerobic conditions, excess nitrogen in the environment will be oxidized to nitrous oxides which will then accumulate in the atmosphere (Denman et al., 2007). Consequently, this rapid increase in these human-made greenhouse gases will trap a higher amount of heat, thus raising sea surface temperatures.

Elevated temperatures can cause several detrimental effects on corals and their symbionts. Being surrounded by water, coral reefs will be directly affected by any changes to the water conditions and thus a rise in temperature will affect corals and their algae symbiont.

The rate of photosynthesis in zooxanthellae rises with elevated temperatures due to an increase in reaction rates (Iglesias-Prieto et al., 1992; Lesser, 1996). The rise in oxygen levels as a result of increasing temperatures (Lesser et al., 1990) produces a hyperoxia environment which facilitates the production of reactive oxygen species (Asada and Takahashi, 1987). Likewise, enhanced temperature-dependent metabolic rates in the mitochondria of the coral and zooxanthellae will cause a simultaneous increase in production of reactive oxygen species (Burdon et al., 1990). Reactive oxygen species in the zooxanthellae can further drive the formation of other superoxide radicals in the corals’ tissues through Fenton reactions (Dykens et al., 1992). Accumulated reactive oxygen species in cells can cause severe damage to DNA which will eventually lead to apoptosis (programmed cell death) unless the DNA is repaired (Imlay and Linn, 1988). Therefore, in order to avoid damages to its tissue, the coral host will resort to the expulsion of its algae symbiont (Lesser, 1997).

Besides causing oxygen toxicity, the rising sea temperatures will also affect the photosynthetic pathway in the zooxanthellae. At elevated temperatures, high amount of oxygen will be produced in the zooxanthellae and the oxygen will then compete with carbon dioxide for reaction with Rubisco, thus affecting the rate of carbon fixation (Yonge and Nicholls, 1931). Moreover, Rubisco found in the zooxanthellae has a low specificity for carbon dioxide as opposed to oxygen (Jordan and Ogren, 1981). The decrease in Rubisco activity in the Calvin cycle will thus cause the accumulation of NADPH which is produced from the electron transport chain (Jones et al., 1998). Over time, there will be a lack in electron carriers (NADPH) as the rate of light reaction exceeds that of the dark reaction (Jones et al., 1998). Electrons will be accumulated in the electron transport chain, causing it to be over-reduced, resulting in significant damages to photosystem two (Jones et al., 1998). Furthermore, reactive oxygen species are able to inhibit Rubisco (Asada and Takahashi, 1987) and cause damage to photosystem two (Asada and Takahashi, 1987; Ritcher et al., 1990). The overall reduction in photosynthetic efficiency (Warner et al., 1996) and damaging effect of reactive oxygen species caused by rising temperatures will lead to the expulsion of the symbiont by its coral host.

Environmental conditions of the ocean have an impact on the organisms living in it. The rise in ocean temperatures has shown to have an influence on marine pathogens, such as Vibrio shiloi, which causes bleaching in the coral Oculina patagonica (Kushmaro et al., 1998). Elevated temperatures can shorten the development time of the pathogen, resulting in an increase in number of generations per year, thereby causing an increase in infected hosts (Harvell et al., 2002). Furthermore, the rise in temperature may increase the susceptibility of the host to pathogens by allowing favorable pathogen growth at their host’s optimum temperature (Harvell et al., 2002).

The bleaching of the coral Oculina patagonica by Vibrio shiloi is similar to that of temperature induced bleaching of other corals (Rosenberg and Ben-Haim, 2002). Infection of the pathogen is temperature dependent and there is an increase in expression of virulent genes at high temperatures (Kushmaro et al., 1998). Elevated temperatures initiated the virulent behavior of V. shiloi, allowing it to produce adhesins which bind to β-D-galactopyranoside residues on the coral surface (Toren et al., 1998). Adhesion of the bacteria to the coral surface allows the penetration into the coral host (Banin et al., 2000) where they will grow and multiply. Inside the coral, V. shiloi produces toxins which affect pH gradients in the zooxanthellae and effectively decreasing photosynthetic efficiency (Banin et al., 2001). In addition, the toxins are able to cause pigmentation loss and also break down the zooxanthellae (Ben-Haim et al., 1999).

Despite being susceptible to the detrimental effects of rising temperatures, corals have shown certain resistance and resilience against climate warming. To combat the increase in reactive oxygen species, corals and their symbionts can increase the amount and activities of antioxidants (Lesser et al., 1990). Enzymes such as superoxide dismutase, catalase and ascorbate peroxidase catalyze detoxification reactions of reactive oxygen species and are able to prevent the formation of free radicals (Lesser et al., 1990). The removal of reactive oxygen species and their harmful effects will thus prevent corals from expelling their zooxanthellae.

Heat-stressed corals are able to perform non-photochemical quenching to prevent accumulation of NADPH and damage to photosystem two (Warner et al., 1996). Excess electrons in the electron transport chain are diverted to oxygen-consuming pathways in which oxygen serves as an electron acceptor (Asada and Takahashi, 1987; Schreiber and Neubauer, 1990). Non-photochemical quenching also removes excess energy obtained through photosystem two in the form of heat (Schreiber and Neubauer, 1990). Therefore, the ability to overcome damage to the photosystem will allow corals hosts to retain their zooxanthellae when under heat stress.

It is also possible for corals to acclimate and adapt to the rising sea temperatures. Corals can acclimatize in hours or days by adjusting their lethal temperature limit through modification of their cellular metabolism and increaseing heat shock proteins (Hoegh-Guldberg, 1999). The expulsion of its zooxanthellae when under stress could allow the coral to take up or host zooxanthellae that are more thermal resistant, thus adapting to the heat stress (Buddemeier and Fautin, 1993). Furthermore, corals can also acquire increased thermal resistance by altering their zooxanthellae composition (Berkelmans and van Oppen, 2006). Dominant zooxanthellae that are stress-prone can be gradually replaced by zooxanthellae types that are more temperature tolerant (Berkelmans and van Oppen, 2006). Lastly, corals can be naturally selected, allowing fitter individuals or species which are able to deal with the increasing temperatures, to survive and reproduce. However, this form of adaptation or evolution will require hundreds or thousands of years.

In conclusion, ocean temperatures will continue to rise over the next few decades unless mitigation measures are taken to reduce the rapidly increasing greenhouse gases. Coral reefs will be affected by the temperature rise and bleaching events will occur at a greater rate. In light of the rapidly increasing temperatures, coral species face an uphill struggle to adapt, evolve and survive. Although corals and their symbionts have the abilities to resist the temperature changes, one can only wonder the extent and effectiveness of these strategies.