Often coined the rainforests of the sea, coral reefs are one of the planet’s most diverse ecosystems. Though they only cover between 260,000 and 600,000 square kilometers (less than 1% of the Earth’s surface), they house approximately one-third to one-fourth of all marine species. Previous studies estimate that up to 3.2 million species may inhabit these coral reef ecosystems (1).
In addition to the biodiversity they provide, coral reefs play an important role in their local economies. Tourism alone provides nearby countries with billions of dollars annually and is now the fastest growing economic sector associated with these ecosystems. Furthermore, coral reef fisheries generate over 6 million metric tons of fish every year and provide about one-quarter of the fish caught in developing nations. Other associated benefits of coral reefs include their part in building materials, coastal protection, and pharmaceutical discoveries (2).
On the downside, however, coral reefs have been on the decline due to stresses resulting from human activity, including pollution and overfishing. Moreover, rises in carbon emissions from increased fossil fuel combustion and cement production have led to ocean acidification and global warming, both of which inhibit the growth of coral reefs (3). How exactly do ocean acidification and global warming affect these underwater ecosystems?
Ocean Acidification and Calcification:
Oceans play an important role in the Earth’s carbon cycle; they act as one of the major carbon reservoirs and readily exchange carbon dioxide with the atmosphere. Holding all other things constant, Henry’s Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas over the liquid. This indicates that the concentration of carbon dioxide in the oceans positively correlates to the atmospheric level of carbon dioxide. In other words, rises in atmospheric carbon dioxide from fossil fuel combustion and cement production increase the concentration of carbon dioxide dissolved in the oceans. This results in a phenomenon known as ocean acidification (4). In the presence of water, carbon dioxide reacts to generate carbonic acid, which consequently releases hydrogen ions to form bicarbonate and carbonate ions. The increased concentration of hydrogen ions lowers the pH, thus giving rise to the name, “ocean acidification” (5). The European Science Foundation reports that oceans have become 30% more acidic since the industrial revolution (6).
Ocean acidification is detrimental to coral reef ecosystems; in particular, it affects the equilibrium between bicarbonate and carbonate ions. Comparing pre-industrial percentages to current ones, studies show that proportions of bicarbonate and carbonate ions in surface tropical seawaters shifted from 88% and 11%, respectively, to 90% and 9%, respectively. These changes had significant implications on calcification, or the process by which organisms secrete calcium carbonate salts to form external structures (5). The reduction in the concentration of carbonate ions lowered the calcium carbonate saturation state and, consequently, decreased the precipitation rate of calcium carbonate. In a recent study, Kleypas and Langdon demonstrate that doubling pre-industrial concentrations of atmospheric carbon dioxide decreases the calcification rate of corals and coralline algae by 10 to 50% (7). Furthermore, the decrease in the calcium carbonate saturation state led to increases in reef dissolution rates. Andersson predicts that dissolution of carbonate salts will more than triple by the year 2300 (8). Altogether, the decline in reef calcification and the rise in reef dissolution contributed to the thinning and disappearance of coral reef ecosystems.
Global Warming and Coral Bleaching:
The mutualism between corals and zooxanthellae constitutes one of the most important relationships within the coral reef ecosystem. Living within intracellular, membrane-bound vacuoles, zooxanthellae pass a majority of their photosynthetic products to their coral hosts; these compounds include amino acids, sugars, carbohydrates, and peptides. Meanwhile, corals excrete nutrients like ammonia and phosphate in their waste, which are consequently absorbed by zooxanthellae (2).
Unfortunately, ocean temperatures have increased by almost 1˚C over the past century, and these elevated water temperatures disrupt this relationship, resulting in coral bleaching. This term refers to the process by which corals lose their color as the zooxanthellae population and their photosynthetic pigments deplete (9). Scientists proposed several mechanisms that account for the reduced algal densities observed during mass bleaching events. These include intracellular degradation of zooxanthellae, the loss of zooxanthellae via exocytosis, and the release of intact, coral cells containing zooxanthellae (10). Several hypotheses have been offered to explain why such events would take place. One proposition is that the symbiosis between corals and zooxanthellae is maintained by a sustained signal that inhibits the host’s defense system against the foreign zooxanthellae. However, under elevated temperatures, the algae are unable to photosynthesize as efficiently. This consequently triggers the host coral cells to expel the algae. Another proposition is that elevated temperatures cause zooxanthellae to release certain compounds or cytoplasmic contents that stimulate the host’s defense system. Ultimately, this process of bleaching damages coral tissues and impairs their capacity to build new skeleton and reproduce (9).
Effects on Marine Ecosystems and Biodiversity:
The detrimental effects of ocean acidification and global warming, among other human activities, can be seen in the numbers. Coral cover in the Caribbean and in the Indo-Pacific declined by 80% and 50%, respectively, over the past few decades (3). Additionally, consider the world’s largest reef: the Great Barrier Reef, which stretches over 2,300 kilometers, houses more than 1,500 species of fish and 400 species of coral, and generates $1.5 billion on an annual basis (2, 11). The Great Barrier Reef experienced multiple episodes of mass coral bleaching, the first one being recorded in 1979; six more events were noted since then. In particular, approximately 60 to 90% of its individual reefs underwent some degree of bleaching in 2002 (3).
Previous findings highlight the incredible toll that coral loss from ocean acidification and global warming take on marine species. For example, Jones et al. recently conducted an eight-year study in Papua New Guinea. Their discoveries reinforced the idea that declines in coral cover and fish biodiversity are positively associated. They observed that approximately 75% of the reef fish species fell in population, and that 50% of the species exhibited counts that were less than half of their originals. In particular, they found that species for which juveniles have a more significant coral reef dependence declined on a greater scale. Furthermore, they predicted that species with more limited geographic ranges, like some coral-dwelling gobies, may become extinct (12).
The loss of fish biodiversity, among other marine species, plays a consequent role in the coral-algal phase shift in coral reefs. When corals die and deplete from ocean acidification and global warming, algae and other benthos – organisms that dwell at the bottom of the ocean – typically occupy the resulting space. Under normal conditions, their spread is limited and controlled by herbivore grazing. However, in the absence of sufficient herbivores, algal communities may establish permanent forms and prevent coral reefs from recovering to their previous forms (13).
Given how important coral reefs are, the disappearance of these ecosystems has had and will continue to have an impact on marine biodiversity and on the local economies. In addition to species of animals, job opportunities and revenue may be lost as these declines in coral reefs persist into the future (11). To combat these downward trends, investigators aim to conduct further research on the effects of ocean acidification and global warming on marine organisms and ecosystems, both macroscopically and microscopically. Ultimately, the hope is to develop strategies for intervention that will reverse the damage already inflicted on coral reefs and allow these ecosystems to grow once again (3).
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5. International Society for Reef Studies, Coral Reefs and Ocean Acidification (2008). Available at http://www.coralreefs.org/documents/ISRS%20Briefing%20Paper%205%20-%20Coral%20Reefs%20and%20Ocean%20Acidification.pdf.
6. Ocean Acidification: Another Undesired Side Effect of Fossil Fuel-Burning (2008). Available at http://www.sciencedaily.com/releases/2008/05/080521105251.htm.
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11. R.A. Butler, Coral reefs decimated by 2050, Great Barrier Reef’s coral 95% dead (2005). Available at http://news.mongabay.com/2005/1117-corals.html.
12. G.P. Jones, M.I. McCormick, M. Srinivasan, J.V. Eagle, P. Natl. Acad. Sci. 101, 8251-8253 (2004).
13. J.W. McManus, J.F. Polsenberg, Oceanography 60, 263-279 (2004).
Danny Wong, Dartmouth College Undergraduate Journal of Science, 11 March 2012. Article.