Ocean Acidification and the Impact on Coral Structures

There is little knowledge on the effects of acidification in the ocean and how that impacts the physiology of coral reefs. Corals build their structures towards the sunlight and thicken them in order to add more strength underneath them. New research is being done in order to understand how acidity in the ocean can cause these skeletal coral structures to be weaker than normal and be more vulnerable to breaking. With this research, scientists are able to predict where corals may become more or less impacted. Overall coral reefs endure multiple stresses, which include rising sea levels, changing nutrient regimes and warming sea temperatures. Ocean acidification is much harder to detect and predict due to the slow and gradual negative affects it deals with these corals. That’s why understanding these effects are crucial in possibly saving these rich biodiverse ecosystems.

Ocean acidification occurs through the absorption of CO2 from the earth’s atmosphere. About 30% of the CO2 emitted into the atmosphere by humans is absorbed into the ocean. This lowers the pH of the surface water levels and in turn, affects the ability of corals to produce calcium carbonate. Coral polyps use CO32- in order to produce CaCO3 for their skeletal structure. The coral takes in seawater that contains HCO3-, CO32- and Ca2+ ions, which in turn can allow the production of CaCO3 for their skeletal structure.

In this particular study, the effects of prolonged exposure to CO2 is analyzed and compared to the bleaching of corals that are usually correlated with the warming sea temperatures. The idea is that the warmer temperature along with the abundance of carbon dioxide will have a symbiotic effect on the overall coral bleaching. Three of the hypotheses looked into are the effects of initial carbon dioxide exposure, how it affects the overall organic productivity and how it changes the rate of calcification.

In order to replicate these variables and how they affect the bleaching of corals a two-step carbon dioxide dosage, an intermediate and high dosage, was given along with a two-step temperature variable in order to reproduce the same real-life situations. As well as the use of a control that only underwent the temperature changes, this allows for comparison purposes.

Figure 1: Effects of experimental ocean acidification and warming

Results showed that the high-level carbon dioxide dosing had an effect on two of the reef-building organisms at up to 40 to 50%. Overall it is determined that prolonged carbon dioxide exposure had bleaching impacts on two of these reef-building organisms. Although, in the Acropora species at the intermediate carbon dioxide rate the net productivity was maximized. This could be due to the abundance of carbon dioxide being directly used in the photosynthesis process, but at the higher dosage of carbon dioxide, the productivity decreased suggesting that abundance still has a negative effect.

The next step of understanding these coral reef deterrents is the combination of all variables and how they deter the growth of these corals. It is obvious that acidification is not occurring in isolation and that corals are not the only organisms that are being affected by this increasing amount of carbon dioxide in the water. It can easily be seen that we are affecting these organisms negatively and it is important to figure out how we are. That is why this study is significant because it challenges previous studies done that suggest the acidification of the ocean doesn’t impact these corals.


Anthony, K. R., Kline, D. I., Diaz-Pulido, G., Dove, S., & Hoegh-Guldberg, O. (2008). Ocean acidification causes bleaching and productivity loss in coral reef builders. Proceedings of the National Academy of Sciences, 105(45), 17442-17446. doi:10.1073/pnas.0804478105