Ocean Acidification

Ocean Acidification

To explain what is ocean acidification is important to understand basic concepts about basic chemistry. pH is a measure of how acidic/basic water is. The range goes from 0 to 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water.

In relation to the ocean, the average pH of ocean surface water fell from 8 to 8.3. this feature is due to the greenhouse gases that are in the atmosphere, mainly Carbon Dioxide (CO2), due to the high absorption capacity of CO2 emissions from the oceans themselves, it is estimated that there is approximately 60 times more CO2 in the oceans than in the atmosphere.

Nevertheless, the CO2 absrobed by oceans, reacts with seawater and forms carbonic acid that acidifies the ocean. This would make the ocean more acidic that any time in the past and this is a dangerous trend because is more difficult for preserve the resilience of marine ecosystems.

Therefore, the effect of acidification in the oceans is the result of CO2 emissions to the atmosphere from human activities, this increase in parts per million of CO2 is absorbed by the oceans and has led to a decrease in the pH of seawater.

While pH decreases, the availability of carbonate decreases at the same time and it produce changes in the chemistry composition of ocean water.

Thus, ocean acidification occurs when high concentrations of COare absorbed by the oceans, a portion of the carbon dioxide reacts with water to form carbonic acid (H2CO3), one of the hydrogens of carbonic acid separates and generates hydrogen ions (H+) and bicarbonate ions (HCO3). Most of that H+ combines with carbonate ion (CO3), to form more bicarbonate ions HCO3 , el cuál genera un desequilibrio en la calcificación de algunos organismos como corales, moluscos para formar conchas y otras partes duras.  A medida que el océano absorbe más CO2, prevents the absorption of CO3 due to the large amount of hydrogen ions associated with this compound.

Calcium carbonate (aragonite, one of the crystallized forms of CaCO3), is the main building compound for coral skeleton structure, and provides food, shelter and substrate for other organisms, therefore, corals have been vulnerable to climatic changes, including ocean acidification which reduces the concentration of carbonates that corals need to build their skeletons due to the association with the availability of hydrogen ions present in the ocean.

Aragonite saturation in tropical climates is higher than in polar climates because of the CO2 is more soluble in cold water. The saturation of aragonite in relation to the increase in high concentrations of CO2 over the ocean, as well as the increase in ocean surface temperature has led to a decrease in the saturation of aragonite in the oceans, this problem will continue until the necessary chemical conditions of the carbon cycle stabilize with respect to its concentrations, and can maintain stable carbonate ion availability for the calcification of some marine organisms, otherwise, the carbonate ion may disappear as we exceed the concentrations of CO2  above the atmosphere above 550 ppm.

In addition to the rate of calcification, growth and reproduction, ocean acidification affects the symbiotic relationship of the algae Zooxanthellae and their productivity of this association, acidification has contributed more to bleaching events than to the decrease in the rate of calcification, in some laboratory studies the high concentrations of CO2 and the increase in temperature have reduced the calcification rate and productivity of these ecosystems.

Some studies indicate that high-dose COand decreased pH disrupt photoproductive mechanisms in zooxanthellae chloroplasts and produce low rates of photorespiration, and increased light levels in some experiments, combined with solar radiation and increased concentrations of CO2 induce bleaching events.

Whitening, Puerto Morelos, Mexico. Photo: Ximena Lizaola

In conclusion, as CO2 concentrations increase in the atmosphere, ocean acidification will continue to affect the development and functioning of coral reefs and other associated ecosystems in different ways.


Anthony, K.R.N; 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 ig the United States of America, Peer-reviewed journal. Uited States. 5 pp.

Carbon in the ocean. Ocean acidification. 2015. https://www.co2.earth/carbon-in-the-ocean.

Doo. S. S; Edmunds, P.J; Robert, C. C. 2019. Ocean acidification effects on in situ coral reff metabolism. Scientifici Reports. Nature research. 8 pp.

Hoegh-Guldberg, O; Mymby, P.J; Steneck, R.S; Greenfield P; Gomez, E; Harvell, C.D; Sale, P.F; Edwards, A. J; Caldeira, K; Knowlton, N; Eakin, C. M; Iglesias-Prieto, R; Muthiga, N; Bradbury, R.H; Dubi, A. Hatziolos, M. 2007. Coral Reefs Under Rapid Climate Change and Ocean Acidification. Science. (318). Wahington. D.C. United States of America. 1737-1742 pp.

Hoegh-Guldberg, O. 2011. The Impact of Climate Change on Coral Reef Ecosystems. The University of Queensland. Australia. Researchgate. 14 pp.

Jiang, L.Q; Feely, R. A; Cartes, B. R; Greeley, A. J; Gledhill, D. K; Arzayus, K. M. 2015. Climatological distribution of aragonite saturation state in the global oceans. Global Biogeochemical Cycles, 29. 1656-1673 pp.

Kleypas, J.A., R.A. Feely, V.J. Fabry, C. Langdon, C.L. Sabine, and L.L. Robbins, 2006. Impacts of Ocean Acidificationon Coral Reefs and Other Marine Calcifiers: A Guide for Future Research, report of a workshop held 18–20 April 2005, St. Petersburg, FL, sponsored by NSF, NOAA, and the U.S. Geological Survey, 88 pp.

Mollica, N. R; Guo, W; Cohen, A. L; Huang, K.F; Foster, G. L; Donald, H. K; Solow, A. R. 2017. Ocean acidification affects coral growth by reducing skeletal density. Proceedings of the National Academy of Sciences ig the United States of America, Peer-reviewed journal. Uited States. 6 pp.

Salinity, sea surface salinity from space. Monitoring total alkalinity and Ocean Acidification. https://salinity.oceansciences.org/highlights03.htm

USGS science for a changing world. pH and Water. Science Water School. https://www.usgs.gov/special-topic/water-science-school/science/ph-and-water?qt-science_center_objects=0#qt-science_center_object

Williamson, P; Turyley, C. and Ostle, C. 2017. Ocean acidification. Marine Climate Change Impacts Partnership: Science Review. 1-14 pp.