Inorder to understand ocean acidification and its possible impacts, oneneeds to understand the behaviour of carbon in nature. Carbon, as otherelements, is circulating in different chemical forms and betweendifferent parts of the Earth system (atmosphere, biosphere and the oceans). These fluxes of carbon in inorganic (e.g. CO₂) and organicforms (sugar and more complex carbohydrates in the biosphere)constitute the carbon cycle. In a very short time span, humanactivities use an old reservoir of carbon (fossil fuels) which tookmillions of year to accumulate, thus creating a new and massive flux ofCO₂ into the atmosphere. The oceans can mitigate thisadditional carbon dioxide flux and thus help moderate global warmingbut this is not without consequences.

The world's oceans play a fundamental role in the exchange of CO₂ with the atmosphere and constitute an important sink for atmospheric CO₂. Once dissolved in sea water, carbon dioxide is subject to two possible fates. It can either be used by photosynthesisor other physiological processes, or remain free in its differentdissolved forms in the water. The latter leads to ocean acidification.

 

Thereis a constant exchange between the upper layers of the oceans and theatmosphere. Nature strives towards equilibrium, and thus for the oceanand the atmosphere to contain equal concentrations of CO₂.Carbon dioxide in the atmosphere therefore dissolves in the surfacewaters of the oceans in order to establish a concentration inequilibrium with that of the atmosphere. As CO₂ dissolves in the ocean it generates dramatic changes in sea water chemistry. CO₂ reacts with water molecules (H₂O) and forms the weak acid H₂CO₃ (carbonic acid). Most of this acid dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO^3-). The increase in H⁺ ions reduces pH(measure of acidity) and the oceans acidify, that is they become moreacidic or rather less alkaline since although the ocean is acidifying,its pH is still greater than 7 (that of water with a neutral pH). Theaverage pH of today's surface waters is 8.1, which is approximately 0.1pH units less than the estimated pre-industrial value 200 years ago(2,3).

Modeling demonstrates that if CO₂continues to be released on current trends, ocean average pH will reach7.8 by the end of this century, corresponding to 0.5 units below thepre-industrial level, a pH level that has not been experienced forseveral millions of years (1). A change of 0.5 units might not sound asa very big change, but the pH scale is logaritmic meaning that such achange is equivalent to a three fold increase in H+ concentration. Allthis is happening at a speed 100 times greater than has ever beenobserved during the geological past. Several marine species,communities and ecosystems might not have the time to acclimate or adapt to these fast changes in ocean chemistry.

Thedissolution of carbon dioxide in sea water not only provokes anincrease in hydrogen ions and thus a decline in pH, but also a decreasein a very important form of inorganic carbon: the carbonate ion (CO₃^2-).Numerous marine organisms such as corals, mollusks, crustaceans and seaurchins rely on carbonate ions to form their calcareous shells orskeletons in a process known as calcification. The concentration ofcarbonate ions in the ocean largely determines whether there isdissolution or precipitation of aragonite and calcite, the two natural polymorphs of calcium carbonate (CaCO₃),secreted in the form of shells or skeletons by these organisms. Today,surface waters are supersaturated with respect to aragonite andcalcite, meaning that carbonate ions are abundant. This supersaturationis essential, not only for calcifying organisms to produce theirskeletons or shells, but also to keep these structures intact. Existingshells and skeletons might dissolve if pH reach lower values, and theoceans turn corrosive for these organisms. Consequently, the results ofthe decrease in carbonate ions might be catastrophic for calcifyingorganisms which play an important role in the food chain and formdiverse habitats helping the maintenance of biodiversity.

Themagnitude of ocean acidification can be predicted with a high level ofconfidence since the ocean chemistry is well known. But the impacts ofthe acidification on marine organisms and their ecosystems is much lesspredictable. Not only calcifying organisms are potentially affected byocean acidification. Other main physiological processes such asreproduction, growth and photosynthesis are susceptible to beimpacted, possibly resulting in an important loss in marinebiodiversity. But it is also possible that some species, like seagrasses that uses CO₂ for photosynthesis, are positivelyinfluenced by ocean acidification. Ocean acidification research isstill in its infancy and more studies are required to answer thenumerous questions related to its biological and biogeochemicalconsequences.

References:

1) Caldeira, K., Wickett, M.E., 2003. Anthropogenic carbon and ocean pH. Nature 425 (6956): 365–365.
2)Key, R.M.; Kozyr, A.; Sabine, C.L.; Lee, K.; Wanninkhof, R.; Bullister,J.; Feely, R.A.; Millero, F.; Mordy, C. and Peng, T.-H. (2004). "Aglobal ocean carbon climatology: Results from GLODAP". GlobalBiogeochemical Cycles 18
3) Orr J. C., Fabry V. J., Aumont O., BoppL., Doney S. C., Feely R. A. et al. 2005. "Anthropogenic oceanacidification over the twenty-first century and its impact oncalcifying organisms". Nature 437 (7059): 681–686.
4) Raven, J. A. et al. 2005. Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society, London, UK.
5) Sabine C. L. et al., 2004. The oceanic sink for anthropogenic CO2. Science 305:367-371.
6) Martin S. et al. 2008. Ocean acidification and its consequences. French ESSP Newsletter 21: 5-16.
7)IPCC 2007. Contribution of Working Group I to the Fourth AssessmentReport of the Intergovernmental Panel on Climate Change. Summary forPolicymakers.

Acclimate - To accustom or become accustomed to a new environment or situation.

Aragonite- An orthorhombic (system of crystallization characterized by threeunequal axes at right angles to each other)  mineral form ofcrystalline calcium carbonate, dimorphous with calcite

Biosphere – The living organisms and their environment

Calcite -A common crystalline form of natural calcium carbonate, CaCO3, that isthe basic constituent of limestone, marble, and chalk. Also called calcspar.

Inorganic -  Involving neither organic life nor the products of organic life

Ocean acidification – The process by which carbon dioxide dissolves in sea water, giving rise to a
decrease in pH and other changes in ocean carbonate chemistry

Organic -  Of, relating to, or derived from living organisms

pH – Measure of acidity (pH= -log[H+])

Photosynthesis -The process in green plants and certain other organisms by whichcarbohydrates are synthesized from carbon dioxide and water using lightas an energy source. Most forms of photosynthesis release oxygen as abyproduct.

Phytoplankton - Minute, free-floating aquatic plants (algae, protists, and cyanobacteria).

Polymorph – Chemistry: A specific crystalline form of a compound that can crystallize in different forms.