Oysters in the acidifying ocean

Posted on December 2, 2011 by

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Gary Dickinson is investigating how shellfish are affected by ocean acidification, the so-called ‘evil twin’ of global warming, but to hear about his work I had to visit my dentist.

That’s because my dentist’s office is alarmingly close to where Gary works, embedded amongst the tissue engineers at the Department of Oral Biology in Pitt’s School of Dental Medicine. Gary is a research associate in the lab of Elia Beniash, who studies biomineralization, the process by which bone and teeth form. Their research is conducted with an eye to ‘bio-inspired’ design and regenerative medicine.  But they also study other kinds of important biominerals, those that form the shells and hard parts of sea creatures like shellfish and barnacles.

In recent years, marine scientists have become alarmed by the likelihood that such creatures, and the ecosystems that depend on them, will be seriously harmed by the increasing levels of carbon dioxide in the atmosphere. Why are they so worried? Around 25% of the carbon dioxide produced by the burning of fossil fuels ends up absorbed by sea water. This extra carbon dioxide is already changing the chemistry of the ocean: it is increasing the acidity of the water and decreasing the availability of carbonate ions, which are needed by many animals for biomineralization. So far, the average pH of the ocean has dropped by about 0.1 (pH decreases as acidity increases). This is equivalent to a 30% increase in the concentration of hydrogen ions (or acid). At current rates of carbon dioxide emission, that pH is predicted to decrease by 0.3 by the end of the century.

Change in sea water acidity pH caused primarily by anthropogenic CO2 between the 1700s and the 1990s by Plumbago via Wikimedia Commons

This unusually rapid drop in pH threatens the ability of many marine organisms to make and maintain their shells. If the concentration of carbonate decreases below a certain level, such structures can dissolve. But even less lethal decreases in pH force organisms to work harder to build their shells and maintain the pH of their bodies. Wasting energy in this way leaves them vulnerable to death by other causes.

One of the delicacies you might miss when dining in our acidified-ocean future is the Eastern oyster. This native of the East coast forms reefs of densely-packed shells and plays many crucial roles in shaping its local ecosystem. In many places, the commercially and ecologically important oyster population has suffered from overfishing and disease, so additional stress due to ocean acidification could prove disastrous. But the oyster is no delicate flower. It has adapted to the harsh and changeable environment of estuaries, where fresh water from the land meets the salt water of the ocean. It must routinely survive dramatic changes of salinity, temperature, water quality, and yes, pH. Because of the natural fluctuations in pH that they routinely experience, estuarine animals should be more tolerant of human-induced ocean acidification. But the Beniash lab and their collaborators demonstrated that this tolerance has its limits.

Oyster bed on Cockspur Island, Georgia.  Image by JohnCub via Wikmedia Commons

That’s because the average oyster doesn’t have the luxury of dealing with one change at a time. It has to balance its responses to decreasing pH with the demands of an already stressful environment, as well as other effects of human activity, like water pollution and changes in rainfall patterns. Because of the competing stresses, the effects of pH could well be different, either better or worse, depending on the context.

For instance, alterations in the volume or variability of freshwater entering estuaries can exacerbate natural fluctuations in salinity. Gary and his colleagues decided to test what happens to oysters when they experience a combination of high atmospheric carbon dioxide, and low salinity. To do this, oysters were stained with a fluorescent dye and then maintained for several weeks at different salinity and carbon dioxide levels in the lab of corresponding author Inna Soklova at the University of North Carolina. Like a fluorescent tree ring, the dye marked the last layer of shell that was laid down before the experimental conditions began, allowing Gary to recognize the newer layers of shell.

Gary examined the structural integrity of the stressed oysters’ shells by pushing on them with a tiny, diamond-tipped rod. At a fixed amount of force, the size of the mark left on the shell is related to its hardness and the length of any cracks extending from the mark is related to the shell’s ability to resist fractures. These tests showed that the oysters were able to maintain shell integrity under conditions of low salinity as well as at the carbon dioxide levels predicted to be reached at the end of the century. However, when the oysters were reared in conditions of high carbon dioxide and low salinity the shell laid down during the experiment was less hard and less fracture resistant, leaving them vulnerable to predation and injury.

The reason that decreased salinity compounded the effect of high carbon dioxide is a matter of speculation, but salinity has both a direct effect on the carbonate and pH chemistry of the water, and an indirect effect by intensifying metabolic stress.

Ask any biologist, and they will tell you that two of the most important parameters in any biological reaction are pH and temperature. The example of the Eastern oysters reminds us that the effect of ocean acidification will vary with local conditions and will be difficult to predict. What is easy to predict is that change is coming.

Interactive effects of salinity and elevated CO2 levels on juvenile Eastern oysters, Crassostrea virginica (Gmelin)

Dickinson, GH, Ivanina, AV, Matoo, OB, Pörtner, HO, Lannig, G, Bock, C, Beniash, E and Sokolova, IM. Journal of Experimental Biology, In press.

Confused about the science of ocean acidification? I recommend this nice summary, from the National Academy of Sciences or this short and sweet video from the National Oceanic and Atmospheric Administration.

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