Bitter Prospects for Shellfish

Increased ocean acidity will make it more difficult for shellfish like mussels and oysters to harden their shells, says a study (1) by French CNRS researcher Jean-Pierre Gattuso (2) and his colleagues from the Netherlands. And this environmental problem will have to be addressed. If the ocean is turning “bitter,” it is due to its property of absorbing some of the greenhouse gas emissions. In fact, during the past two centuries of industrialization, it has soaked up half of the carbon dioxide (CO2) produced by human activity. “This is a boon for mankind but not for the ocean creatures,” says Gattuso. The acidity, or pH, measures the concentration of H+ ions in the water, with low pH meaning high H+ concentration. When CO2 combines with water H2O, it forms carbonic acid H2CO3. This acid releases H+ ions into the water, causing the pH to drop. And some marine organisms are not reacting well at all. It makes the process of shell calcification more difficult for those that need calcium carbonate to build their “body” structure, such as the skeleton of corals or the shell of mollusks.

The average pH level of the ocean is presently just above 8, but more than 25 million tons of CO2 combine with seawaters everyday. The Intergovernmental Panel on Climate Change (IPCC) predicts a decrease in pH of up to 0.35 units by the year 2100. Following this scenario, the scientists showed that the speed at which the edible mussel (Mytilus edulis) and the Pacific oyster (Crassostrea gigas) produce their shells will decrease respectively by 25 and 10%. “At higher CO2 concentrations, the mussel shells actually dissolve!” adds the biologist.

This calcification decrease will have major socio-economic consequences. Mollusk aquaculture production reaches 12 million tons a year, a market value of $10.5 billion. But there could be far-reaching consequences as shellfish play a major ecological role. They create habitat for other species, filter the waters, are a food source for birds, and also act as bioindicators for the quality of the environment.

“The next step will be to study the possible long-term genetic adaptation of mussels and oysters to this acidification and to consider other parameters,” explains Jean-Pierre Gattuso. “Rising sea temperatures combined with this CO2 input could make this calcification problem worse.” A bitter wake-up call.

Samantha Maguire, CNRS International, July 2007. Article.

1. F. Gazeau et al., “Impact of elevated CO2 on shellfish calcification,” Geophys. Res. Lett. 34: L07603, doi:10.1029/2006GL028554. 2007.
2. Oceanographic laboratory in Villefranche-sur-Mer (LOV) (CNRS / Université Pierre et Marie Curie).

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