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As CO2 rises, common blue mussels' shells get more brittle on the outside, and softer on the inside. Photo:  Marcel Theisen via Flickr, CC-BY.
As CO2 rises, common blue mussels' shells get more brittle on the outside, and softer on the inside. Photo: Marcel Theisen via Flickr, CC-BY.
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Carbon dioxide threat to mussels' shells

The Ecologist

24th December 2014

The world's mussel population could be under threat as rising CO2 levels in atmosphere and oceans makes their shells weaker and more brittle shells - making them more vulnerable to stormy seas, and predation.

The calcite outer shells of the mussels became stiffer and harder, making it more brittle and prone to fracture under pressure, and the aragonite inner shell became softer.

In a new paper published today in the Royal Society's journal Interface, researchers from the University of Glasgow describe how mussels' shells become more brittle when they are formed in more acidic water.

The world's oceans are becoming increasingly acidic as they absorb some of the atmospheric carbon dioxide which contributes to climate change.

The water reacts with the carbon dioxide to form carbonic acid, which is gradually lowering the pH of the oceans (indicating an increase in acidity). Scientists expect the pH of the world's oceans to have dropped from 8 today to 7.7 by the end of the 21st century.

"What we've found in the lab is that increased levels of acidification in their habitats have a negative impact on mussels' ability to create their shells", said research team leader Susan Fitzer of the University's School of Geographical and Earth Sciences.

As oceans get more acid, less bicarbonate for shell-making

Mussels' shells are composites of calcium carbonate and organic material created by the mussels through a process known as biomineralisation.

Mussels draw bicarbonate ions from seawater and use proteins in their bodies to make crystals of calcium carbonate to form their two-layer shells. In more acidic water, there are less bicarbonate ions available for the mussels to make their shells.

"This could mean that mussels growing in the wild in the future could be more vulnerable to attack from predators, as well as from the effect of ocean forces", explained Dr Fitzer.

"As blue mussels are commonly used for human consumption, it could also have an effect on the yields of mussels available for the fishing industry."

The mussels do have way to resist the more acidic water once their shells have formed. Their shells' outer later is composed of calcite, a form of calcium carbonate that is more resistant to acid decay. Only the inner layer is made of the more soluble aragonite.

But even that mechanism is under threat, says Dr Fitzer: "What we found was that the calcite outer shells of the mussels past a certain threshold of acidity was stiffer and harder, making it more brittle and prone to fracture under pressure, and the aragonite inner shell became softer."

Ocean conditions replicated in the lab

The research, carried out with colleagues in our School of Engineering, was designed to examine the toughness of the shells of the mussels in the more acidic water against those in control conditions.

Common blue mussels, Mytilus edulis, were housed in laboratory tanks. The researchers controlled and altered the temperature and CO2 levels of the water in the tanks to simulate four different types of ocean waters at CO2 levels projected to occur in the coming decades (380, 550, 750, 1000 ppm).

Ocean conditions were also simulated as closely as possible by changing light levels over time to mimic the changing of the seasons.

Another finding was that the impact of the increased acidity reduced as temperatures increased: "The effect on the mussels' shells was reduced when the temperature of the water was increased by 2°C. This might suggest that the mussels are reverting to ancestral evolutionary mechanisms to mitigate the effects of increased acidity."

Now the team is planning to extend its research to include other marine organisms, says Dr Fitzer: "We're planning to continue our research in this area in the future and expand its scope to look at the effects of more acidic water on the shells of other marine organisms including oysters and abalone."



The paper: 'Ocean acidification alters the material properties of Mytilus edulis shells', is published in Interface.

The research was funded by the Leverhulme Trust awarded to the research team including Professor Maggie Cusack, Dr Nick Kamenos and Dr Vernon Phoenix.



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