Mussel shells become more brittle due to climate change

Mussel’s shells become more brittle as climate change causes oceans to become increasingly acidic, a team of scientists from the University of Glasgow wrote in the Royal Society’s journal Interface.

Our oceans are becoming more acidic as they absorb some of the carbon dioxide (CO₂) in the atmosphere that contributes to climate change.

The seawater reacts with the CO₂, which is slowly reducing the pH of the oceans. By the end of the 21st century, scientists believe the pH of the world’s oceans will have dropped to 7.7, compared to 8 today.

The shells of mussels are composites of calcium carbonate and organic material created by the bivalve molluscs through a process known as biomineralization.

Mussels form their two-layer shells by drawing bicarbonate ions from seawater and proteins in their bodies to make crystals of calcium carbonate. The outer layer of their shells are composed of calcite, while the inner layer is made of aragonite.

When the pH level of water falls (or acidity rises), there are fewer bicarbonate ions available for the mussels to make their shells.

Team leader, Dr. Susan Fitzer of the University’s School of Geographical and Earth sciences, and colleagues kept common blue mussels in laboratory tanks.

The scientists altered the temperature and pH levels of the water contained in the tanks to simulate four different levels of acidity. Their aim was to create environments that reflected what seawater would be like in the coming decades.

They also simulated ocean conditions with different levels of light depending on the seasons.

With weaker shells, mussels will find it harder to withstand the ocean forces. (Photo: NOAA)

Dr Fitzer said:

“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.”

“We worked with colleagues in our School of Engineering to examine the toughness of the shells of the mussels in the more acidic water against those in control conditions. 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.”

This may mean that mussels growing in the wild could be more vulnerable to attack in the future from predators, as well as the effects of the elements.

The fishing industry will likely notice a significant decline in the availability of blue mussels, the type most commonly consumed by humans.

Dr. Fitzer added:

“However, we also found that 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.”

“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.”

Citation:

“Ocean acidification alters the material properties of Mytilus edulis shells,” Susan C. Fitzer, Wenzhong Zhu, K. Elizabeth Tanner, Vernon R. Phoenix, Nicholas A. Kamenos, Maggie Cusack. The Royal Society Interface. DOI: 10.1098/rsif.2014.1227. Published 24 December 2014.