A hard shell: how mussels are affected by ocean acidification

Scientists studying calcium in a marine environment have discovered a direct link between the acidification of the seas in a changing climate and the rate at which mussels develop their calcified outer shell. The shell of a mussel protects it from predators and is formed at a very early stage of development. At this point, they are particularly sensitive to low pH levels in the ocean caused by increasing uptake of carbon dioxide from the atmosphere dissolved in seawater.

An EU-funded study is looking at how acidification affects the largely unexplored mechanisms behind the calcification, growth, malformation and dissolution of mussel shells. The findings of CACHE (Calcium in a CHanging Environment) help explain how the larvae of bivalves such as mussels form shells under moderate acidification scenarios and provide a direct link between ocean carbonate chemistry and the rate at which larvae calcify.

A new look at how mussel shells develop

Mussels begin to form their shells when they are just a day old. In a report published in ‘Nature Communications’, the researchers outline how they used micro-electrodes and fluorescent dyes to track the deposition of calcium carbonate in one- to two-day-old shelled larvae. They found that calcium was not formed intracellularly, as previously thought, but is more likely extracted from seawater and transported via specific proteins before calcium carbonate is formed.

They then studied the conditions directly beneath the shell. This examination showed that the larvae were able to increase the pH and carbonate concentration beneath their shell, leading to higher rates of calcification. In more acidic conditions, the larvae’s ability to calcify decreased; at the highest CO2 concentrations, researchers noted greater dissolution of shells, leading to increased mortality.

Bivalves provide several essential ecosystem services, including as bioindicators to monitor the levels of pollutants in a body of water, and as biofilters. They are particularly vulnerable to ocean acidification, but until now, little has been known about how they regulate calcium to produce a shell, how this process might be affected by changing environmental conditions, and what the consequences are for the population. This lack of knowledge restricts the ability to predict future biodiversity and the consequences for the aquaculture industry. The research carried out under CACHE suggests that the reason for the high sensitivity of mussel larvae to acidification is their limited capacity for ion regulation.

Because molluscs take a soluble form of calcium in seawater and turn it into an insoluble compound, without requiring huge amounts of energy, understanding the process may have potential applications for biotechnology. The aim of CACHE is to increase knowledge about calcium production in the marine environment and train young scientists to tackle complex biological problems using multidisciplinary approaches. Understanding how commercially important molluscs produce their shells and regulate shell production under different environmental conditions should also provide insight into how the species will fare in a changing climate and provide resilient stocks for aquaculture.

For more information, please see:
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