One significant effect of climate change is that the ocean is becoming increasingly acidic. This has implications for marine life, including farmed shellfish such as oysters and mussels. Dr Susan Fitzer at the University of Stirling is investigating what climate change means for shellfish farming, and how aquaculture could adapt to keep thriving.
Scotland is a world leader in aquaculture, including the farming of marine bivalves. These farms are found in sea lochs and bays around Scotland’s coast, where mussels, oysters and scallops are grown in seawater. Once harvested, the shellfish are sold to restaurants and retailers in the UK, Europe and beyond. Bivalve farming is often considered one of the most sustainable forms of animal protein production, but there are concerns that the industry could be threatened by the effects of climate change.
As more carbon dioxide enters the atmosphere, a significant proportion is absorbed by the ocean. Whilst this helps reduce the effects of global warming, it instead makes the ocean more acidic, which threatens a variety of marine ecosystems and the species that are found in them. This also applies to aquaculture, as shellfish are potentially some of the most vulnerable species to ocean acidification. Dr Susan Fitzer, who works at the University of Stirling’s Institute of Aquaculture, is studying the effects of ocean acidification on shellfish to help the aquaculture industry prepare for the future.
OCEAN ACIDIFICATION
Shellfish produce their shells using calcium and carbonate minerals as building blocks. This process of a living organism producing a non-organic product is known as biomineralisation. “Biomineralisation is used by marine invertebrates such as corals, sea urchins, mussels and oysters to produce protective structures in the form of calcium carbonate shells and skeletons,” says Susan. As well as serving a useful biological function, these shells are also important for aquaculture: strong shells lead to greater yields as fewer shellfish are lost to predators or storms, and fewer are damaged in the harvesting and transport process, too.
“Ocean acidification is caused by the ocean uptake of atmospheric carbon dioxide, which dissolves into seawater to form carbonic acid,” says Susan. “This reduces the pH of seawater, as well as reducing the carbonate available to produce calcium carbonate shells and skeletons.”
Ocean acidification affects shellfish in three main ways:
- Hypercapnia is the retention of carbon dioxide in an organism’s tissues. For shellfish, hypercapnia leads to reduced shell growth as energy is diverted to metabolic processes that are impacted by this carbon dioxide increase.
- Reduced carbonate in the seawater means there are fewer minerals available for shellfish to take up from the environment to build their shells.
- Acidic conditions can actively dissolve calcium carbonate shells and skeletons.
“There are two main forms of calcium carbonate shells: aragonite and calcite,” says Susan. “Species can use one or the other, or a mixture of the two.” Research has indicated that aragonite is more affected by ocean acidification than calcite, which means that species that primarily use aragonite are likely more vulnerable.
CARBONATE PATHWAYS
Susan’s lab is investigating how bivalves respond to ocean acidification, in particular how their shell-building process is altered. “Marine animals can source carbonate from several different routes,” she explains. “They can source it directly from seawater; produce hydrogen carbonate through a protein-mediated process; or take carbonate produced from metabolic processes directly from tissues.” Susan collects bivalves from the field and grows mussels and oysters in the lab to see how they respond to seawater of different acidities.
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THE STATE OF PREY
However, given that any lab experiment is by necessity a simplification of the real world, it is possible that this is only part of the story. The ocean is a complex environment, and any one change will lead to a cascade of other changes – not just for shellfish, but also for the organisms they eat, for instance. “Mussels eat micro-algae – tiny single-celled plants,” says Susan. “Increased carbon dioxide in seawater may boost micro-algae growth, since they use carbon dioxide for photosynthesis.” For mussels, this may counteract some of the negative effects of ocean acidification, since they would have more food (and therefore more energy) available.
Susan grew micro-algae in the lab to understand how they respond to different levels of ocean acidification: “I found that when mussels were fed more micro-algae under more acidic conditions, the mussels were able to use this food source to continue to grow shells through metabolic routes of carbon uptake.” This is promising, but given the many interacting factors of the marine environment, more research is needed to accurately predict future changes.
BUILDING MUSSEL MASS
“Selective breeding is a promising route for growing larger and stronger mussels and oysters under more acidic conditions in the future,” says Susan. “In future research, I will examine the toughness of mussel shells throughout the harvest and transport process, and whether this will change as the environment changes.”
Susan’s research has resulted in plenty of useful insights for aquaculture. The different responses of aragonite and calcite indicates that different species will likely respond in different ways to ocean acidification, which may lead to changing species or breeding preferences within the aquaculture industry. Her research also indicates that metabolic routes of carbon uptake are preferable for shell growth, and can be promoted through increased concentrations of micro-algae. By modifying aquaculture practices to pre-empt these changes, shellfish farmers can have a head start in adapting to the effects of climate change.
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Futurum, 20 September 2021. Full article.
