Musseling through climate change

Our planet’s climate is pretty old — an estimated 3.5 billion years old, in fact. Understanding how Earth’s climate has changed since then is important for predicting and coping with climate change today and in the future. But, because it is hard to know exactly what happened a couple of billion years ago, climate scientists use mathematically constructed models that take into account abiotic, or non-living, factors like carbon dioxide levels and ocean chemistry in the past to predict weather patterns in the future. Ultimately, the goal is to predict how biotic factors — living things like us — will be affected. 

These mathematical models are a work in progress. Often, they are made using data from field studies conducted over periods of only one to two years. Additionally, many models do not factor in biological mechanisms for plasticity that allow organisms to adapt to changing environmental conditions. These gaps were the impetus for a study conducted by Dr. Luca Telesca and colleagues, recently published in Global Change Biology. Their work investigated shell shape and body structure in archival specimens (read: preserved in ethanol, not fossilized) of the blue mussel (Mytilus edulis) collected roughly every decade between 1904 and 2016 along 15 kilometers of Belgium’s coast (Fig. 1). Measurements of the mussels themselves were coupled with extensive long-term datasets of coastline environmental conditions over the past century, all of which were obtained from collections at the Royal Belgian Institute of Natural Sciences

Fig. 1: Study location, along 15 km of Belgian coast between the cities of Ostend and Nieuwpoort (starred). Image source: Google Maps.

The blue mussel is not your typical specimen in an archival collection. Common animals often aren’t considered worth preserving for the historical record. However, it’s precisely because they are common that species like the blue mussel make great barometers for environments gone by. The blue mussel in particular is an example of a “calcifying foundation species,” species so named for their ability to sequester and store calcium and carbon from the surrounding water (see Fig. 2), and their habitat on shallow marine floors. This calcifying ability, or biomineralization, is the process by which living organisms convert non-living organic substances into still-non-living inorganic derivatives. It is an astoundingly ubiquitous process: all six taxonomic kingdoms from single-cell organisms in Archaea to mammals like us — we’re in the kingdom Animalia — contain organisms capable of biomineralization. The bones in our bodies are an example of this, the result of binding calcium phosphate from our diets into a different, crystallized form of calcium called hydroxyapatite. Furthermore, because biomineralization is an easy-to-measure, direct interaction between biotic and abiotic factors, it is an ideal study for climate scientists. 

Fig. 2: A typical blue mussel shell and cross-section. After calcium carbonate crystals are absorbed from the surrounding water, they become layered with secreted structural proteins from the mussel’s body tissue, or mantle. These layers of calcium carbonate and secreted proteins form the mussel’s shell, the thickness of which can vary depending on how much calcium carbonate is absorbed. Image created using Biorender.com.

One of the most pressing concerns presented by rapid climate change today is ocean acidification, characterized by an increase in oceanic carbonic acid resulting from elevated levels of carbon dioxide in the atmosphere. Excess carbonic acid increases the acidity of ocean water, which can dissolve shells, and decreases the availability of calcium carbonate, the nutrient that mussels and other ocean biomineralizers use to form shells in the first place. Ocean acidification has had a negative impact on many species; one notable impact is on coral in the Great Barrier Reef. Given these known effects of climate change and ocean acidification on many ocean calcifers, the authors predicted that they would observe a steady decrease in shell size between 1904 and 2016. 

Instead, to their surprise, they observed a marked increase in blue mussel shell size since 1904. The team’s results hold a number of implications for predictive climate change modeling. First, the findings signify that archival collections of organisms from the past can and should be used to influence our current predictions about what’s to come in any given biome, 10, 20, or 100 years from now. Second, and quite hopefully, the findings suggest that mussel populations somehow acclimated to shifting environmental conditions along the Belgian coast over the past century. The authors speculate that this could be because rising ocean temperatures could actually increase calcification, combating dissolution induced by acidic conditions, or that rising water temperatures may have increased the availability of a specific food source. Altogether, the potential for compensatory mechanisms in this study population of blue mussels points to the same potential in other species for coping with rapid environmental change over the next century. As we continue to update predictive models with data from the past and study and protect the populations most vulnerable to rapid climate change, we may find ways to help them mussel through yet. 

 

Dr. Telesca is a postdoctoral research scientist affiliated with Columbia University’s Earth Institute and the Lamont-Doherty Earth Observatory.

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