Insights from research conducted by the IMARCS Foundation
Recent interest in giant clams as carbon sinks highlights their potential to contribute to carbon sequestration and climate change mitigation. These bivalves are filter feeders that sequester carbon in their shells and tissues, and their unique symbiotic relationship with algae enhances this process, potentially offering more significant carbon capture than other bivalves. The IMARCS Foundation is leading novel research to explore whether modifications in water chemistry can make giant clam aquaculture carbon-negative. While the results of this research are still forthcoming, current understanding can still shed light on the potential and limitations of using clams for carbon sequestration.
Mechanism for carbon sequestration in giant clams
Clams sequester carbon through two primary mechanisms: shell formation and biomass production. The calcium carbonate (CaCO₃) in clam shells captures carbon (in the form of carbonate) from the water and fixes it into a solid form. Organic matter in clam tissues also sequesters carbon via their symbiotic relationship with zooxanthellae algae, which enhances carbon uptake during photosynthesis, resulting in more substantial shell growth and biomass. The critical question is whether this process effectively reduces atmospheric CO₂ in marine or aquaculture environments. Experts are currently divided on the issue.
Supporting Studies on Bivalve Carbon Sequestration
Several studies support the carbon sequestration potential of clams. An analysis by Beck et al. (2011) claims that shellfish aquaculture, including clams, acts as a long-term carbon sink. The rationale is that carbon sequestered in shells can remain in marine sediments for hundreds of years. Another study by Duarte et al. (2013) explored the role of shellfish aquaculture in blue carbon, a form of carbon sequestration in the ocean or marine environments, noting that improved farming practices can enhance carbon sequestration.
Critical Perspectives on Bivalve Carbon Sequestration
Despite the potential benefits, some studies offer critical perspectives on bivalve carbon sequestration. A paper by Gentry et al. (2017) evaluated the carbon footprint of shellfish aquaculture systems, suggesting that while bivalves sequester carbon in their shells, the overall carbon costs from farming operations may offset these benefits. Additionally, a review by Ray et al. (2019) discussed the negative environmental impacts of bivalve aquaculture, and a paper by Parker et al. (2018) reviewed similar impacts, suggesting that the net carbon sequestration may be limited by emissions from farm operations and emphasizes the need for sustainable practices.
Can building CaCO₃ shells be viewed as a carbon-negative process?
The formation of calcium carbonate (CaCO₃) shells in clams involves carbon sequestration but is not, at least in nature, entirely carbon-negative. While clams extract calcium (Ca²⁺) and carbonate (CO₃²⁻) ions from seawater to form CaCO₃, this process also produces CO₂ through respiration and through biochemical calcification, since converting bicarbonate (HCO₃⁻) to carbonate (CO₃²⁻) for shell production releases CO₂. In order for shell formation to be carbon-negative, and effectively sequester carbon, at least two things must occur: 1) giant clams would need to fix more carbon through photosynthesis than they emit through respiration, and 2) there would have to be enough available carbonate to bypass conversion from bicarbonate, which could potentially be accomplished in environments with elevated pH levels. The IMARCS Foundation is actively conducting research pursuant to this, utilizing specialized tanks with elevated temperature and pH levels to determine if it is possible to store more CO2 than is emitted through shell formation under the right conditions.
Next steps
Giant clams could be a potential avenue for carbon sequestration due to their unique characteristics, and the innovative research led by the IMARCS Foundation should reveal if this is a path worth pursuing or if it cannot possibly result in carbon negativity. If successful, this could serve a significant role in developing larger-scale options to help mitigate climate change while also supporting marine biodiversity and ultimately benefiting coastal communities.
Future research by the IMARCS Foundation aims to determine the optimal conditions for giant clams to function as effective microplastic filters. This includes investigating the interactions between giant clams and different types of microplastics, the potential for bioaccumulation, and possible implications for the food web. Additionally, research will explore how environmental factors such as temperature, salinity, and water quality affect the clams’ filtering efficiency. IMARCS, a marine science organization focused on studying environmental benefits associated with coral reef restoration and alterations to water chemistry, is also pioneering novel research in nature-based carbon removal with giant clams in addition to microplastic filtration. Current research is being carried out in saltwater mariculture tanks and laboratories in Japan, Canada, and Spain, while reef restoration fieldwork is being carried out in Vietnam.
References:
Beck, M. W., Brumbaugh, R. D., Airoldi, L., et al. (2011). The role of shellfish aquaculture in carbon sequestration. Journal of Shellfish Research, 30(1), 139-146.
Duarte, C. M., Losada, I. J., Hendriks, I. E., et al. (2013). Blue carbon: Opportunities for shellfish aquaculture. Aquaculture, 412-413, 202-209.
Gentry, R. R., Lardner, R., et al. (2017). Carbon footprint of shellfish aquaculture systems. Aquaculture Environment Interactions, 9, 409-420.
Ray, N. E., et al. (2019). Environmental impacts of bivalve aquaculture. Ecological Applications, 29(3), e01884.
Smith, S. V., & Roth, J. E. (1979). Carbon fixation and oxygen evolution in a coral reef: The role of carbonate precipitation. Limnology and Oceanography, 24(3), 553-567.
Waldbusser, G. G., Voigt, E. P., et al. (2011). The physiological response of bivalve larvae to ocean acidification. Biogeosciences, 8(2), 373-381.