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. Analyses by Gu et al. (2022) and Lai et al. (2022) claim 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 Feng et al. (2023) claims that shellfish aquaculture is a carbon-negative technology, taking into account LCA (life cycle analysis) of the global potential of this practice. Furthermore, a slightly difference approach was taken by Alonso et al. (2021) by claiming that CaCO3 in bivalve shells can replace part of the current supply for this mineral, resulting in a more circular carbon economy and therefore more stored carbon.
Critical Perspectives on Bivalve Carbon Sequestration
Despite the potential benefits, some studies offer critical perspectives on bivalve carbon sequestration. A paper by Ray et al. (2017) suggested that while bivalves sequester carbon in their shells, they release CO2 during shell production – and this must be taken into consideration when assessing the true LCA of shellfish aquaculture. Additionally, a paper by Pernet et al. (2024) claims that there is no scientific basis behind the idea that bivalve farming is a carbon sink and that there are no observational or experimental studies that sufficiently refute this. The title of this paper states it bluntly: Bivalve farming is not a CO2 sink.
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, fully 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.
References:
Gu, Y., Lyu, S., Wang, L., Chen, Z., & Wang, X. (2022). Assessing the carbon sink capacity of coastal mariculture shellfish resources in China from 1981–2020. Frontiers in Marine Science, 9, 981569.
Lai, Q., Ma, J., He, F., Zhang, A., Pei, D., & Yu, M. (2022). Current and future potential of shellfish and algae mariculture carbon sinks in China. International Journal of Environmental Research and Public Health, 19(14), 8873.
Feng, J.-C., Sun, L., & Yan, J. (2023). Carbon sequestration via shellfish farming: A potential negative emissions technology. Renewable and Sustainable Energy Reviews, 171, 113018
Alonso, A. A., Álvarez-Salgado, X. A., & Antelo, L. T. (2021). Assessing the impact of bivalve aquaculture on the carbon circular economy. Journal of Cleaner Production, 279, 123873.
Pernet, F., Tremblay, R., Royer, S., Salvo, F., Saurel, C., Bernard, I., Paillard, C., Robbins, I., Jeffrey, N., & Morvezen, R. (2024). Cracking the myth: Bivalve farming is not a CO₂ sink. Reviews in Aquaculture.
Ray, N. E., Al-Haj, A., Wallace, R. B., & Gobler, C. J. (2017). Consideration of carbon dioxide release during shell production in LCA of bivalves. International Journal of Life Cycle Assessment, 22(11), 1799–1810.
