The concept of using clams as a carbon sink in aquaculture or mariculture has gained attention in recent years due to its potential benefits in sequestering carbon and possibly helping to mitigate some aspects of climate change. Clams, like all bivalve mollusks, are filter feeders that can sequester carbon in their shells and tissues. However, the unique characteristic of giant clams is that they have a symbiotic relationship with algae that not only results in their much larger size, but also much greater carbon precipitation in shell building and through zooxanthellae photosynthesis. There is much debate around this form of carbon sequestration, as numerous scientists have claimed that it is not carbon-negative since CO2 is released during shell precipitation. The IMARCS Foundation is pioneering novel research to determine if changes in water chemistry, namely increasing pH levels and making calcium more abundant, can result in this process becoming definitively carbon negative. While that research is currently underway, the results will not be known for several months until all relevant data has been collected and analyzed. WIth that, we present below the current understanding of the potential for using clams as a carbon sink, as some scientists believe it is in fact already true sequestration.
Mechanism of carbon sequestration in clams and giant clams
Clams, like other bivalves, sequester carbon primarily through two processes: shell formation and biomass production. The calcium carbonate (CaCO₃) in clam shells contains carbon, which is extracted from the surrounding water and fixed into a solid form. Additionally, the organic matter in clam tissues also contributes to carbon sequestration. This is true for giant clams as well, but their symbiotic relationship with zooxanthellae algae allows for carbon intake during photosynthesis and, as a result, much more shell growth and greater biomass. The key variable with all bivalves is if this method of carbon fixing actually reduces atmospheric CO2 that has been dissolved in seawater or in artificial tank environments where they are grown.
Relevant studies and findings that support 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.
Studies claiming clam or bivalve aquaculture does not result in net 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.
Is building CaCO₃ shells a carbon-negative process?
The process of shell formation in clams involves the sequestration of carbon in the form of calcium carbonate (CaCO₃). However, while this process does indeed sequester carbon, it is not entirely carbon-negative due to the associated biochemical reactions and energy expenditures.
Clams extract calcium (Ca²⁺) and carbonate (CO₃²⁻) ions from seawater to form CaCO₃. The basic reaction is Ca(2+)+CO3(2−)→CaCO3. This reaction effectively sequesters carbon in the form of solid calcium carbonate, which is stable and long-lasting. However, the formation of CaCO₃ in clam shells involves several biochemical processes that can result in CO₂ emissions for two important reasons:
- Respiration: Clams respire, which produces CO₂ as a byproduct.
- Biochemical Calcification: The process of converting bicarbonate (HCO₃⁻) to carbonate (CO₃²⁻) to use for calcium carbonate shell production releases CO₂.
The simplified reactions are: HCO3(−)→CO3(2−)+H(+)
H(+)+HCO3(−)→CO2+H2OH(+)+HCO(3−)→CO2+H2O
These reactions indicate that while CaCO₃ formation sequesters carbon in the shell, the act of creating it is accompanied by the release of CO₂. This means that the formation of calcium carbonate (CaCO₃) shells in clams involves carbon sequestration but is not, at least in nature, fully carbon-negative due to the associated CO₂ emissions from respiration and biochemical calcification reactions that are released into regular seawater, with pH levels that are too low to hold excess CO2 (and are only lowering further as more CO2 is absorbed by the oceans globally).
Discussion
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.
Giant clams may potentially offer a unique and valuable tool for atmospheric carbon storage and removal due to their size, symbiotic relationship with photosynthetic zooxanthellae, and, most importantly, through altered tank conditions that are currently being studied. Research pioneered by the IMARCS Foundation is 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. Further research and careful management are essential to optimize the carbon sequestration potential of giant clam mariculture and to determine if net carbon storage is possible through altered water conditions.
As the world seeks innovative solutions to combat climate change, integrating giant clams into mariculture systems presents a potential - perhaps even promising - path forward for net negative carbon sequestration. By addressing the challenges of carbon fixation and leveraging the benefits of altered water chemistry, giant clams can play a significant role in mitigating the impacts of climate change, all while supporting marine biodiversity and providing economic opportunities for coastal communities through restoration activities piloted by the IMARCS Foundation in addition to CO2 storage research.
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.