Cracking the myth: Bivalve farming is not a CO_ 2 sink

Bivalve farming was usually considered as a CO 2 source through respiration and calcification, but recent studies suggest its potential as a CO 2 sink, prompting exploration of its inclusion in carbon markets. Here we reviewed the scientific basis behind this idea and found that it is not supported by observational and experimental studies. This idea indeed arises from carbon budget models that are based on theoretical misconceptions regarding seawater carbonate chemistry. The main misunderstanding consists of assuming that the carbon trapped in the shell originates from atmospheric CO 2 when it mostly comes from (bi)carbonate ions. While these ions originate from atmospheric CO 2 through the erosion of minerals over geological time scales, their incorporation into shells does not prompt short-term CO 2 compensation. The opposite occurs — calcification releases CO 2 in seawater and limits or even prevents the uptake of atmospheric CO 2 . Some authors suggest that considering the bivalve farm ecosystem could change the perspective on the source/sink issue but there is no evidence for that now. Most ecosystem-based carbon budget models rely on several unverified assumptions and estimates. Although challenging, field measurements must be conducted for monitoring, reporting, and verifying atmospheric CO 2 uptake before qualifying for carbon credits. To achieve scientific consensus, we need reinforcing measurement-based studies of CO 2 fluxes in shellfish ecosystems, integrating carbon balance models with observational and experimental science, and fostering interdisciplinary collaboration. Although bivalve farming provides numerous environmental benefits and is vital for sustainable aquaculture, there is currently no evidence that it contributes to CO 2 capture.


| INTRODUCTION
The latest Intergovernmental Panel on Climate Change (IPCC) report delivers a clear message: urgent actions are required to reduce carbon dioxide (CO 2 ) emissions, mitigate climate change, and ensure the future habitability of our planet. 1At the current rate of CO 2 emissions, the remaining carbon budget for a 50% chance to reach the Paris Agreement will be spent in 2029. 2 In practical terms, netzero CO 2 emissions must be reached by 2050, necessitating the implementation of carbon dioxide removal (CDR) strategies in addition to deep cuts of CO 2 emissions.CDR is necessary to balance residual emissions that are technically difficult to stop, such as agricultural and industrial emissions.Among the most well-known CDR methods are afforestation, reforestation, carbon-sequestering agricultural practices, wetland restoration, as well as blue carbon approaches.Blue carbon is defined by the IPCC as biologically driven carbon fluxes and storage in marine systems that are amenable to management. 3 Coastal blue carbon currently focuses on rooted vegetation, such as tidal marshes, mangroves, and seagrasses. 4ile the traditional view is that shellfish farming (mainly bivalves) acts as a CO 2 source due to respiration and calcification processes, an alternative is becoming increasingly popular.Some recent studies suggest that bivalves could serve as CO 2 sinks with the potential to combat climate change in a way comparable to terrestrial forests or seaweeds (e.g., References [5, 6]).This idea has resonated within the shellfish farming community and made its way into policy in government departments, ministries, and professional organizations.Consequently, there is now a serious exploration of the possibility of incorporating shellfish into carbon markets and, perhaps, offering subsidies or credits to shellfish farmers who may contribute to CO 2 capture and the battle against climate change. 7Here, we evaluate the concepts behind the idea that bivalve aquaculture acts as a CO 2 sink and conclude that it is based on theoretical misconceptions and, to the best of our knowledge, not supported by observational and experimental studies.

| PRINCIPLES GOVERNING AIR-SEA CO FLUXES AND CARBONATE CHEMISTRY
The air-sea CO 2 exchange is driven by the difference in the partial pressure of CO 2 (pCO 2 ) between these two compartments.Overall, the surface ocean has lower pCO 2 than the atmosphere, allowing it to be a net CO 2 sink capturing about 25% of anthropogenic emissions. 8hancing this sink through marine CDR (mCDR) requires pCO 2 reduction in the surface ocean to allow for more uptake.
The capacity of the ocean to act as a CO 2 sink primarily stems from the transformation of the majority of aqueous CO 2 into carbonic acid (H 2 CO 3 ) which rapidly dissociates in other forms of inorganic carbon, bicarbonate (HCO 3 À ) and carbonate (CO 3

2À
) ions that are not readily re-released into the atmosphere.Reactions are summarized by the following reversible equations: The sum of these three species is referred to as dissolved inorganic carbon (DIC): The uptake of CO 2 by the ocean leads to its acidification (increase in hydrogen ions = decrease in pH), it also increases the Note, however, that CO 2 is the only form of inorganic carbon that is exchanged with the atmosphere.
Total alkalinity (TA), is a measure of the capacity of seawater to resist sudden changes in pH by absorbing hydrogen ions using available bases such as HCO 3 À and CO 3 2À .Note that CO 3 2À can absorb twice as many hydrogen ions as HCO 3

À
. This means that CO 3 2À has twice the impact on total alkalinity compared with HCO 3 À .Other ions also contribute to TA, but they are minor contributors.TA is given by the following equation: Other things being equal, TA is an indicator of the ocean's capacity to store CO 2 . 9Differential changes in DIC and TA, mediated by physico-chemical or biological processes, can modify pCO 2 and control the direction of the air-sea CO 2 flux 9,10 (Figure 1).If DIC decreases more rapidly than TA, seawater pCO 2 decreases, leading to an increased uptake of atmospheric CO 2 by surface waters.In contrast, if TA decreases more rapidly than DIC, pCO 2 increases, diminishing the capacity of surface waters to absorb atmospheric CO 2 .
The impact of photosynthesis and respiration on air-sea CO 2 fluxes is intuitive.Indeed, primary production through photosynthesis consumes CO 2 (and thus DIC) to produce organic carbon by releasing dioxygen (O 2 ), while conversely, respiration uses organic carbon and O 2 to produce energy and release CO 2 (Equation 4).Photosynthesis and respiration have a negligible effect on TA 12 (Figure 1) allowing the changes in DIC to be reflected in changes to pCO 2 and air-sea fluxes. 10Summarizing, photosynthesis tends to increase while respiration tends to decrease the capacity of surface waters to absorb atmo- The impacts of calcium carbonate (CaCO 3 ) precipitation and its reverse process, dissolution, are less intuitive.Marine organisms utilize inorganic carbon to synthesize shells, tests, or skeletons composed of CaCO 3 .Calcification is described by the following reversible reaction: This equation shows that CO 2 is generated because of CaCO 3 precipitation (Figure 1).The production of 1 mole of CaCO 3 consumes 2 moles of HCO 3 À (and TA).Quantitatively, however, <1 mole is released because of the seawater buffering capacity 13 (see Appendix A).The dissolution of CaCO 3 yields the opposite effects.
Overall, CaCO 3 production tends to decrease while its dissolution tends to increase the capacity of surface waters for atmospheric CO 2 uptake (see Appendix A for further details).The impact of biogenic calcification on the CO 2 cycle and climate is not trivial: the deposition of CaCO 3 within coral reefs may have contributed to the increase in atmospheric CO 2 during the last glacial-interglacial transition (References [13,14] and references therein).

| CHALLENGES IN ATTRIBUTING A CO 2 SINK OR SOURCE ROLE TO SPECIES AND ECOSYSTEMS
The intricacy of biogeochemical and physical processes complicates the assessment of whether a species is a net CO 2 source or sink.
Although primary producers taken in isolation are CO 2 sinks, and animals, especially heterotrophic calcifiers, are sources, the reality is more nuanced at the ecosystem scale.The distinction between CO 2 source and sink status depends, among other things, on the fate of organisms in the ocean at the end of their life (sequestration or remineralization) and their ecological role in driving other processes.An illustrative example is the case of seagrass beds.Typically, they are considered as net primary producers contributing significantly to blue carbon in coastal ecosystems. 4However, recent findings reveal that the decrease in pCO 2 resulting from seagrass photosynthesis may be surpassed by the CO 2 released by the calcification of organisms living in association with them. 15In that case, the ecosystem becomes a source of CO 2 .Conversely, calcifying species acting as CO 2 sources as taken individually can theoretically favour the growth of primary producers such as seaweeds and therefore promote CO 2 uptake at the ecosystem level. 16While a species can act as either a source or a sink of CO 2 , its effect can be mitigated, or even reversed by the other species that exert an opposite effect on the carbonate chemistry.

| MARINE SHELLFISH AQUACULTURE AS A BLUE CARBON STRATEGY?
Several review papers have explicitly identified ecosystems dominated by calcifiers, such as natural shellfish beds or aquaculture production areas, as potential blue carbon ecosystems. 17,18We, therefore, carried out the most exhaustive literature review possible and found a total Distribution of the partial pressure of CO 2 ( pCO 2 ) in total alkalinity (TA) and dissolved inorganic carbon (DIC) phase space, calculated at temperature = 15 C and salinity = 35 using the R package seacarb. 11The black lines are drawn at the pCO 2 values labelled on the colour legend.Vectors represent the effect of biogeochemical processes.Uptake and release of CO 2 into/from the ocean changes only DIC (and therefore pCO 2 ), whereas photosynthesis and respiration also slightly change TA owing to nutrient uptake and release.Calcium carbonate (CaCO 3 ) formation decreases TA and DIC in a ratio of 2:1, and increases pCO 2 .CaCO 3 dissolution has the reverse effect.For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.
of 51 articles on the role of bivalves in the carbon cycle and their impact on CO 2 levels (Table 1).For each paper, we listed the taxonomic or functional group, the approach used (in situ observation, experimentation, and carbon budget modelling), the type of ecosystem (natural vs. farmed populations), and the conclusion of the study concerning the role of bivalves on the source/sink status.
The idea that bivalve farming is a CO 2 sink originated from the study by Tang et al., 6 which investigated carbon removal in a coastal ecosystem dedicated to shellfish and seaweed harvesting.Their estimates relied on the carbon content of these organisms and annual production statistics.From these estimates, authors concluded that cultivating shellfish significantly contributes to carbon sequestration, thereby enhancing coastal ecosystems' ability to absorb atmospheric CO 2 .Then, several other papers used similar theoretical carbon budget models and reached similar conclusions (Figure 2).
Of the studies examined, 43 are theoretical budget models and only 8 are based on observational or experimental measurements.
Among the theoretical carbon budget studies, 28 conclude that bivalves are CO 2 sinks, while 15 conclude the opposite.Seven out of eight observational and experimental studies also indicate that bivalves are not CO 2 sinks.Among these observational and experimental studies, the sole exception is the paper by Fodrie et al., 28 which concludes that wild oyster reefs can operate as either sources or sinks based on their tidal position.We will see that the majority of the budget model studies were built upon theoretical misconceptions regarding seawater carbonate chemistry.

| Shell formation is a carbon sink but not a CO 2 sink
Most of the theoretical budget models mentioned in the preceding section consider that the carbon contained in the CaCO 3 shell comes from atmospheric CO 2 rather than HCO 3
This misconception arises from the belief that HCO 3 À in the ocean primarily comes from atmospheric CO 2 .Overall, HCO 3 À comes from two sources: direct CO 2 capture at the ocean surface and chemical weathering of sediments and rocks.
The direct CO 2 capture at the surface ocean represents a minor contribution to the oceanic pool of HCO 3 À .The atmosphere is indeed a relatively small reservoir of carbon compared with the ocean (ca.600 vs. 38,000 Gt C, respectively).To illustrate this, considering pre-industrial, atmospheric CO 2 concentrations at 280 ppm and in equilibrium with the ocean, HCO 3 À represents 89.5% of the oceanic pool of DIC.At the current atmospheric CO 2 concentrations of 420 ppm, the concentration of CO 2 in seawater increases by 50% compared with the pre-industrial era, while that of HCO 3 À increases by only 7%, the latter now representing 92% of the DIC pool.Therefore, only 6% of the HCO 3 À pool available to bivalves originates from anthropogenic CO 2 .However, since equilibrium is not achieved, this figure is an overestimate.These calculations were obtained using a TA of 2300 μmol kg À1 , a temperature of 15 C, and a salinity of 35 using the R package sea carb. 11 contrast to direct atmospheric CO 2 capture, mineral weathering is the primary source of HCO 3 À for the ocean, as it involves the most important reservoir of carbon on Earth (sediments and rocks: ca.90 Â 10 6 Gt C). 68 Weathering is the reaction of atmospheric CO 2 and H 2 O with silicate and carbonate rocks.Briefly, CO 2 and rainwater produce carbonic acid (H 2 CO 3 ) which can dissolve silicate and carbonate rocks.The formation of biogenic CaCO 3 is the primary process through which a portion of CO 2 sequestered during land weathering cycles returns from the ocean to the atmosphere.However, weathering operates on timescales exceeding 10,000 years and this process is considered stable on shorter timescales such as decades or centuries. 69Therefore, the HCO 3 À used to build calcareous shells is not compensated by accelerated weathering and capture of atmospheric CO 2 in the short-term.Yet, the emission of CO 2 as a result of CaCO 3 production is immediate, and air-sea CO 2 equilibration occurs on timescales of weeks to years. 70Consequently, calcification is a shortterm CO 2 -releasing process, with compensation only occurring through accelerated weathering in the long term.
Overall, converting the carbon stored in CaCO 3 to CO 2 equivalent is incorrect and reflects a lack of consideration for the carbon cycle and more particularly of the size of carbon reservoirs and the time scales involved in the exchanges between them.For a more comprehensive understanding of the carbonate biogeochemistry, we recommend consulting Ridgwell and Zeebe 71 and Zeebe. 69te, however, that bivalves can convert metabolic CO 2 into (bi) carbonate ions through the action of the enzyme carbonic anhydrase, 72,73 thereby contributing to shell formation.While the proportion of metabolic CO 2 integrated into the shell is generally below 10%, it can reach up to 37% depending on factors such as species, age or size, and environmental conditions. 74In specific cases, such as clams exposed to ocean acidification over multiple generations, the contribution of metabolic CO 2 may range from 45 to 61%. 75This pathway reduces the amount of CO 2 released into seawater through respiration.
However, in theory, calcification using metabolic CO 2 has the same impact on seawater carbonate chemistry as calcification using HCO 3

À
or CO 3 2À (see Appendix B).Both processes result in a 2:1 decrease in the TA to DIC ratio and increase in pCO 2 .Despite the potential significance of metabolic CO 2 incorporation into bivalve shells, its contribution is markedly variable and challenging to quantify.

| Shell dissolution compensates the release of CO 2 from calcification
The above misconception leads to another one: assuming that exporting the carbon contained in the shells through harvesting qualifies as a T A B L E 1 Summary of the analysis of the scientific literature on the role of bivalves in the carbon cycle and their impact on CO 2 levels.Note: For each paper (n = 51), we listed the taxonomic or functional group studied, the approaches implemented (in situ observation, experimentation and modelling), the type of ecosystem (natural or farm environment) and the conclusion of the study concerning the role of bivalves as sources or sinks of CO 2 .
For carbon balance model studies, we noted whether the shell and/or the flesh were considered as CO 2 sinks.For experimental studies, no a priori hypothesis concerning the role of shells/tissues as a sink/source of CO 2 is formulated.These a priori hypotheses are therefore considered not applicable.Abbreviations: Exp, experiment; N, no; NA, not applicable; Y, yes.form of CO 2 sink or sequestration.This idea contradicts the definition of a CO 2 sink, which refers to any process, activity, or mechanism that removes CO 2 from the atmosphere. 3Removing carbon from the ocean is not necessarily equivalent to removing CO 2 from the atmosphere.
In fact, the opposite is true: shells should be left in seawater where their dissolution would absorb CO 2 , thereby compensating for the emissions from calcification. 34,42,76Dissolution of shells in seawater raises TA, reduces pCO 2 and increases CO 2 uptake 77,78 as demonstrated in the sediment of the intertidal zone. 79Some papers suggest that shells should be removed from the sea to prevent dissolution (see, e.g., Ref. [17]) or that the integrity of CaCO 3 should be preserved as long as possible. 80 Not only are shellfish not a CO 2 sink, but the inorganic carbon stored in their tissues and shells is often not sequestered on climate-relevant time scales (100 years or more).In major bivalve-producing countries like China and Europe, shell wastes from human consumption are incinerated as organic wastes, where they undergo high-temperature combustion that releases CO 2 in the atmosphere. 42,80For these countries, the amount of CO 2 released into the atmosphere each year would be equivalent to the emissions from nearly 1 million cars. 80The carbon stored in the flesh of seafood intended for human consumption is respired and therefore transferred as CO 2 in the atmosphere as well.Therefore, carbon in tissues and shells cannot be considered as sequestered, as reported in some carbon budget models (Table 1).
Advocating for carbon sinks is insufficient.It is indeed imperative to go beyond solely extracting CO 2 from the atmosphere; the focus lies on sequestration within geological, terrestrial, or oceanic reservoirs, or in usable products on climate-relevant timescales.Carbon sequestration demands secure, dependable, and verifiable storage.
Ideally, carbon sequestration permanently confines CO 2 , preventing its future release into the atmosphere. 87,88This is not what bivalve aquaculture does.

| Toward an ecosystem-based carbon budget for bivalve farming
Filgueira et al. 89 proposed considering bivalve aquaculture within broader ecosystems to account for benthic-pelagic interactions and phytoplankton dynamics in carbon budgets.Secondary effects of bivalve cultivation such as enhanced primary productivity and nutrient cycling as well as the burial of organic carbon in the sediment through bio-deposits should indeed be considered in carbon budgets.According to the authors, a comprehensive analysis of all forms of carbon, as well as significant ecological relationships, feedbacks, and habitats, are required to rigorously quantify the role of cultured bivalves in the CO 2 budget of coastal ecosystems. 33,89 present, this framework exclusively focuses on assessing the influence of shellfish farming on organic and inorganic carbon, overlooking its potential effects on TA and air-sea CO 2 fluxes.This omission is obvious in the few studies that claim using such a comprehensive approach. 5,33,37,50This limited perspective provides an incomplete picture of the carbon cycle and requires further consideration before including other ecosystem compartments.The only study employing an ecosystem-based approach to carbon budget, considering both DIC and TA budgets, reveals that mussel production is a CO 2 source. 42Other studies that investigate in situ air-sea CO 2 flux, pH, and TA in farming ecosystems demonstrate that, in comparison to unfarmed control areas, shellfish sites exhibit a lower TA, higher seawater pCO 2 and a lower capacity to absorb atmospheric CO 2 39,41,47,51 as also reported in wild oyster reefs. 57e use of the ecosystem approach often relies on unverified assumptions.For example, it is sometimes assumed that the CO 2 released by respiration and calcification is rapidly used to stimulate primary production, thereby ignoring it in the carbon budget (e.g., Reference [6]).The main problem with this statement is that if photosynthesis uses the CO 2 generated by the bivalves, it will not absorb the seawater CO 2 it would have absorbed in the absence of bivalves.It is also noteworthy that primary production is rarely constrained by CO 2 .Many species of primary producers possess carbonconcentrating mechanisms that render them not responsive to CO 2 enrichment, [90][91][92] and limiting factors such as inorganic essential nutrients, particularly nitrogen and phosphorus, seawater temperature, and light could play a more decisive role. 93While the ammonia excretion of bivalves can enhance nutrient cycles and primary production in nutrient-limited ecosystems and the biodeposition can increase denitrification, 94 thereby potentially acting as indirect CO 2 sinks, it is crucial to verify these effects locally rather than making assumptions.
Likewise, bio-deposits are considered as CO 2 sink because they accumulate in the sediment and are therefore permanently separated from the marine water and biosphere. 5,46,53However, a fraction of the bio-deposit undergo remineralization, leading to CO 2 release.The other fraction can store carbon if the environment is suboxic or anoxic.Only a small fraction of benthic organic carbon may ultimately be sequestered within the sediment. 95While the rate of carbon burial can be elevated under certain conditions, such as in deep anoxic fjords 33 or shallow-subtidal oyster reefs compared with intertidal ones, 28 incorporating bio-deposits into the bivalve aquaculture carbon budget requires a careful assessment of their fate locally.
Filgueira et al. 89 also propose to make distinct CO 2 budgets for shells, separate from the overall organism, to evaluate the feasibility of incorporating shells into carbon trading systems.This involves allocating the CO 2 emissions associated with respiration to the shell's CO 2 budget (ca., 10%).This approach isolates the carbon footprint of the shells from the rest of the organism and ignores the interdependence between shell formation and soft tissue metabolism, which seems incongruous in the context of the carbon budget of shellfish farming.It also contradicts the fundamental principle of the carbon budget which must include all greenhouse gas fluxes of the manufacturing of the product.
Determining the shellfish ecosystem's net CO 2 contributionwhether as a source or sink-depends on seasonality and local factors like rearing practices, temperature, hydrodynamics and bathymetry, phytoplankton, nutrients, and ecological feedback.Hence, extensive studies on typical shellfish aquaculture ecosystems that consider direct measurement of carbonate chemistry (DIC, but also TA/pCO 2 ) are needed.Currently, the model-based ecosystem approach of the carbon budget has limited practical utility and opens the door to various approximations and subjective decision-making regarding the inclusion or exclusion of CO 2 emissions in the carbon budget.

| CONCLUSION
The claim that shellfish farming can contribute to atmospheric CO 2 sequestration is not scientifically grounded.Stating that bivalves and their culture act as CO 2 sinks does not arise from controversies but from misconceptions, largely stemming from an inaccurate or incomplete understanding of seawater carbonate chemistry.This statement is exclusively based on inaccurate theoretical models and it is not supported by observational and experimental studies.Spreading such misconceptions in the scientific literature and the public could lead to counterproductive policy-making, both for climate protection and for the development of sustainable marine aquaculture.
Most of the carbon contained in the CaCO 3 comes from HCO 3 À originating from the long-term weathering process on rocks.
The contribution of atmospheric CO 2 is negligible.In the short term (<10,000 years), CaCO 3 production does not accelerate weathering or atmospheric CO 2 capture.Instead, it increases oceanic pCO 2 , reducing the ocean's capacity to store atmospheric CO 2 and thereby contributing to climate change.Moreover, harvesting and storing shell wastes on land does not contribute to CO 2 sequestration.Shells should be returned to the seawater where their dissolution absorbs CO 2 and offsets emissions resulting from calcification.Currently, most shells are incinerated leading to more CO 2 in the atmosphere.Therefore, the fate of shells following flesh consumption must be considered.
An ecosystem-wide evaluation of the bivalve carbon budget will unlikely change this perspective.There is currently no evidence challenging the idea that bivalve aquaculture acts as a source of CO 2 .In fact, the few CO 2 air-sea flux measurements conducted in the field suggest that bivalve farms are net CO 2 sources.Current ecosystembased carbon budgets exclusively focus on organic and inorganic carbon, overlooking changes in total alkalinity and air-sea CO 2 fluxes, and often rely on unverified assumptions that can minimize CO 2 emissions.Most carbon budgets of bivalve farming are based on models whose parameter estimates are not verified under field conditions.
Although challenging, field measurements must be conducted for monitoring, reporting, and verifying CO 2 capture to reliably and honestly qualify for carbon credits. 87,88,96 acknowledge that our perspective will encounter opposition, While bivalve farming is not a CO 2 sink, it should not impede its future development.Cultivated bivalves are among the sources of animal protein with the lowest CO 2 emissions, 97,98 and their carbon footprint can be further reduced by either returning their shells in the ocean or by adopting cocultures with algae to partially offset CO 2 emissions. 29,35,39,47,99reover, efforts to tackle climate change must be assessed in a broad context that include the preservation of ecological health, ecosystem services, and biodiversity.In this regard, bivalve farming provides many services such as effective seawater filtration and clarification, regulation of nutrient cycles and eutrophication, and the creation of habitats for diverse species. 18Therefore, the overall positive impact of bivalves on marine ecosystems makes them an impor- The simulation was conducted under conditions of 15 C and salinity 35 and the assumption that seawater is equilibrated with the atmosphere with respect to CO 2 (i.e., 397 μatm).Initial TA and DIC concentrations were set at 2300 and 2080 μmol kg À1 , respectively.These two parameters, together with salinity and temperature, allow determining the remaining parameters of the carbonate chemistry 102,103 (Table A1).
The production of 100 μmol of CaCO 3 results in a reduction of Dissolving 100 μmol of CaCO 3 leads to opposite results: TA, DIC, and pH increase, while pCO 2 decreases (Table 1).Upon equilibration with the atmosphere, the water will have absorbed 71 μmol kg À1 of DIC and pH T will remain slightly higher compared with the initial condition.

APPENDIX B
As many calcifying organisms, bivalves have the ability to use metabolic CO 2 produced through the respiration as an alternative carbon source for shell production.This energy consuming pathway involves the activity of the carbonic anhydrase, an enzyme which catalyses the transformation of bicarbonate ions (HCO 3

À
) to CO 2 .Here we show that using metabolic CO 2 has the exact same consequence on seawater carbonate chemistry than by using seawater HCO 3 À or carbonate (CO 3 2À ) ions.
The reaction of incorporating metabolic CO 2 that was enzymatically transformed to HCO 3 À , into the shell is: This reaction releases 2 hydrogen ions into seawater.The equilibration of the carbonate system involves the titration of these newly formed hydrogen ions by the most abundant base in seawater, HCO 3 À according to the following reversible reaction T A B L E A 1 Seawater carbonate parameters under initial conditions and following production or dissolution of 100 μmol of calcium carbonate (CaCO 3 ) before and after equilibration with the atmosphere., concentration in bicarbonate ions; pCO 2 , partial pressure of CO 2 ; pH T , pH on the total scale; TA, total alkalinity.
The overall reaction can therefore be written as: This equation shows that calcification based on metabolic CO 2 leads to the consumption of 2 moles of HCO 3 À (decrease of total alkalinity by 2 moles) and the consumption of 1 mole of dissolved inorganic carbon (2 HCO 3 À consumed À 1 CO 2 produced), therefore shifting the system to a higher proportion of CO 2 in seawater.

3 2À.
concentrations of CO 2 and HCO 3 À and decreases concentrations of CO At the ocean surface (considering a temperature of 15 C, a salinity of 35, a pH level of 8.1 (on the total scale), and a DIC concentration of 2300 μmol kg À1 ), DIC mainly consists of HCO 3 À (90.9%) and CO 3 2À (8.5%), and CO 2 is a relatively minor component (0.6%).

F
I G U R E 2 Graphical summary of the literature review on the role of bivalves in the carbon cycle and their impact on CO 2 levels.(a) temporal evolution of the cumulative number of articles and (b) current relative proportion of observational/experimental (Obs./Exp.)versus modelling studies (numbers indicate the number of studies).Modelling studies are distinguished depending on whether the shell and/or the flesh were considered as CO 2 sinks.Panel b categorizes studies according to the reached conclusions, indicating whether bivalves are identified as sources and/or sinks of CO 2 (in bold).
reflecting the current division in the scientific literature.To achieve a scientific consensus, we propose three lines of action.First, while studies based on laboratory and field measurements indicate that shellfish are sources of CO 2 , even when the whole ecosystem is considered, these studies are relatively rare and often outdated compared with carbon balance models.It is therefore necessary to reinforce observational and experimental studies, including the measurement of CO 2 fluxes at the interfaces of shellfish farming ecosystems and within them.Second, carbon balance models are frequently dissociated from observational and experimental studies.To bridge this gap, integrative approaches that include observation, experimentation, and modelling are essential.Finally, the aforementioned misconceptions are likely the consequence of a lack of integration between disparate research areas, including biogeochemistry, ecology, physiology, and aquaculture engineering.By fostering collaboration between disciplines, we can advance our knowledge and hopefully resolve the current debate.
TA and DIC by 200 and 100 μmol kg À1 , respectively, reaching concentrations of 2100 and 1980 μmol kg À1 .In a closed system without contact with the atmosphere, this leads to an increase in pCO 2 by 241 μatm and a decrease in pH by 0.22 units.In an open system, pCO 2 in seawater tends to equilibrate with the atmosphere.Consequently, seawater degasses, allowing excess CO 2 to escape to the atmosphere until equilibration is attained (i.e., 397 μatm) and a new equilibrium is reached in the seawater carbonate chemistry.Under these conditions, final concentration of DIC is reduced by 171 μmol kg À1 , of which 100 μmol kg À1 are due to CaCO 3 production and 71 μmol kg À1 are due to CO 2 degassing to the atmosphere.The latter corresponds to the value ψ, defined by Frankignoulle et al.13 as CO 2 released into the atmosphere: precipitated carbonate ratio of 71/100 = 0.71.It is also worth noting that pH remains lower compared with the initial condition.