Lithium isotopes reveal impaired ion transport in tropical corals exposed to high <em>p</em>CO<sub>2</sub>

Vigier, Nathalie; Thibon, Fanny; Rodolfo Metalpa, Riccardo; Montanes, Maryline; Jouini, Arbia; Counillon, Laurent; Poet, Mallorie; McCulloch, Malcolm; Tanvet, Clément; Télouk, Philippe; Comeau, Steeve

doi:10.5194/egusphere-2026-3326

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https://doi.org/10.5194/egusphere-2026-3326

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19 Jun 2026

| 19 Jun 2026

Lithium isotopes reveal impaired ion transport in tropical corals exposed to high pCO₂

Nathalie Vigier, Fanny Thibon, Riccardo Rodolfo Metalpa, Maryline Montanes, Arbia Jouini, Laurent Counillon, Mallorie Poet, Malcolm McCulloch, Clément Tanvet, Philippe Télouk, and Steeve Comeau

Abstract. Ocean acidification, driven by rising atmospheric CO₂, threatens the ability of corals to build their skeletons by reducing their capacity to maintain an elevated pH at the calcification site (pH_cf), a process essential for calcium carbonate precipitation. Boron isotopes have commonly been used to show that the response of pH_cf to ocean acidification is highly species-specific. However, the physiological mechanisms underlying this variability remain poorly understood. Recently, lithium (Li) isotopes have been used to trace the activity of ionic transport involved in cellular pH regulation and calcification (e.g. H⁺, Na⁺and Ca²⁺), and may therefore help resolve these mechanisms. Here, we investigate multiple coral species from Tutum Bay (Papua New Guinea), a natural CO₂ seep system creating pH gradients (mean pH_T = 7.66 at seeps vs. 8.01 at control sites) analogous to future ocean acidification scenarios. Our results show a relationship between seawater pH, calcifying fluid chemistry, and lithium isotopic composition. Corals exposed to low seawater pH exhibit significantly altered δ⁷Li values relative to colonies from the control site, with some species becoming enriched in ⁷Li (up to 2‰) as pH_cf declines. This isotopic shift is consistent with reduced efficiency of Na⁺/H⁺ exchangers (NHEs), active transporters that preferentially incorporate the lighter ⁶Li isotope under optimal conditions but may become less effective under elevated proton concentrations. By linking Li isotopes to calcifying-fluid chemistry, these results provide geochemical evidence that ocean acidification may disrupt ionic regulation in corals and that Li isotopes can help to resolve biogeochemical controls of carbonate-systems.

Received: 08 Jun 2026 – Discussion started: 19 Jun 2026

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Nathalie Vigier, Fanny Thibon, Riccardo Rodolfo Metalpa, Maryline Montanes, Arbia Jouini, Laurent Counillon, Mallorie Poet, Malcolm McCulloch, Clément Tanvet, Philippe Télouk, and Steeve Comeau

Status: final response (author comments only)

RC1:
'Reviewer comment on egusphere-2026-3326', Anonymous Referee #1, 06 Jul 2026
Vigier et al. present an intriguing dataset regarding lithium isotope ratios of coral carbonate in coral reefs that grow under low and normal pH conditions in Papua New Guinea. They use the difference in δ⁷Li of coral aragonite from open ocean and low pH seep sites, and suggest the coral δ⁷Li imply impairment of ion channels and transporters in corals that grow under a low pH. The quality of the analytical data is excellent, and the experiment planning chose a good site to test the research question. However, the paper has three major flaws that prevent me from supporting its publication in the current version:
The entire discussion is based on the difference between the δ⁷Li of seawater and coral aragonite. The authors report a single data point for δ⁷Li at the seep site, which is significantly different than the seawater δ⁷Li. There is no reason to assume steady state conditions at this site yet the paper does not show that the reported δ⁷Li is representative even of a single diurnal cycle, let alone the period during which the corals precipitated their skeletons. It is possible that the measured difference in δ⁷Li_carbbetween the control and seep sites is only due to the variability in the δ⁷Li of the ambient water.

I don’t think the statistical analyses support a correlation between the lithium and boron isotopes and between δ⁷Li_carb and calculated calcification fluid pH. See specific comments about this below.

The results are discussed in the context of a calcification model that suggests full control over the composition of the calcification fluid by a system of ion transporters and channels. In my opinion there is stronger evidence in the literature in favour of a competing model that suggests corals initially create internal vacuoles of seawater and then modify their composition (e.g. Gagnon et al., 2012; Ram and Erez 2023). The different models lead to different interpretation of the experimental results but these possibilities were completely ignored in the paper.

Specific comments:
Methods section: Which instrument and methods were used to determine element ratios? What were the accuracy and precision of these analyses? How was seawater collected and handled?
Lines 339-342 claim δ⁷Li and δ¹¹B covary, however, according to Table S2 there is no correlation between pH_cf or δ¹¹B and δ⁷Li at the control site, and the correlation coefficients for the seep site are weaker than for each of these parameters with other analysed parameters.
Do you have any information regarding the temporal variability in seawater δ⁷Li at the seep site? Is it possible that the variability measured in the composition of the skeletons is due to variability in the ambient [Li] and δ⁷Li? It will not take large changes in seepage fluxes to make it.
Lines 384-386: If seawater is the initial calcification fluid (the Erez model), low Li/Ca ratios suggest that coral species that precipitate aragonite with low Li concentrations work harder to remove Li from the initial seawater vacuoles. Under the assumptions of this model, you expect larger δ⁷Li deviation from the seawater ratio in the low Li carbonates based on simple Rayleigh distillation, which agrees with the reported observations.
Figure 7 and lines 419-423: I don't see how the PCA indicates covariation between pH_cf and δ⁷Li_carb. First, the figure should include information regarding the percent of the variability explained by each of the principal components, often the contribution of PC4 is negligible. Second, the arrows of the discussed variables are perpendicular, suggesting that they do not covary but rather controlled by different processes.
Lines 453-487: This part of the discussion is highly speculative.
Line 493 contradicts Figure 9, which suggests that the isotopic difference between the sites is roughly constant.

Minor comments:
Please add a column for δ⁷Li_sw in Table S1.
Figure 5: I think you should draw both axes over the same range (-9 to -13 or -14). This will show more clearly the difference between the control and seep sites.
Citation: https://doi.org/10.5194/egusphere-2026-3326-RC1
RC2: 'Comment on egusphere-2026-3326', Anonymous Referee #2, 06 Jul 2026

Manuscript Summary

Vigier et al. present a unique physiological perspective of the impacts of ocean acidification on coral calcification efficiency. Using corals from a “natural laboratory” in the tropical Pacific proximate to volcanic CO₂ seeps and home to a gradient of seawater pH, the authors investigate skeletal lithium (Li) elemental and isotopic variability across a wide variety of coral taxa and morphologies. They then use their results to comment on the extent to which skeletal-bound Li dynamics covary with ambient seawater / calcifying fluid pH, is species specific, and better showcase a mechanistic link between ambient pH and ion pumping channels within the coral calcifying space.

Contribution Significance

In summary, the work presented here has the potential to provide an important and highly novel physiological framework for understanding ocean acidification’s impact on coral biomineralization. By pairing traditional boron-derived pH_cf calculations with lithium stable isotopes across a natural CO₂ gradient, the authors offer a compelling look past simple calcification rates and into active cellular ion-transport disruption. This dual-proxy approach helps bridge the gap between structural geochemistry and cellular physiology. However, the manuscript requires significant clarifications to its methodologies and interpretative frameworks before these promising mechanistic claims can be fully supported.

Strengths & Weaknesses

The authors do a great job establishing the biological and theoretical motivation for using lithium isotopes as an analog for Na/H exchange pathways. Utilizing a natural volcanic seep system provides an ideal, long-term ecological context that standard laboratory experiments often struggle to replicate. Furthermore, the correlation shown between specific-specific vital effects and calculated internal calcifying fluid parameters represents a genuinely exciting development in the coral proxy community.

However, in its current form, the manuscript suffers from significant clarity issues regarding sample replication and data aggregation. The text suggests highly unbalanced sampling (down to single-colony representatives for certain taxa), yet many primary conclusions rely on pooled statistical tests that obscure this species-specific variability. Mechanistically, the authors leap into heavy speculation regarding transmembrane ion-channle dynamics while operating on a highly limited suite of skeletal endmembers. Grounding these interpretations in explicit, quantitative mass balance models (and double-checking their carbonate chemistry equations) will make for a more impactful, rigorous argument.

Suggestions

Line 109: Materials and Methods header should be Section 2, no?

Lines 111 – 134: I understand not wanting to repeat too many of the major findings/results from a past study (Pichler et al., 2019), but I think it may be worth expanding discussion on what is known / has already been characterized for this study site. What is the benthic community composition (e.g., % coral coverage, % turf/fleshy macroalgae, % CCA, % sediments, etc.) and the dominant genera/species present across each sampling site? What is known about water residence time (if anything) in Tutum Bay, and how does this knowledge/lack of knowledge influence interpretations of ambient seawater chemistry? You mention that the whole bay has a mean pH ranging from 7.6 – 7.7 year-round, but isn’t it more important to comment on the extent to which the parameter is variable over daily to seasonal timescales? Do the CO₂ seeps overwhelm the characteristic diel and seasonal “heartbeat” in pH and carbonate chemistry that shallow reef ecosystems typically display? Also, I think it’s important to discuss the precision and accuracy of the sensor-based measurements of pH and discrete bottle samples for total alkalinity (e.g., what technique / instrument was used to measure TA, and what is its precision?). Furthermore, with these two parameters (pH and TA), you could calculate the remaining carbonate chemistry parameters for the timescales relevant to this study (assuming you also have some constraints on ambient salinity variability). How might these measurements provide additional constraints on the coral-based back-calculations?

Lines 127 – 130: “Both” is confusing in this context as it refers to Sites 1, 3, 4, 5, 6 and offshore. What is the significance (and/or variability between) the selected sites, and why was Site 2 excluded?

Lines 137 – 140: This is confusingly worded. From which sites in Figure 2 were corals collected, how many total corals, and which species? A table might be a better way to summarize these important takeaways as Figure 3 doesn’t show up until much later in the text.

Lines 144 – 146: I’d add the phrase “using methods detailed below” somewhere in here to preempt questions about analytical strategies. Also, while you discuss the specifics behind the determination of δ¹¹B and δ⁷Li, there is no discussion of how B/Ca and Li/Mg measurements are made.

Lines 150 – 151: Instead of just saying “mQ” (more of a “brand”), I’d first report the resistivity (e.g. MilliQ 18.2 MΩ.cm ultra-high purity water (mQ)). You could then use “mQ” in subsequent references.

Lines 160 – 164: Purity of HNO₃ used? Also, 163 – 164 read as if you performed B/Ca measurements on a chromatographically-purified aliquot. Some clarity in the language would be useful here to make it clear that you ran 1 aliquot of sample at 10 ppm Ca through ion exchange columns to purify boron for isotope measurements, while a separate 10 ppm Ca aliquot was used (not treated with columns) for trace element analyses (inclusive of B/Ca)… if I understand correctly? Also, what resin was used for your boron purification?

Line 181: Equation 3 should be Equation 2, no? Also, shouldn’t (K₁ + K₂) in the [CO₂]_aq term be a product (e.g., K₁ * K₂) rather than additive?

Lines 195 – 199: If seawater samples were filtered with 0.45 µm filters, why poison with HgCl₂ and not acidify to pH < 2 with HNO₃ or HCl? Acidification is usually used to preserve filtered seawater samples for trace element analyses, while poisoning is typical for carbonate chemistry analyses (where filtering can induce gas exchange and alter DIC concentrations, for example).

Line 205: Check consistency on equations being embedded in text versus being their own line.

Line 225: Missing “s” on species.

Lines 243 – 252: Here is another place where I think more discussion on the temporal patterns of variability in pH and carbonate chemistry parameters should be discussed for the study location. Also, seawater DIC, pCO₂, and saturation states are reported as if they are measured variables, but I’m assuming these are all calculated from measured TA and pH? If so, the calculation approach needs to be discussed briefly in the Methods.

Lines 253 – 257: I think the same comment on temporal patterns of variability could apply to your measurements of Li dynamics, particularly in regard to how representative the seep samples are of year-round Li behavior in the Bay. How can you be sure (without this information) that residence time of the water in the Bay isn’t influencing the magnitude of the observed δ⁷Li_sw difference between the Bay and open ocean on variable timescales, and that the corals are just recording this physical signature rather than a physiological one?

Lines 257 – 260: This is a little confusing. If there are hydrothermal contributions at the seep site, shouldn’t the δ⁷Li value be higher here compared to the open ocean? Do you mean depleted instead of enriched?

Figure 3: Does Panel C need to be a part of this figure? If so, consider renaming it A. Regardless, scale bars are needed, as is a more descriptive caption / annotations of what the major structural and/or morphological features of interest are in the context of this study. Can Figure 3A and B be combined with Figure 4? They don’t immediately seem to be showcasing entirely different data.

Lines 278 – 294: There’s a considerable number of findings here that may warrant tables / figures in the main text versus being in the Supplementary Materials (given the primary conclusions and central hypotheses of the manuscript). Consider including some version of Fig. S2 and/or Table S1 here.

Lines 311 – 312: Ah! I think “systematically enriched in the light ⁶Li isotope” is confusing and relates back to an earlier comment w.r.t the hydrothermal vent isotopic behavior. Consider consistently using “enriched” and “depleted” to refer to the change in δ⁷Li from the perspective of the heavy isotope (as is typically convention with other traditional stable isotope systems).

Lines 345 – 347: See comment on Lines 253 – 257. I think this may need to be toned down / qualified a bit based on the limits of your dataset (if you aren’t going to comment on the temporal variability or representative nature of the water samples).

Lines 349 – 352: It may be worthwhile to provide a brief discussion on why one may have expected there to be species-specific and/or morphological differences in Li systematics in the first place. You observe that this isn’t the case and is in agreement with previous studies, but why was this a hypothesis worth testing / for which elemental systems and physiological mechanisms of relevance is this a known issue?

Lines 419 – 423: I feel like these statements are too strong for the data presented in the PCA and in Table S2. pH and δ⁷Li have slightly significant correlation only in the seep sites, while other parameters demonstrate stronger correlations. I feel like these differences warrant deeper discussion.

Lines 453 – 487: It feels like this section is a key piece to the central argument of the paper, but I think the authors should tread carefully w.r.t. the strength of the conclusions drawn from a lot of mechanistic speculation given their handful of *endmember* skeletal samples. Obviously, it’s unreasonable to expect that the authors present calcifying fluid Li measurements (as that would likely be a totally separate suite of experiments), but I think this section could benefit from drawing more on calculated/measured carbonate chemistry parameters of the calcifying space and explicit, quantitative links to back-of-the-envelope style models of ion channel reaction mechanisms could ground the plausibility of all of this conjecture a little more.

Citation: https://doi.org/10.5194/egusphere-2026-3326-RC2
RC3: 'Comment on egusphere-2026-3326', Anonymous Referee #3, 23 Jul 2026

This manuscript presents measurements of δ¹¹B, δ⁷Li, B/Ca and Li/Mg in several coral species collected from both control seawater and hydrothermal seep environments. Using these data, the authors discuss the potential mechanisms controlling Li transport during coral calcification.
The overall idea behind this study is interesting and represents a logical extension of previous work on Li systematics in biominerals, notably that of Vigier et al. (2015) on foraminifera. The dataset also has the potential to improve our understanding of the relationship between coral biomineralization and seawater chemistry.
However, I have significant concerns regarding the current version of the manuscript. The discussion remains largely descriptive, several interpretations are insufficiently supported by the data, and a number of methodological details are either missing or require clarification. In addition, the manuscript contains many inaccuracies, inconsistencies and omissions that make it difficult to fully assess the robustness of the conclusions. Overall, I do not find the manuscript sufficiently convincing in its present form and therefore cannot support publication at this stage.
Specific comments
L51–52: If I am not mistaken, Sr/Ca has historically been much more widely used than Mg/Ca. Why has it not been included?
L53: Please add "e.g." before the references.
L56–58: The use of Li/Mg as a paleothermometer remains debated, and several studies have highlighted its limitations (e.g. Cunny-Guirrec et al., 2019; Rollion-Bard & Blamart, 2015; Hathorne et al., 2013). Please adopt a more cautious wording.
L66: Please also cite earlier studies such as Rollion-Bard et al. (2003) and Allison et al. (2010).
L67: Which environmental conditions are you referring to?
Figure 1: Please provide references supporting the reported pH values.
L86–87: Why should Na⁺ behave similarly to Li⁺? Is this behaviour restricted to biological systems? If so, please explain why.
From the beginning of the manuscript, the study focuses exclusively on the effect of pH. However, DIC also differs between the two sites and is later invoked in the Discussion. With the present experimental design, it is impossible to disentangle the respective effects of pH and DIC. Therefore, DIC should be introduced in the Introduction as another environmental parameter that may influence the observed geochemical signatures.
L111–116: Please report the salinity at both sampling sites.
L142: fascicularis instead of Fascicularis.
Please explain how B/Ca and Li/Mg were measured and report the associated analytical uncertainties.
L168 and L214: Please use a consistent notation for JCp-1 throughout the manuscript.
L170: Please use δ instead of d.
L171: Please remove the reference to Trotter et al. (2011). This paper did not firstly define the equation used here.
L173: Please provide a reference for the calculation of the dissociation constant.
L177: Please specify that the calculations are based on B/Ca ratios.
Please include a section describing the propagation of uncertainties in the calculations of pHcf and DICcf.
L178: What do "(15, 17, 18)" refer to?
L185: Please remove the "s" from "isotopes".
L205: Please remove the reference to Burton & Vigier.
L211: What were the procedural blank levels relative to the measured sample signals?
Please be consistent throughout the Methods when reporting uncertainties (SE versus SD).
L216–220: This paragraph would fit better at the end of Section 3.1.
L225: Please add an "s" to "species".
L236: Please use Δ instead of D.
Were B/Ca, Li/Mg and δ¹¹B measured in seawater at both sampling sites? If so, please report these values.
L245: Please report the Ca²⁺ concentration used to calculate Ωarag. What Ksp value was used? Please also provide the corresponding reference.
Figure 2: Please revise the formatting of the species names. Panel C is missing. Please add a scale bar. Species names should be italicised throughout.
L270–276: Please remove this paragraph.
L283: "sp." should not be italicised.
L284: Please add an "s" to "species".
L286: Please include uncertainties for the pHcf values.
L289: "others" instead of "other".
L299: Tissues? I may have missed it, but tissue analyses are not introduced previously and are not discussed afterwards. What is the purpose of these measurements? How do they contribute to the objectives of this study?
L305–306: Why were the datasets from Fowley et al., Hathorne et al., and Rollion-Bard & Blamart not included in the comparison?
L312: Please remove "for all corals".
Figure 4: Please report the calculated isotope fractionation factors. In the figure legend, please explain how the inorganic aragonite values were determined and provide the corresponding references.
L342: "...or enriched in DIC."
L356: What kind of regulation do you mean? Does this imply that the extracellular calcifying fluid is depleted in Li relative to seawater? If so, what would be the expected consequences for the δ⁷Li of the calcifying fluid?
L361–363: I do not understand this sentence. Is the aquarium effect really relevant to the present study?
Figure 5: Please revise the formatting of the species names. What do you mean by "each purple dot"?
L375: Although a review paper can be cited, the original studies should also be referenced.
L375–381: Please remove this section. It is not directly relevant to the objectives of this study.
L386: Which elements?
L388: Same comment as for Line 375.
L394: Please add "e.g." before the references.
L396: What do you mean by "closely"? Please provide the range of calculated temperatures. Using the values reported in your table together with the calibration of Montagna et al., I obtain temperatures ranging from approximately 25.9 to 34.0 °C. Compared with the measured seawater temperature of 30.5 °C, this agreement does not appear to be particularly close.
L418: "...whether pH or DIC."
Figure 7: Please define all abbreviations used in the figure legend (Li/Ca, AVG Li, etc.).
Figure 8: Please use the same terminology in the figure legend as that introduced around Line 485.
I do not understand the statement referring to "corals living under high pCO₂" .
In the figure, "sp." should not be italicised, and fascicularis should begin with a lower-case letter.
L464: Please add a reference.
L465: Has it been demonstrated that corals precipitate exclusively from HCO₃⁻? Why are carbonate ions not considered?
L469: Tambutté instead of Tambutte.
L472: Please add an "s" to "supports".
L478: If activity is enhanced, why is there no corresponding increase in Li concentration?
L482: Albarède instead of Albarede.
Figure 9: Please also present the data as a function of DICcf, which is already mentioned in the figure legend.
L497–499: Same comment as above. Why would this process not affect Li concentration?
L507: There appears to be an extra underscore.
L514: Why use the term "altered"?
L515: By "calcifying fluid chemistry", do you mean only pH and DIC? If so, please state this explicitly.
L519–520: This statement is too broad. Please either remove it or support it with appropriate examples.

Citation: https://doi.org/10.5194/egusphere-2026-3326-RC3
RC4: 'Comment on egusphere-2026-3326', Anonymous Referee #4, 25 Jul 2026

Why does this/their question matter?
Ocean acidification is likely to disrupt calcifier biomineralization by interfering with maintenance of pH at their sites of calcification. However those physiological responses are variable and difficult to consistently predict. This study proposes to use Lithium isotopes to better understand ion transport at the sites of calcification enabling better predictions of calcifier response to projected ocean acidification (OA).
General comments:
Abstract: I feel this would benefit from some more clarity towards the end on the specific impact on the organisms’ ability to calcify. It is currently a little unlcear with a mix of the Li response and the organismal physiology mixed together.
Introduction: To help the reader grasp the importance more effectively, I would suggest structuring the intro (and the wider paper) in one of two ways. I do appreciate my suggestion which follows can be difficult as the authors are trying to achieve two objectives, 1) first show the approach works (or at least what it is measuring), and then 2) to understand the physiological aspect. If possible, my suggestion would be to either a) have the question/paper’s main storyline be about the physiological responses and the Li isotopes are the “just” the detection approach, or b), have the storyline about finding an approach for answering physiological questions. At present the storyline tends to weave between both approaches making the paper somewhat tricky to follow in some places. For me, (a) is a more interesting question so for example, you could lay out the introduction as: OA is a problem for calcifiers, the physiology of the problem and the components we don’t understand, natural laboratories provide a great place to test your questions, which approach (7Li) you will use to understand the physiology. Then tune the last paragraph of the introduction accordingly with what questions will be answered.
Methods: Some parts of the methods have good detail (analytical aspects) although other parts (in particular the field collection) would benefit from more details. Also, some more space should be assigned to exploring / demonstrating what the measurement are recording. I understand that the authors needed to account for the different environmental background concentrations in L at the different sites (and it is good they have done so) so also good to assure the reader that this “normalized” approach is still robust (lines 216-220).
Results: In general I found the results easy to follow other that in a few places where I think they may benefit from clarification of which “pool” the analyte came from (e.g. sw, carb, etc). Also seem my comment below on the discussion where it may be that some of the more descriptive aspects of the discussion move to the results to help the discussion.
Discussion: I think the discussion does a good job of describing the observations, although I feel the unpacking of the “how” aspects and then synthesising into the “why” should be expanded earlier on in the discussion. For example, lines 401-403 seem like they are a key finding and would benefit from an earlier synthesis of the “how” and “why” aspects. They do come in section 4.4 which is at the end but that is probably a bit too late. One way to approach this would be to move some of the more descriptive aspects of the discussion to the results and keep the discussion more focused on the “why”.
From a wider layout perspective in the discussion, I agree with the use of subheadings. I know sometimes this means the flow is not quite there, but I believe the subheadings do help the reader for multifaceted analytical descriptions such as those required by this paper.

Specific comments:
L36-43; I feel this part of the abstract would benefit from some rewording so it is more specifically about the calcification process (ie expand the last sentence) rather than a mix of the proxy approach and the calcification process.
L52: have been widely used
L59-60: It may be the rendering on my system, but if not there, suggest adding a paragraph break here.
L60-73: suggest separating the physiological process from the geochemical approach used to determine the physiology.
L124: Suggest including an indication of how long the venting has been taking place. This is important contextual information to help understand if they corals are likely to be acclimated to their environment.
L137: How were the corals sampled?
L138: The word “either” is a little confusing here as it could be interpreted that you don’t know what species the corals were and it is one of the ones on the list you provide. Of course you do know, so I would suggest rewording that part of the sentence to reduce the uncertainty.
L141: Suggest not starting the sentence by referring to the figure. Easier to read if you make your point then place the figure number in brackets after your point.
L141-146; Suggest pointing to the detail which follows or move the sentence to the start of the following paragraph. At present it could be interpreted that no more information will be provided.
L170: Greek delta rather than letter d
L196: Suggest using the word conducted rather than done.
L199: Fine to use term “were dried” without the word “up”.
L206: ratios
L216-220: See above suggestion for addition information here.
L219: For completeness suggest clarifying the seawater referred to is the same “treatment” (control or seep) as the corresponding coral.
L223: I found this section a little confusing due to the first sentence; I was expecting different samples being treated differently but maybe I misinterpreted what was meant. I was thus not completely sure what the two approaches were; was this referring the Mann-Whitney and PCA? Suggest adding subheadings to this section to help the reader.
L225: species
L226: values
L240-243: Suggest reversing the sentence starting with the observation then cite the refs and database in brackets.
L245-246: I may have missed it earlier in the text, but if not, please specify how the seawater DIC & pCO2 equivalents were calculated, likely using Seacarb or co2sys?
L284: species
L298: Suggest clarifying here you are referring to the aragonite (ie Licarb) not the tissue. Later in this sentence reference to tissue (in the literature) appears so good to avoid any confusion.
L351-352: If the analytical approach is being included for one ref (SIMS) the corresponding approach should be included for the other reference also.
L344-356: If found this a little repetitive with it coming back to the biological control point a few times. Suggest that is mentioned once at the start and then number x pieces of evidence that support the observation. Would also suggest expanding on the “how” the biological control operates here in the discussion. See point above about how you might do that.
L357-365: suggest making this bit clearer as I struggled to tie the culturing part back the methods without some head scratching. This could be that the methods also need some more detail about the culturing aspects.
L368: For the figure key, have the genus names in lowercase (first letter upper case) and in italics to follow Latin naming convention. Species name should be all lowercase and Latin if the name is provided.
L373: As above this paragraph is a bit repetitive and would benefit from the individual points being listed as lines of evidence corroborating the main point being made at the start of the paragraph.
L382-390: I think paragraph would benefit from unpacking the “how” and “why” aspects of the observations (e.g. possible physiological mechanisms) as at present it is somewhat descriptive. They do appear later on in section 4.4 but see my suggestion in the general comments above about how to incorporate it together.
L421-439: This is an interesting finding and I think would benefit from some further unpacking of possible mechanisms. I appreciate that may require some speculation but I think this can be done without going too far.
L441: Suggest labelling the coral in plate B as well.
L536: Funding rather than fundings

Citation: https://doi.org/10.5194/egusphere-2026-3326-RC4

Nathalie Vigier, Fanny Thibon, Riccardo Rodolfo Metalpa, Maryline Montanes, Arbia Jouini, Laurent Counillon, Mallorie Poet, Malcolm McCulloch, Clément Tanvet, Philippe Télouk, and Steeve Comeau

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https://doi.org/10.5194/egusphere-2026-3326-supplement

Nathalie Vigier, Fanny Thibon, Riccardo Rodolfo Metalpa, Maryline Montanes, Arbia Jouini, Laurent Counillon, Mallorie Poet, Malcolm McCulloch, Clément Tanvet, Philippe Télouk, and Steeve Comeau

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Short summary

Tropical corals along a natural CO₂ gradient reveal how acidification affects calcification. Low pH alters their lithium isotopes, enriching ⁷Li as calcifying fluid pH drops, indicating reduced proton transport. Inverse links between lithium isotopes and fluid chemistry show acidification disrupts ionic regulation, and lithium helps decode carbonate system controls.


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Lithium isotopes reveal impaired ion transport in tropical corals exposed to high pCO2

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Lithium isotopes reveal impaired ion transport in tropical corals exposed to high pCO₂