the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jun 2026
Lithium isotopes reveal impaired ion transport in tropical corals exposed to high pCO2
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 (pHcf), a process essential for calcium carbonate precipitation. Boron isotopes have commonly been used to show that the response of pHcf 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 pHT = 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 pHcf 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.
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Status: final response (author comments only)
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RC1: 'Reviewer comment on egusphere-2026-3326', Anonymous Referee #1, 06 Jul 2026
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AC1: 'Reply on RC1', Nathalie Vigier, 20 Oct 2026
Thank you for your constructive review. Please find our responses below each comment respectively. The revised version of figure 7 is attached as a supplement.
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 δ7Licarb between the control and seep sites is only due to the variability in the δ7Li of the ambient water.
Response : We agree that the original manuscript did not make the seawater sampling and averaging clear. The reported Li concentrations and δ⁷Li values for filtered seawater are means of four separate samples collected at each site. The standard deviation was below 10% for Li concentration and below 0.5‰ for δ⁷Li, within analytical uncertainty. We will clarify this in the manuscript and provide the individual seawater measurements in a table in the Supplementary Information.
Lithium has a long residence time in seawater (more than one million years) and its concentration in seawater is approximately two orders of magnitude higher than in river or aquifer waters. Published measurements indicate that Li chemistry is homogeneous across the oceans. Although marine organisms and calcifiers can incorporate Li (e.g., Thibon et al., 2021), calcium-carbonate formation is not a major sink for oceanic Li (Misra and Froelich, 2012). These considerations make substantial diurnal variation in seawater Li unlikely. The measured variability at our sites is consistent with the range reported for seawater, within the analytical uncertainties of our measurements.
2. I don’t think the statistical analyses support a correlation between the lithium and boron isotopes and between δ7Licarb and calculated calcification fluid pH. See specific comments about this below.
Response : Indeed, we believe there has been a misunderstanding, which we will clarify in the revised manuscript, should the editorial/reviewing team consider our responses favorably. As explained in our responses to the following comments, we do not claim that the individual δ⁷Li and δ¹¹B values covary. Rather, we argue that isotopic differences among species covary with boron-derived estimates of calcifying-fluid chemistry, as shown in Figure 8. We have also corrected an error in the dataset used for the original PCA and rerun the analysis. The revised results are now consistent (see the revised figure attached).
3. 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 interpretations of the experimental results but these possibilities were completely ignored in the paper.
Response : We apologize for not discussing this alternative model. The revised manuscript will include the model in which internal vacuoles initially contain seawater, as well as the suggested references. This model has also been used to describe foraminiferal shell formation (Erez, 2003). The formation of seawater-filled vacuoles does not, however, exclude ion transport. As in foraminifera (Vigier et al., 2015), the pH and carbonate chemistry within the vacuoles need to be modified to permit precipitation at the calcification site; this process requires ion channels and transporters.
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?
Response : Trace elements, including B/Ca, were measured by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS; X-Series II, Thermo Fisher Scientific) using solutions containing approximately 10 ppm Ca in 2% HNO₃, following Holcomb et al. (2015). We will add this information to the Methods section.
Lines 339-342 claim δ7Li and δ11B covary, however, according to Table S2 there is no correlation between pHcf 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.
Response : We did not intend to claim that individual δ⁷Li and δ¹¹B values covary. Rather, we meant that isotopic differences among species covary with boron-derived estimates of calcifying-fluid chemistry, as illustrated in Figure 8. We will rephrase the sentence to make this distinction clear.
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.
Response : As noted above, the reported Li concentrations and δ⁷Li values for filtered seawater at each site are means of four separate samples. The standard deviations were below 10% for Li concentration and below 0.5‰ for δ⁷Li. Li concentrations varied more at the seep site than at the control site (9% versus 4%), whereas δ⁷Li varied slightly less (0.4‰ versus 0.5‰, respectively). The long residence time and high concentration of Li in seawater make substantial variability at the control site unlikely. We will include the individual seawater data in the Supplementary Information so that this variability is transparent. In addition, the relationships between Δ⁷Li and pH or DIC at the calcification site, estimated using independent parameters, are consistent with a biological effect rather than a source effect. We will ensure that the revised text presents this interpretation with appropriate caution.
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.
Response : Thank you, it is an interesting point and we will add this model and interpretation in the manuscript. This also would reinforce a key role of ionic transport for CaCO3 precipitation even in the case of seawater vacuoles formation, as we discuss in our previous answers.
Figure 7 and lines 419-423: I don't see how the PCA indicates covariation between pHcf and δ7Licarb. 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.
Response : We apologize for the error in the original PCA and have corrected the analysis. The revised PCA was conducted using the prcomp function in R on 10 geochemical parameters, after centering and scaling all variables to unit variance.
The revised PCA revealed a clear multivariate differentiation between coral communities from control and seep sites, with the first two axes explaining 62.7% of the total variance (PCA1: 42.6%; PCA2: 20.1%). While PCA1 primarily reflected variation in B/Mg, BCa, DICcf and Mg/Ca, PCA2 was strongly structured by Δ⁷Licoral-SW and pHcf, which contributed 38.2% and 23.4% to this axis, respectively. These two variables showed opposite loadings on PCA2 (−0.62 and +0.48, respectively), highlighting a strong contrast between coral Li fractionation and carbonate chemistry. We will revise the text and figure caption accordingly.
Lines 453-487: This part of the discussion is highly speculative.
Response : This section offers a possible mechanistic explanation for the relationships between lithium isotopes and internal pH/DIC shown in Figure 8. It draws on studies of pH regulation by ion transporters in living cells and evidence that these transporters are active in corals. A link between Li isotopes and Na⁺/H⁺ exchangers was reported in 2023, and recent isotope studies of marine species also point to a possible role for Na⁺ transporters in explaining Li-isotope variability (Chen et al., accepted in ACS OMEGA; Jeong et al., accepted in ES&T; Thibon et al. papers). A recent study further reported that coral skeletal Li-isotope composition may be influenced by ion transport at the cellular level (Yoshimura et al., 2026). We agree that further work is needed to identify which transporters are involved when corals are exposed to high pCO₂. We will make the speculative nature of this proposed mechanism clearer in the revised Discussion. Nevertheless, we consider the combined use of lithium and boron isotopes a promising approach for investigating calcification chemistry.
Line 493 contradicts Figure 9, which suggests that the isotopic difference between the sites is roughly constant.
Response : Thank you for pointing this out. We will correct the sentence so that it is consistent with Figure 9.
Minor comments:
Please add a column for δ7Lisw in Table S1.
Response : Thank you for the suggestion. We will include all of our seawater Li data in the revised Supplementary Information.
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.
Response : We will use the same range for both axes in the revised figure.
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AC1: 'Reply on RC1', Nathalie Vigier, 20 Oct 2026
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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 pHcf 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 δ7Lisw 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 -
AC2: 'Reply on RC2', Nathalie Vigier, 21 Oct 2026
We greatly thank the reviewer for this constructive assessment. Please find our responses below each comment respectively and new tables or revised figure in the attached document.
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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 pHcf 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-channel 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.
Response : We agree that the original manuscript did not describe the sampling design and the scope of our mechanistic interpretation with sufficient clarity. We will considerably complete the Methods as detailed in our responses below, and will add new tables displaying clearly the representative nature and variability of the carbonate chemistry parameters and of our measurements (see attached documents). We agree that a quantitative mass-balance or transport model could strengthen future work. However, relating metabolic activity or the energetic costs of exchanger activity to our isotopic observations would require assumptions about several parameters that are currently too uncertain to support reliable estimates. We have therefore chosen not to present speculative calculations that could give a misleading impression of precision. We hope that our study will provide a foundation for future experiments that would be designed to obtain the measurements needed for such estimates.
Suggestions
Line 109: Materials and Methods header should be Section 2, no?
Response : Yes. We apologize for this error and will correct the section numbering if the Editorial/Reviewing team is favorable to a revised version.
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?
Response : We agree that the information provided in the original manuscript was insufficient to fully characterize environmental conditions at the study sites. Although measurements were collected during four expeditions between 2016 and 2018, the present study reports only data collected before and during coral sampling (2016–2017). These measurements nevertheless capture the pH and temperature conditions relevant to the coral collection.
Since the first field campaign in 2016, the sites have been visited six times for 10-day campaigns. During these visits, pH and other physical and chemical parameters were measured at high temporal resolution using SeaFET instruments. All measurements, including total alkalinity (TA), followed theGuide to Best Practices for Ocean Acidification Research issued by the EU EPOCA project. Comeau et al. (2022) reported most of the data collected from September 2016 to October 2019, including diel and multi-day variability; these data are publicly available. We have added further methodological details to the Methods section.
We believe that Section 3.1 already presents the principal pH, temperature, and seawater carbonate-chemistry data relevant to coral calcification. Nevertheless, in response to the reviewer’s suggestion, we will add a table to the Supplementary Information summarizing the measurements and calculated parameters (including temperature, pH, pCO₂, HCO₃⁻, and Ω) for each station (see attached document). The data show that diel pH variability is substantial and that mean pH at the seep sites is markedly lower than at the control sites.
Regarding benthic community composition, previous studies of CO₂ seeps at Normanby, Papua New Guinea (e.g., Fabricius et al., 2011), have documented effects of ocean acidification on reef composition. At Ambitle, however, coral cover and species diversity were reported to be unaffected (Pichler et al., 2019), while turf algae were abundant in the shallow area with the lowest pH. We began collecting quantitative benthic-community data only in 2019, with further surveys in 2024 and 2025. These later data will be presented in a future publication.
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?
Response : We thank the reviewer for identifying this oversight. Station 2 was not excluded; data from this station were included in our analysis of pH variability in Tutum Bay. We apologize for the unclear wording. The new table in the Supplementary Information will report the statistical comparisons of pH among all considered Tutum Bay stations (see attached document).
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.
Response : We agree that the original sentence was unclear. For each of the eight coral species, we collected one branch tip or skeletal fragment (3–7 cm long) from each of three colonies separated by approximately 5 m. We will clarify this sampling design in the Methods section.
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.
Response : We will provide additional information on the analytical methods. Trace elements, including B/Ca, were measured by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS; X-Series II, Thermo Fisher Scientific) using solutions containing approximately 10 ppm Ca in 2% HNO₃, following Holcomb et al. (2015).
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.
Response : We will follow this suggestion in the revised version.
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?
Response : Thank you for identifying this ambiguity. We will clarify the analytical procedure in the revised Methods. Two separate aliquots were used: trace elements, including B/Ca, were measured by Q-ICP-MS on a non-purified aliquot, while a separate aliquot was processed by ion-exchange chromatography to purify boron before δ¹¹B analysis by MC-ICP-MS. Boron was purified using stacked AG50W-X8 cation-exchange and AG1-X8 anion-exchange resins, following D’Olivo et al. (2019). We will also specify the grade of HNO₃ and how it was purified (see below):
Proposed revised manuscript text: “In the clean laboratory of the Advanced Geochemical Facility for Indian Ocean Research (AGFIOR, University of Western Australia), 10 mg of each coral aragonitic powder was cleaned and dissolved in 0.5 N HNO₃. The HNO₃ was distilled from analytical-grade acid using a Teflon sub-boiling distillation system. Separate aliquots of each sample solution were used for trace-element and boron isotope analyses. For trace-element analyses, an aliquot was diluted to approximately 10 ppm Ca in 2% HNO₃ and analyzed by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS; X-Series II, Thermo Fisher Scientific), following Holcomb et al. (2015). B/Ca ratios were determined from this non-purified aliquot.
A separate aliquot was used for boron isotope analysis. Boron was purified by ion-exchange chromatography using stacked AG50W-X8 cation- and AG1-X8 anion-exchange resins, following D’Olivo et al. (2019), and analyzed using a Nu II multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). Boron isotope compositions are expressed in conventional delta notation relative to the boric international standard (NBS951): δ11B (‰) = (11B/10Bsample / 11B/10Bstandard -1)*1000. Measurements of the international carbonate standard JCP-1 (nominal value of 24.33 ± 0.11 ‰ (SE)) yielded a mean value of 24.43 ± 0.08 ‰ (mean ± SE, n = 10).”
D’Olivo, J. P., G. Ellwood, T. M. DeCarlo, and M. T. McCulloch. 2019. Deconvolving the long-term impacts of ocean acidification and warming on coral biomineralisation. Earth and Planetary Science Letters 526: 115785. doi:10.1016/j.epsl.2019.115785
Holcomb, M., T. M. DeCarlo, V. Schoepf, D. Dissard, K. Tanaka, and M. McCulloch. 2015. Cleaning and pre-treatment procedures for biogenic and synthetic calcium carbonate powders for determination of elemental and boron isotopic compositions. Chemical Geology 398: 11–21. doi:10.1016/j.chemgeo.2015.01.019
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?
Response : we will correct this
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).
Response : We used HgCl₂ to preserve the seawater samples because acidification would have altered the carbonate chemistry that we aimed to preserve. We will clarify this rationale in the revised manuscript.
Line 205: Check consistency on equations being embedded in text versus being their own line.
Response : We will check the formatting and ensure that equations are presented consistently throughout the manuscript.
Line 225: Missing “s” on species.
Response : Thank you for noting this. We have corrected the missing “s.”
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.
Response : Thank you for this, yes, they were calculated, as it will be better described in the Methods and in our new table in the Sup. Information (see attached document). We will add a short discussion on the variability measured at the study sites, as described in our second response.
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 δ7Lisw 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?
Response : We have not been able to measure water residence time in Tutum Bay directly. However, Pichler et al. (2019) showed that seawater pH was more strongly associated with tidal cycles than with other environmental drivers, such as wind or changes in seawater currents. This is because, unlike other CO2 vents, not only does the bay has hundreds of sparse seeps bubbling CO2, but 3-4 large vents (real volcanoes) constantly add immense volumes of gases. So, according to the tide direction, this mass of acidified seawater moves more or less into the bay, therefore explaining the pH variation. Only rarely does the seawater in the bay benefit from water at “normal” pH values (Pichler et al., 2019). With that in mind, and although speculative, we assume that δ7Lisw difference between the Bay and open ocean is strictly linked to the constant gas released.
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?
Response : all publications reporting d7Li values in hydrothermal waters indicate a much lower d7Li value (< 10 ‰) compared to that of the ocean (31‰) - We will revise the sentence to make clear that hydrothermal input is associated with lower, not higher, δ⁷Li values.
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.
Response : We prefer to retain Figure 3 in its current location rather than combine it with Figure 4. However, we will revise the panel labels and add scale bars, as suggested.
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.
Response : we intend to add a new Table to the Sup. Information reporting details about the carbonate chemistry (already published in Pichler et al. 2019), as well as a new Table reporting all data that we measured in the seawater samples collected at both sites.Because the seawater boron data have already been published and interpreted (Comeau et al. 2022), we consider it more appropriate to report those data in the Supplementary Information.
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).
Response : We understand the reviewer’s point and will use “enriched” and “depleted” consistently with reference to the heavy isotope, ⁷Li, as suggested.
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).
Response : We will ensure that the revised text presents this interpretation with appropriate caution. Please note that beyond our previous response to Lines 253-257, the original manuscript did not make the seawater sampling and averaging clear. The reported Li concentrations and δ⁷Li values for filtered seawater are means of four separate samples collected at each site (see attached document). The standard deviation was below 10% for Li concentration and below 0.5‰ for δ⁷Li, within analytical uncertainty. In addition, the relationships between Δ⁷Li and pH or DIC at the calcification site, which were estimated using independent parameters, are consistent with a biological contribution rather than a source effect.
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?
Response : Published data on Li concentrations and Li-isotope compositions across coral species remain limited, which constrains rigorous statistical comparisons. Nevertheless, testing for species-specific and morphological effects is important because such “vital effects” are known to influence other geochemical proxies in coral skeletons. Based on Bastian et al. (2018), some previously reported differences may have resulted from methodological artifacts, potentially related to the low Li concentrations in coral skeletons. We will clarify this rationale in the revised discussion.
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.
Response : We identified and corrected an error in the dataset used for the original PCA and reran the analysis. The revised results are consistent with our interpretation (see the revised Figure 7 in the attached document). The revised PCA was conducted using the prcomp function in R on 10 geochemical parameters, after centering and scaling all variables to unit variance.
The revised PCA revealed a clear multivariate differentiation between coral communities from control and seep sites, with the first two axes explaining 62.7% of the total variance (PCA1: 42.6%; PCA2: 20.1%). While PCA1 primarily reflected variation in B/Mg, BCa, DICcf and Mg/Ca, PCA2 was strongly structured by Δ⁷Licoral-SW and pHcf, which contributed 38.2% and 23.4% to this axis, respectively. These two variables showed opposite loadings on PCA2 (−0.62 and +0.48, respectively), highlighting a strong contrast between coral Li fractionation and carbonate chemistry. We will revise the text and figure caption accordingly.
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.
Response : We agree that additional quantitative modeling could be informative. However, estimating metabolic activity or the energetic costs associated with exchanger activity from our isotopic observations would require assumptions about several parameters that are currently too uncertain to support reliable calculations. Poet et al. (2023) developed a cell-level transport model that highlights potentially important aspects of Li transport, including a dimeric mechanism, competition with Na⁺, and a strong pH effect. Further quantitative modeling in our system would require additional measurements, particularly of the Li/Na ratio at the calcification site, which are not currently available while Na is strongly linked to ATP. Several other parameters such as the number of polypes and of calicoblastic cell per polyp, the energetic cost of the biomineralisation process display important roles too. We have therefore avoided presenting speculative calculations that could give a misleading impression of precision.
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AC2: 'Reply on RC2', Nathalie Vigier, 21 Oct 2026
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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 -
AC3: 'Reply on RC3', Nathalie Vigier, 21 Oct 2026
We thank you for your insightful and constructive feedback, which has helped us refine our manuscript. In response to your comments, we have carefully addressed each point by:
- Expanding the methodology section with additional technical details and justifications,
- Incorporating supplementary data and references
- Correcting inaccuracies and clarifying ambiguous passages.
While we understand that a revised manuscript cannot be submitted at this stage, we outline below our proposed modifications for each of your requests. We hope that these changes enhance the rigor, clarity, and overall robustness of our work, and we look forward to implementing them fully in the next iteration.
Please find our responses below each comment respectively and new tables or revised figure as attached documents.
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?
Response : Indeed, Sr/Ca has also been used, this will be specified with appropriate references
L53: Please add "e.g." before the references.
Response : this will be done
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.
Response : We will replace ‘recognized’ by ‘proposed’
L66: Please also cite earlier studies such as Rollion-Bard et al. (2003) and Allison et al. (2010).
Response : These references will be added
L67: Which environmental conditions are you referring to?
Response : for instance temperature, pH, salinity, and parameters relative to water chemistry
Figure 1: Please provide references supporting the reported pH values.
Response : this will be done (Comeau et al. 2022)
L86–87: Why should Na⁺ behave similarly to Li⁺? Is this behaviour restricted to biological systems? If so, please explain why.
Response : both elements are alkali elements and monovalent, and in biological systems, for ionic transport in particular, the valence appears as an important parameter. In the environment, often Li covary with Mg because they have similar ionic radii and can enter into similar crystalline sites (Vigier et al 2008) - We will add this information to the text.
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.
Response : we fully agree and will add DIC from the Introduction section.
L111–116: Please report the salinity at both sampling sites.
Response : from Pichler et al. 2019, the salinity was 34 and did not differ among the sites
L142: fascicularis instead of Fascicularis.
Response : this will be corrected
Please explain how B/Ca and Li/Mg were measured and report the associated analytical uncertainties.
Response : We will provide additional information on the analytical methods. Trace elements, including B/Ca, were published by Comeau et al. (2022), they were measured by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS; X-Series II, Thermo Fisher Scientific) using solutions containing approximately 10 ppm Ca in 2% HNO₃, following Holcomb et al. (2015). Uncertainties were estimated to be 10%.
L168 and L214: Please use a consistent notation for JCp-1 throughout the manuscript.
Response : this will be done
L170: Please use δ instead of d.
Response : this will be done
L171: Please remove the reference to Trotter et al. (2011). This paper did not firstly define the equation used here.
Response : this will be done
L173: Please provide a reference for the calculation of the dissociation constant.
Response : These calculations were performed by Comeau et al. 2022.
L177: Please specify that the calculations are based on B/Ca ratios.
Response : this will be done
Please include a section describing the propagation of uncertainties in the calculations of pHcf and DICcf.
Response : These calculations were already performed by Comeau et al. 2022.
L178: What do "(15, 17, 18)" refer to?
Response : this will be removed
L185: Please remove the "s" from "isotopes".
Response : this will be done
L205: Please remove the reference to Burton & Vigier.
Response : this will be done
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).
Response : Li blanks were determined to range routinely between 5 and 10 pg, about one per mil of the Li signal. We will report consistent uncertainties in the revised version.
L216–220: This paragraph would fit better at the end of Section 3.1.
Response : this will be moved to section 3.1
L225: Please add an "s" to "species".
Response : this will be corrected
L236: Please use Δ instead of D.
Response : this will be corrected
Were B/Ca, Li/Mg and δ¹¹B measured in seawater at both sampling sites? If so, please report these values.
Response : Boron chemistry is from Comeau et al. (2022) and we will add a new table in the Sup. Info. displaying Li chemistry for four seawater samples collected at each site (see attached document)
L245: Please report the Ca²⁺ concentration used to calculate Ωarag. What Ksp value was used? Please also provide the corresponding reference.
Response : Aragonite saturation state was calculated using the R package seacarb, assuming a salinity of 34. The Ca²⁺ concentration was calculated from salinity following the relationship implemented in seacarb, [Ca²⁺] = 0.01028 × S/35 (Dickson et al., 2007), corresponding to 9.99 mmol kg⁻¹ at S = 34. The aragonite solubility product (Ksp) was calculated as a function of temperature and salinity using the formulation of Mucci (1983), with the pressure correction of Millero (1995), as implemented in seacarb.
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.
Response : this will be done
L270–276: Please remove this paragraph.
Response : sorry we do not understand why we should remove this paragraph that appears important to us
L283: "sp." should not be italicised.
Response : this will be corrected
L284: Please add an "s" to "species".
Response : this will be corrected
L286: Please include uncertainties for the pHcf values.
Response : this will be corrected
L289: "others" instead of "other".
Response : this will be corrected
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?
Response : comparing Li chemistry in soft and in calcified tissues may bring some keys to better understand the Li transfer, accumulation or excretion by living organisms.
L305–306: Why were the datasets from Fowley et al., Hathorne et al., and Rollion-Bard & Blamart not included in the comparison?
Response : we compare our data to the range of values estimated from the literature and we will include these references
L312: Please remove "for all corals".
Response : this will be done
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.
Response : this will be added.
L342: "...or enriched in DIC."
Response : this will be added.
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?
Response : Li concentrations in coral aragonite is one order of magnitude lower than in inorganic aragonite grown under similar conditions , which suggests a biological regulation of Li. This part is largely discussed in the following parts of the discussion section.
L361–363: I do not understand this sentence. Is the aquarium effect really relevant to the present study?
Response : we will correct this sentence : the variability displayed by d7Li values of corals grown at the Ambitle control site ( 1.45 ‰ ) is twice that given by seven species living in an aquarium fed by Mediterranean seawater (0.7 ‰, see Bastian et al. 2018). This also indicates an "aquarium effect” potentially related to a biological regulation of lithium that may be affected by experimental conditions.
Figure 5: Please revise the formatting of the species names. What do you mean by "each purple dot"?
Response : we will remove ‘purple’
L375: Although a review paper can be cited, the original studies should also be referenced.
Response : we will cite original references
L375–381: Please remove this section. It is not directly relevant to the objectives of this study.
Response : it appears important to us to provide arguments that studying corals sampled from their natural environment is key for lithium studies.
L386: Which elements?
Response : Sr, Ba, Li for instance
L388: Same comment as for Line 375.
Response : this reference was not appropriate and we will change it
L394: Please add "e.g." before the references.
Response : this will be done
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.
Response : the details are shown in figure 6 . we will rephrase this sentence.
L418: "...whether pH or DIC."
Response : this will be added
Figure 7: Please define all abbreviations used in the figure legend (Li/Ca, AVG Li, etc.).
Response : this will be done.
Figure 8: Please use the same terminology in the figure legend as that introduced around Line 485.
Response : this will be done.
I do not understand the statement referring to "corals living under high pCO₂" .
Response : this will be corrected (corals from the seep site).
In the figure, "sp." should not be italicised, and fascicularis should begin with a lower-case letter.
Response : this will be corrected.
L464: Please add a reference.
Response : this will be done
L465: Has it been demonstrated that corals precipitate exclusively from HCO₃⁻? Why are carbonate ions not considered?
Response : The precipitation of calcium carbonate (CaCO₃) by corals does not occur exclusively from bicarbonate ions (HCO₃⁻), but these ions play a central role in the process, particularly due to the chemical conditions of their internal environment (the calicoblastic layer or calcifying tissues). Carbonate ions (CO₃²⁻) are rarely considered the direct source.
L469: Tambutté instead of Tambutte.
Response : this will be corrected.
L472: Please add an "s" to "supports".
Response : this will be corrected.
L478: If activity is enhanced, why is there no corresponding increase in Li concentration?
Response : This is a good question and probably because the transporters activity not only concerns Li uptake but also Li elimination, this is why all our arguments in the previous part concerning indirect demonstration of Li regulation appear important to us.
L482: Albarède instead of Albarede.
Response : this will be corrected.
Figure 9: Please also present the data as a function of DICcf, which is already mentioned in the figure legend.
Response : this will be done
L497–499: Same comment as above. Why would this process not affect Li concentration?
Response : we can complete our previous answer by adding that maintaining Li concentration at a similar level despite changes in the proton or in the carbonate chemistry implies a change in ionic transport that induces a change in Li isotopic compositions (6Li being transported more rapidly than 7Li, e.g. Poet et al. 2023).
L507: There appears to be an extra underscore.
Response : this will be corrected.
L514: Why use the term "altered"?
Response : this will be completed by ‘compared to the control site’.
L515: By "calcifying fluid chemistry", do you mean only pH and DIC? If so, please state this explicitly.
Response : yes, this will be done
L519–520: This statement is too broad. Please either remove it or support it with appropriate examples.
Response : we will complete our sentence since our work shows that Li isotopes can bring complementary information compared to boron isotopes
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AC3: 'Reply on RC3', Nathalie Vigier, 21 Oct 2026
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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 -
AC4: 'Reply on RC4', Nathalie Vigier, 21 Oct 2026
We thank you for your insightful feedback, which has helped us refine our manuscript. While we understand that a revised manuscript cannot be submitted at this stage, we outline below our answers for each of your requests. We hope that these changes will enhance the clarity of our work, and we look forward to implementing them fully in the next iteration.
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 unclear with a mix of the Li response and the organismal physiology mixed together.
Response : we propose to simplify the last part of the abstract to clarify it.
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.
Response : This proposition is exactly the one that we follow in our abstract, and therefore we wanted to start the introduction section as a continuity, describing in more details the geochemical approaches, such that we do not repeat the same sentences of the abstract. We propose in the revised version to delete the first sentence of the Introduction such that the first point of this section directly concerns the ocean acidification.
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).
Response : as answered to the other reviewers, we will add several pieces of information to the Methods section, including the sampling, the instruments used, analytical uncertainties, as well as a new table of data for the seawater samples and their Li chemistry.
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.
Response : thank you for this positive comment. We will clarify the pool throughout the text.
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”.
Response : thank you for this suggestion. Lines 401-403 concern mostly the Mg transport, which is not the main focus of this study, but we still think that it deserves to be highlighted to some readers who investigate Li/Mg as a proxy for past environments. We are not sure to understand what you mean by the ‘why’ aspect given that intentional behaviour may not be easy to demonstrate here.
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.
Response : many thanks for your support as, indeed, we wanted to present several aspects in the Discussion that we think are relevant for readers who may come from different disciplines (e.g. coral physiology, isotope geochemistry, paleoceanography).
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.
Response : yes we will reword this part to make it clearer
L52: have been widely used
Response : this will be corrected
L59-60: It may be the rendering on my system, but if not there, suggest adding a paragraph break here.
Response : this will be done
L60-73: suggest separating the physiological process from the geochemical approach used to determine the physiology.
Response : this will be done
L124: Suggest including an indication of how long the venting has been taking place. This is important contextual information to help understand if the corals are likely to be acclimated to their environment.
Response : the venting system was first described by Pichler and Dix in 1996 and is believed to have existed for approximately 4,000 years.
L137: How were the corals sampled?
Response : we apologize for this missing information that we will add to the revised version. For each of the eight coral species, we collected one branch tip or skeletal fragment (3–7 cm long) from each of three colonies separated by approximately 5 m.
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.
Response : we will delete ‘either’ and will be more specific (note that pictures of all skeletons are shown in figure 3)
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.
Response : this will be done
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.
Response : this will be done
L170: Greek delta rather than letter d
Response : this will be corrected
L196: Suggest using the word conducted rather than done.
Response : this will be corrected
L199: Fine to use term “were dried” without the word “up”.
Response : this will be corrected
L206: ratios
Response : this will be corrected
L216-220: See above suggestion for addition information here.
Response : this will be considered
L219: For completeness suggest clarifying the seawater referred to is the same “treatment” (control or seep) as the corresponding coral.
Response : this will be done
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.
Response : we will delete the first sentence that was confusing indeed
L225: species
Response : this will be corrected
L226: values
Response : this will be corrected
L240-243: Suggest reversing the sentence starting with the observation then cite the refs and database in brackets.
Response : this will be done
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?
Response : as described in Comeau et al. (2022), we used the R package seacarb, and more information is written in the caption of our new Table (see our attached document), which reports all the carbonate chemistry data and parameters for each sampling station.
L284: species
Response : this will be corrected
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.
Response : this will be specified
L351-352: If the analytical approach is being included for one ref (SIMS) the corresponding approach should be included for the other reference also.
Response : sorry we do not understand what you mean here
L344-356: I 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.
Response : it may appear a little repetitive but for most geochemists, it is not widely accepted that the biology may influence Li chemistry and Li isotopes in biogenically formed calcium carbonate. As a consequence, it appears important to us to describe these evidences here. The plausible mechanisms at play are presented in the following sections.
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.
Response : we have not done some cultures but compare our results to cultured corals. We will make this clearer in the text.
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.
Response : this will be done
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.
Response : this will be done for further clarification of this part
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.
Response : we will improve the wording of section 4.1 as you advise, but still prefer to discuss the plausible mechanisms at play in a separate section as for isotope geochemists, it remains important to group together all the arguments pointing towards a link between lithium chemistry and biology. Then, for biologists and ecologists, the section 4.4 will be of interest since it provides some explanation to the observations.
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.
Response : the unpacking of possible mechanisms starts just after, in section 4.4. Please see also our answers to other reviewers as we provide some explanations but we do not want to be too speculative by providing under constrained calculations.
L441: Suggest labelling the coral in plate B as well.
Response : this will be done
L536: Funding rather than fundings
Response : this will be corrected
Citation: https://doi.org/10.5194/egusphere-2026-3326-AC4
-
AC4: 'Reply on RC4', Nathalie Vigier, 21 Oct 2026
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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:
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 pHcf 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 pHcf and δ7Licarb. 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 δ7Lisw 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.