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: open (until 31 Jul 2026)
- RC1: 'Reviewer comment on egusphere-2026-3326', Anonymous Referee #1, 06 Jul 2026 reply
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RC2: 'Comment on egusphere-2026-3326', Anonymous Referee #2, 06 Jul 2026
reply
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
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.