Dear Dr. de Winter,



I was pleased to review manuscript # 2025-2626 “The influence of irradiance and interspecific differences on δ11B, δ13C and elemental ratios in four coralline algae complexes” by Guillermic and colleagues. The manuscript represents a significant contribution to the understandings of coralline algal geochemistry and calcification mechanisms, which are still poorly understood, especially given the growing number of identified species and the small research group that studies them. I agree with the authors that this study represents part of the groundwork required to validate the use of certain paleoenvironmental proxies. The major findings of insignificant effects of irradiance on δ11B, δ13C and elemental ratios in four coralline algae species, but also notable differences in DIC modulations between geniculate and non-geniculate species represents an important step towards understanding calcification mechanisms and biological processes among diverse coralline algal morphologies and species. The manuscript is well presented, clear, and data support the findings. Except for a few technical corrections, figures clearly demonstrate findings and support interpretations. It is rare that a paper includes this abundance of data collected and from multiple species. I recommend that the manuscript be accepted subject to minor corrections.

I remain available if you have any questions. 

Regards

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Scientific signifiance: Excellent

Scientific quality:  Excellent

Presentation quality: Good

Reviewer Recommendation: Accepted subject to minor revisions

* I would not be willing to review the revised manuscript.

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General Comments:

• I think the manuscript would benefit with a higher impact statement in the introduction and abstract on what the potentially risks are of not understanding irradiance impacts on calcification and geochemistry (e.g., erroneous paleoenvironmental reconstructions, linking the wrong parameter to calcification rates, providing conflicting data between different archives and creating doubt in paleo-environmental timeseries, and potentially even more importantly, environmental forecasts which allow us to put in place the proper environmental policies and protections, etc.). This would better elucidate the importance of the study.

Specific Comments:

• The point made on line 456 is very interesting: “These differences could be explained by the competition experienced by non-geniculate species to not be overgrown (e.g. turf algae) which must also rely on fast calcification while geniculate species must compensate for a more dynamic environment and prioritize other needs (e.g. grazing, repairs)”.
  • A short description on morphologies and “behaviour” (e.g., rolling / mobile vs. encrusting / immobile) could be highlighted somewhere in the introduction to “foreshadow” this point.
• In the methods section 2.3., I was unclear about the temporal span represented by these algae samples that were powdered. Were spores deposited in the water column to cultivate the algae from scratch or were small crusts/branches collected and placed in tanks experiments? If the latter, provide growth rates of species and the time represented by each species / samples. Clarifying which growth layers are powdered for analysis would also be important to know (specific language on whole samples including epithallus and perithallus). Could different temporalities represented by the different species be responsible for some of the geochemical differences reported here?
• The introduction would benefit from a short theoretical explanation on boron, carbon, Li and Mg fractionation and how it is affected by seawater and calcification etc. Some of this information is found in the discussion and methods, but should be discussed in the introduction.
• Line 114: Ref (Gaillardet et al., 2001, Wang et al., 2008). Indicate if the more specific procedure is found in these papers. If not, add the time(s) of dissolution etc. 
• In the introduction and discussion (maybe section 4.6), there is a missing statement on why this specific study was conducted. Something like, “In other words, previous studies such as Anagnostou et al. found […] but lacked understandings about […]. This understanding is critical because without it we risk […].” Or focus on Comeau et al. 2019’s findings and state something like “To further test Comeau et al’s hypothesis we investigated calcification differences between faster and slower growing coralline algae species using geochemical tracers.
  • We need something clear that states how this paper builds on what is already known.
  • In the discussion: a comment can be added about how internal pH has been studied here and additional studies on how coralline algae modulate pH CF, DIC CF calcification would be helpful to capture the limits of plasticity of photosynthesis and calcification modulation with increasing ocean acidification as to provide limits or warnings for policy application?
• Section 4.7: could you add that this study also supports that well defined DNA work might be required to calibrate geochemical data to the species for paleoenvironmental reconstructions?
• Figure 4: Provide a more elaborate description especially for panels A-D. Consider also adding a label to colour bar (axis)
• Figure 7: Explanation for larger symbols and error bars.

Technical Corrections:

• Line 70: “CCA” This is the only place where this acronym is used. It is also not explained anywhere
• Line 162: “However, Krieger et al. (2023) presented two significant relationships, one non-linear for Corallina and one non-linear for Spongites when the full dataset was taken into account”.  Clarify if this refers to a relationship between net calcification and irradiance. If so, consider inversing the order of the two last sentences of the paragraph.
• Line 180 to 181, when possible, always add geniculate or non-geniculate adjectives before species to orient readers. You do this most of the time, but check throughout
• The two back-to-back sentences “There are no significant linear relationships between δ11B and irradiance (Tables S3 and S4). No significant linear or non-linear regression was observed between δ11B and irradiance” are a bit redundant
• Sometimes e.g., is used instead of the correct i.e., (e.g., lines 369 and 371 .
• Line 412: make sure to define OA (ocean acidification). I think this is the only place the acronym is used
• Figure 3: correct the alignment of B)
  • I think the font of the axes and axes titles might also be smaller

Guillermic et al. (2025) seek to address a crucial literature gap by studying experimental evidence of the effect of varying levels of irradiance on growth in geniculate versus non-geniculate species complexes of algae. They use data from tank studies conducted by Krieger et al. (2023) where two geniculate complexes and two non-geniculate complexes of coralline algae were collected from two sites in Te Moana-o-Raukawa Cook Strait, Te Whanganui a Tara Wellington, Aotearoa New Zealand. All complexes were subjected to four different irradiance treatments representing naturally occurring levels at the site (0.6, 1.2, 1.8 and 2.3 mol photons/m²/day) with corresponding fluctuations in irradiance to account for the diurnal insolation pattern. Various isotope measurements and elemental ratios were calculated including d¹³C, d¹⁸O, d¹¹B, Mg/Ca, Li/Ca, Sr/Ca, U/Ca and Li/Mg. Additionally, gross photosynthesis along with other parameters of photosynthetic efficiency, net calcification and d¹¹B-derived pH_CF were measured and/or calculated.

All complexes, except the non-geniculate Phymatolithopsis showed a significant positive correlation between irradiance and parameters measuring photosynthetic activity. Significant positive correlations were also observed between d¹³C_mineral and irradiance, except in one geniculate complex, Corallina/Arthrocardia robust whose d¹³C_mineral remained stable across all treatments. d¹¹B-derived pH_CF generally stayed constant across all treatments. No significant differences in net calcification were observed across irradiance treatments. However, the most pertinent results contributing to addressing gaps in literature were the different calcification regimes observed between morphologies, where non-geniculate complexes showed higher net calcification and lower pH_CF than geniculate complexes despite the latter having higher gross photosynthesis. Further, the authors speculate that the differences in d¹³C_mineral based on morphology may be due to differences in DIC pools available to the geniculate versus non-geniculate morphologies and possibly individual complexes or differential uptake of CO₂ passively or through internal recycling as supported by B/Ca and U/Ca results. Overall, the authors have collected and analyzed a robust set of isotope and elemental data in addition to calculating various parameters for photosynthetic efficiency and calcification to effectively support their conclusions. The sample size and methods used are appropriate for the analyses being conducted and interpretations being made. I would suggest minor edits and request clarification to the text prior to publication to contextualize the study within broader algal literature and ensure balanced communication of the study’s results.

Title: The title is accurate, concise and descriptive.

• Lines 1 & 2: I would either add that specimens are from a mid-latitude location and temperate climate or indicate the study site/country (i.e., Aotearoa New Zealand)

Abstract: Abstract offers an effective and concise summary of the results and discussion.

• Line 13: Change the Arabic numeral “4” to the written “four” following writing conventions (i.e., numbers below nine that are not statistics are spelled out whereas numbers >10 are written as numerals).

Introduction:

General comments: The introduction is generally well-written and offers a logical progression discussing the ecosystem-level importance of coralline algae then further expanding on the proxy measurements of pH_CF, followed by discussion on the regulation of biogenic calcification by pH, light and photosynthesis. A main point for revision in the introduction would be to adjust the paragraph at lines 48-55 to summarize and reflect more on coralline algae literature and associated knowledge gaps, following which the summary of coral literature could be brought forward (i.e., indicating that the current gap in literature being addressed on the impact of light on coralline algal calcite formation has been explored more extensively in coral literature).

Specific Recommendations:

• Lines 37 & 38: Awkward phrasing here with the use of the word “some” twice. I would remove the phrase “but with some evidence” and simply write “evidence suggests”.
• Line 41: Coralline algae have already been used for paleoclimate reconstruction. It may be more accurate to write something to the effect of “To increase the reliability of coralline algae paleoclimate reconstructions, a good understanding…”
• Line 43: As pH_CF refers to pH of calcifying fluid, explicitly define the acronym as “pH of calcifying fluid (pH_CF)” in accordance with writing conventions.
• Lines 46-47: More recent reference to include would be Cornwall et al. (2020): A coralline alga gains tolerance to ocean acidification over multiple generations of exposure.
• Lines 48-55: This is a good summary of existing coral research, however we are missing the direct connection to coralline algae in this paragraph. I would suggest a more concise explanation of coral research in favour of additional background on pH geochemical tracers in coralline algae including studies cited in the introduction already (e.g., Donald et al., 2017) or linking existing coral research to emerging coralline algae research.
• Lines 56-62: I would also add more explicitly here that there may be differences in light adaptation and calcification mechanisms for species in tropical vs temperate vs polar environments (i.e., latitude and climate play a role) explaining some of this variability, possibly before the sentence on line 58. Gould et al. (2022) and Williams et al. (2018) are some examples for Arctic studies examining the relationship between light and calcification that have not been cited and show that calcification is reduced during periods of low irradiance but still occurs at decreased rates.
  • Williams et al., 2018: Effects of light and temperature on Mg uptake, growth, and calcification in the proxy climate archive Clathromorphum compactum;
  • Gould et al., 2022: Growth as a function of sea ice cover, light and temperature in the arctic/subarctic coralline C. compactum: A year-long in situ experiment in the high arctic).
• Line 70: Ensure the acronym, “CCA” is defined prior to using it. There do not appear to be other instances where “CCA” is used in the text, therefore the term can be written in its full form.
• Line 75: δ¹⁸O is not discussed in the body of the paper, so should either be excluded here or if there are any relevant results they may be briefly discussed.

Methods:

General comments: The methods section is clear and concise. Appropriate methods are used to address the defined research questions with redundancies built into the methodology for robust interpretation. Most concerns here are related to clarity of writing. I would recommend minor changes listed below to follow formal/academic writing conventions. The only concern of significance here is the inclusion of the Mantel test methodology which does not seem to be well explained. While the figure itself offers a useful summary, there is little reference to it in the text. I would recommend elaborating on its purpose in the context of interpretation of results or see further recommendations in the comments on the results section.

Specific recommendations:

• Line 84: “Latter” is used incorrectly here. This typically refers to the second of two items in a list (i.e., latter vs former) or the last item in a list. The phrase prior to the comma can be removed and Krieger et al. (2023) can be cited at the end of the sentence in parentheses. 
• Lines 86-87: The list of species complexes reads awkwardly here as the conjunction “and” is incorrectly placed. These lines could be rephrased as follows: “For clarity, non-geniculate species will be referred to as Phymatolithopsis and Pneophyllum, while geniculate species will be referred to as Corallina/Arthrocardia fine and Corallina/Arthrocardia robust.”
• Line 96: Were the irradiances related to minimum and maximum values at the site, why were these specific intervals chosen? This does not seem to be detailed in Krieger et al. (2023) beyond indicating that these levels were observed at the site.
• Lines 98-99: It would be clearer to simply write that “eight header tanks each supplied six different experimental tanks…” The way it is currently written, on first read, the sentence suggests that header tanks only supplied six tanks in total.
• Lines 130-135: Based on the way line 134 defines δ¹¹B_sw, presumably the equation should show δ¹¹B_sw instead of δ¹¹B_seawater. “a” is also not defined as the equilibrium isotopic fractionation factor.
• Line 140: Phrasing reads awkwardly, may want to change to “best described the data”.
• Lines 149-150: What was the purpose of the Mantel test, did it assist in interpreting results or was it simply for data presentation? This section does not discuss how it was used, only describes the general definition of Mantel tests.
• Lines 150-154: Keep to a single convention when referring to figures, either Fig./Figs. or Figure/Figures. Variation occurs throughout the text when referencing figures.

Results:

General comments: The results are overall well-communicated with specific differences between morphologies highlighted as well as across irradiance treatments. The major concern in this section is apparent contradictions in Section 3.7 to other sections in the results and the PCA figure. I recommend reviewing this section for accuracy and adjusting to align with the other results sections.  

Specific recommendations:

• Lines 173 & 211: Ensure consistency with how all complexes are referred to (i.e., Corallina/Arthrocardia fine is repeatedly referred to as Corallina which can be confusing to the reader and require re-referencing figures/tables multiple times).
• Line 230: Mantel test results are not discussed at all, are they relevant to include? If the test was conducted for data presentation alone, a short summary of relevant correlations could be included at the end of the section (i.e., switch 3.6 and 3.7) or it could be incorporated into the PCA section of the results. If the the authors agree that this would be a redundancy, the section could be removed altogether, and the Mantel test figures could be moved to the supplemental materials or to the discussion as a summary figure.
• Lines 234-238: If interpretations of relationship in the PCA are made based solely on angles of vectors as indicated by the reference to Figure 4 at the end of the sentence, irradiance and net calcification show an obtuse angle, indicating a minor negative correlation between irradiance and net calcification contrary to previous interpretations of results and what has been written here. δ¹¹B and F_v/F_m seem to show a similar correlation in magnitude and direction to δ¹³C and net calcification (i.e., indicating that δ¹¹B and F_v/F_m correlation may not be minor if that is the case for net calcification and δ¹³C_mineral). Either specific references to correlation coefficients should be made to address the mismatch between the biplot and section 3.7 or the sentence in Lines 237-238 needs to be amended.
• Lines 239-241: These results appear to relay the exact opposite of what is indicated in section 3.2 (Lines 182-186) and section 3.3 (Lines 197-200). I would assume that this is an error, please amend to reflect the correct results (i.e., geniculate and non-geniculate should be switched to indicate that non-geniculate show higher net calcification, higher δ¹³C_mineral, and lower δ¹¹B, while geniculate coralline algae show lower net calcification, lower δ¹³C_mineral, and higher δ¹¹B.

Discussion:

General comments: The discussion is generally well-written, particularly sections 4.5 and 4.6. The main recommendations are to provide some clarification on certain claims and references to ensure that they are applicable. Otherwise, comments include minor corrections for grammar, flow and accuracy of statements.

Specific recommendations:

• Line 245: Section title should likely be Carbon isotopes (δ¹³C) as trace element discussion occurs towards the end of the discussion section.
• Line 247: Is this meant to say δ¹³C in both instances rather than ¹³C?
• Lines 252-255: Based on the results sectionand relationships shown in figure fS1 (i.e., δ¹³C_organic results) for geniculate coralline algae, δ¹³C_tissue and irradiance do not show a positive relationship. Only non-geniculate complexes show significant relationships, one of which is a positive, linear correlation. Therefore, it is unclear whether the second point in this paragraph can be inferred or supported by the given results. Please review and adjust section for accuracy or clarity of communication. 
• Line 263: Authors may consider rephrasing here to indicate that photosynthesis impacts the δ¹³C of the available DIC for calcification. “Enhancing” suggests that photosynthesis increases rate of net calcification (e.g., through increase of pH).
• Lines 273-276: Please review if Mao et al. (2024) is relevant to this case. The reference seems to indicate that CO₂ produced from calcification is recycled for photosynthesis and not vice versa.  However, as it is written here, the explanation suggests that carbon used for photosynthesis is recycled internally for calcification thereby affecting δ¹³C_mineral. HCO₃^- is actively pumped into cells for calcification and photosynthesis. CO₂ produced by calcification or respiration may be recycled for photosynthesis, and products of photosynthesis like ATP are used for calcification, however it would not follow that carbon from photosynthesis would be directly recycled for calcification. Re-wording of this section or clarification may be necessary. It may be possible for HCO₃^- released from respiration to be recycled for calcification, thereby reducing δ¹³C_mineral, and as inputs for respiration are derived from photosynthesis, δ¹³C of DIC available for calcification could be indirectly affected by photosynthesis.
• Lines 279-280: Rather than “allows us”, it may be more accurate to write something to the effect of: “Our results show that the geochemical signatures of the mineral are impacted by changing irradiances indicating potential changes in pH_CF, which we analyzed by boron isotope proxy.”
• Line 376: “Few differences”, should likely be changed to “a few differences,” or simply “differences between morphologies…”.
• Line 415: I would restructure this section as encrusting species are much more commonly and successfully used for paleoenvironmental reconstructions than rhodoliths in coralline algae literature. Records produced from encrusting individuals are less impacted by differential light exposure since they are anchored to an unmoving substrate unlike free-living rhodoliths where a face of the organism is always buried in sediment.
• Line 420: Adjust this section of the sentence to make grammatical sense: “which does not produce additional complexity the use of the proxy.”
• Line 425: It may also be relevant to include that a multi-proxy approach could be applied to proxies like Mg/Ca that are affected by multiple variables. Additionally, light availability would likely affect species adapted to different latitudes and depths uniquely in addition to differences in effects by morphology, so it would be beneficial to indicate that the results could possibly apply to other mid-latitude species but not all coralline algae (e.g., Arctic species are adapted to much lower light conditions where it has been suggested that stored photosynthates can be used to support calcification during winter months as indicated by Adey et al. (2013) and Gould et al. (2022)).

Conclusion:

General comments: The conclusion provides an excellent summary of the research and pertinent results as well as interpretations. The only recommendation would be to acknowledge that as study results may be species-specific and morphology specific, they could be cautiously generalized to mid-latitude species but additional replication of the study is necessary for species adapted to different light regimes.

Specific recommendations:

• Line 471: Additional studies should also be repeated with different coralline algal species that experience different irradiance regimes and environments (i.e., are there differences between algal species that are adapted to living at greater depths and higher/lower latitudes with lower access to light).

Figures:

• Figures 4, 6, 7, 8: Figures require more detailed figure captions, including drawing reader attention to pertinent results accompanied by applicable statistics. All figures should follow the same format in describing sub-figures in the caption.
• Figures 5, 7, S3 & S5: Color schemes should be accessible and consistent across figures (e.g., Figure 8 is not accessible to those with blue-yellow color blindness, Figures 5, 7, and S5 are not accessible to those with red-green color blindness).
• Figures 5 & 7: The changes between irradiance are quite difficult to distinguish with the size of the data points. Either the size must be increased, or figures should be separated. Alternatively, the four average data points could be colored to represent irradiances and individual data points in the background could be eliminated if sample size was indicated in the legend or figure caption.
• Figure 5: Are each of the four black-filled and black-outlined geometric shapes representing individual coralline algae averages per species, morphology type and irradiance as shown in previous figures while the smaller data points are all individual measurements taken at each irradiance as in Figure 3? Please include this in the figure caption to clarify in more detail. Observing trends based on irradiance as described in body of paper is difficult in these figures, for example in Figure 5A the highest irradiance for Pneophyllum showes lower δ¹³C_mineral than at the second highest irradiance. Does this indicate that at 2.3 mol photon/m²/day the point at which photochemical quenching is at its maximum has been exceeded? If so, this should be noted, as it appears to be inconsistent with the claim in the discussion.
• Figures S1& S2: Y-axis labels are missing for these figures. Ensure to be consistent with the inclusion of R² and p-values across the figures. At minimum, both should be included for significant results if not all.