the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2026
Light availability controls multi-decadal trends in crustose coralline algal cell elongation
Abstract. Crustose coralline algae (CCA) are globally distributed calcifying macroalgae that can grow as free-living rhodoliths, act as ecosystem engineers by creating complex three-dimensional habitats, and contribute to the carbon cycle. The dimensions of cells in the carbonate skeleton influence rhodolith structural integrity and, consequently, the ecological functions these rhodoliths provide, yet CCA cellular responses to environmental change remain poorly resolved. This study quantifies multi-decadal variability in skeletal cell dimensions of the rhodolith-forming CCA Sporolithon nodosum from New Zealand and evaluates the relative importance of sea surface radiation (SSR), sea surface temperature (SST) and sea surface CO2 partial pressure (pCO2) as potential drivers. The length and width of 2975 cells were measured along a 38-year transect (1985–2022) using stitched scanning electron microscope images. Cell length declined significantly over time (R2adj = 0.105, p = 0.027), whereas cell width showed no temporal trend (R2adj = -0.019, p = 0.583). Among the environmental variables, SSR was the strongest predictor of cell length (R2adj = 0.235, p = 0.001), while SST and pCO2 explained comparatively little variance. These results identify light availability as a primary correlate of cell elongation in S. nodosum, consistent with a role for irradiance-driven changes in photosynthetic energy supply. Because long-term SSR trends are spatially heterogeneous, light-mediated shifts in CCA cell dimensions – and their potential implications for rhodolith structure and ecosystem function – are likely to vary regionally. This study highlights the central role of light availability in shaping CCA cell morphology under changing surface-ocean conditions and motivates multi-site comparisons to assess broader ecosystem implications.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-2316', Anonymous Referee #1, 03 Jun 2026
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AC1: 'Reply on RC1', Bastian Seidl, 26 Aug 2026
Dear Reviewer,
Thank you very much for your careful evaluation of our manuscript, your positive assessment of the relevance of our study, and your constructive comments. We also appreciate your suggestions for future research on variability in cell dimensions and their potential controls.
We agree that the analysis of a single protuberance from a single rhodolith represents an important limitation of our study and was not sufficiently addressed in the original manuscript. This prevents us from assessing intra- and interspecimen variability and limits the extent to which the observed relationships can be generalised beyond the analysed specimen. We will therefore add a dedicated limitations paragraph addressing the lack of biological replication and revise the Discussion and Conclusion to restrict our interpretations to the analysed specimen where appropriate.
Our responses to the specific comments included in the reviewer’s PDF are provided below.
Line 29: (e.g.: Caragnano et al., 2016)
We agree and will add “e.g.,” to clarify that the reference is included as an example.
Line 30: (e.g.: Whisshak et al., 2017; López Correa et al., 2023)
We agree and will add “e.g.,” to clarify that these references are included as examples.
Line 44: Please add Reference to Foster et al., 2001. Foster, M. S. (2001). Rhodoliths: Between Rocks and Soft Places. Journal of Phycology, 37(4), 659-667.
Thank you for pointing this out. We will add Foster (2001) to the relevant statement.
Line 242: This is defintely too much defintiive to me, given that you measured only one protuberance of one specimen. It is for sure relevant and the most well correlated in your results, but I might be a bit more cautious. As well indicated in the Discussion (well written) what is clear is that the correlation between growth parameters (cell width and lenght in this case) and specific environmental control factors depends on highly site-specific conditions and even on the species involved.
We agree that our interpretation is too definitive given that the analysis is based on a single protuberance from one specimen. While SSR was the strongest predictor of cell length among the environmental parameters analysed, we cannot determine whether this relationship extends beyond the analysed specimen. We will therefore revise this statement and similar statements throughout the Discussion and Conclusion to restrict our interpretations accordingly.
Line 296: This is particularly interesting and may be related to other changes (cell wall thickness/lumen size?). I recommend that you look into this further in the future.
We agree that cell wall thickness and lumen size may vary even if total cell width remains relatively stable. Investigating these parameters could reveal additional aspects of cellular variability and represents an interesting direction for future research.
Line 326: This, combined with cyclical turbidity, might suggest that some form of SSR attenuation is indeed necessary for CCA survival. I would not rule out this possibility outright, as it would call into question everything that has been said so far; however, it remains a hypothesis, given that the SSR ultimately explains “only” 23.5% of the variability.
We agree that our statement that the observed correlation likely underestimates the influence of light availability is too speculative, as higher light availability may not necessarily have an increasingly positive effect on cell elongation. We will therefore remove this interpretation and instead focus on the uncertainty regarding the in situ irradiance experienced by the analysed rhodolith. We will also acknowledge that attenuation through turbidity or shading may be beneficial under high-irradiance conditions, although this cannot be evaluated with the available data.
Line 333: That is a valid point, but I don’t think measuring protuberances along the three axes will resolve the issue, because that would still amount to describing a snapshot at the moment of sampling. Perhaps a microstructural study of the evolution of the branches and filaments (high-resolution micro-CT?).
We agree that analysing protuberances along the three axes would provide only a snapshot at the time of sampling and would not reveal their past exposure conditions. Our intention was rather to suggest that analysing multiple protuberances across a rhodolith could help quantify intraspecimen variability. We will clarify this in the revised manuscript.
Thank you again for your helpful comments and suggestions.
Citation: https://doi.org/10.5194/egusphere-2026-2316-AC1
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AC1: 'Reply on RC1', Bastian Seidl, 26 Aug 2026
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RC2: 'Comment on egusphere-2026-2316', Anonymous Referee #2, 04 Aug 2026
This manuscript investigates variability in cell sizes in red coralline algae over a 38 year period from a Sporolithon specimen.
This is a valuable and understudied topic that will help us to understand past, present and future growth of coralline algae. The conclusion that light availability is most related to physically cellular structure is not surprising given previous work and the photosynthetic nature of coralline algae, but it is important to continue to build support for this as it will be useful information for understanding how this algal group may fare in the future. The strength of this relationship with light is significant but not strong (r=0.41), suggesting there are other factor(s) also regulating cell size parameters in this coralline alga.
Although I see value in the concept of this manuscript, I have two significant concerns that, in my view, must be addressed before this manuscript is formally published – the first can be addressed in a revision, the second requires further data analysis, or a complete revision of the manuscript’s objectives.
- Terminologies
Throughout the manuscript there is a confusion and mix between CCA and rhodoliths – indeed the title states “crustose coralline algae” but the methods explicitly state (and visually show; Figure 1) that this study is focused on data collected from a rhodolith branch. Both are coralline algae, but they are distinct morphologies with separate terminologies and should not be used interchangeably.
This is important because confusion between terminologies cascades into confusion in the conclusions and subsequent use of the results (e.g informing studies projecting future growth, or identification of conservation priority areas).
The whole manuscript *(including the title) must be revised to reflect the focus here on rhodoliths not CCA, and inaccurate sentences should be edited (e.g. line 36: CCA do not provide “complex 3D habitats such as rhodolith beds” – rhodoliths do). This also applies to the references cited in the Introduction, which predominately bias towards CCA studies and exclude many highly relevant rhodolith studies (e.g. pH proxies, biogeochemical importance).
Please consult Jardim et al (2025) Aquatic Conservation for the most up-to-date morphological organismal typology for coralline algae:
Jardim, Victor L., et al. "A common terminology to unify research and conservation of coralline algae and the habitats they create." Aquatic Conservation: Marine and Freshwater Ecosystems 35.3 (2025): e70121.
- Methodological robustness
My biggest concern is the use of just one branch, despite collecting multiple rhodoliths and preparing five branches for analysis. Based on the methodologies, just one was subsequently selected for analysis because it presented the ‘nicest’ banding structure. Surprisingly, there is not even an attempt at addressing the consequence on this on methodological robustness and the strength of the conclusions in the discussion, despite the fact that we know there can be considerable intra and inter-individual variability in the growth of coralline algae, and the relationship between that growth and environmental variability.
‘Tilted’ cells are a reality of rhodolith cross-sections because of their non-linear growth (ie bends in branches), and is precisely why cell structural parameters are not typically used in rhodolith proxy investigations – because one cannot avoid tilted cells in a rhodolith branch, and one cannot know the extent (or not) of ‘tilt’ from a longitudinal cut along the branch; just because the banding looks regular does not mean you have cut perpendicular through all the bands. A perfectly perpendicular cut through all 38 bands of this data set is highly unlikely to have occurred. Figure 2a supports this – the cells are not perfectly vertically aligned (and so why the assumption they are not tilting in another orientation?), and there are many areas where the inside of the cell lumen is not visible and therefore orientation of the pit connection cannot be determined. ´
Similarly, the location of the transect in the branch will also affect the results, particularly the ‘growth increment thickness’. Based on Figure 2d it appears that the authors sought maximum growth increment thickness (should be stated), but visually from Fig2d, the banding to the right of the transect appears wider.
In some cases, the outcome of this may only be small (small tilt = small measurement error), but the challenge is that this error cannot be quantified, and by selecting only the ‘best’ sample the robustness of the data collection is significantly reduced – through reduced data points and a bias towards a branch that grew particularly straight. It cannot be determined how representative the environmental-coralline relationships identified here are for other branches, other individuals nor other areas.
With this in mind, further data collection with multiple replicate branches (within and between branches and individuals) is necessary to achieve sufficient robustness in the data and to understand inter/intra-variability.
Sorry for the delay in submitting my response. Hopefully the comments can help to support further data collection, which would significantly improve the manuscript’s methodological robustness and strengthen the weight of the conclusions.
Citation: https://doi.org/10.5194/egusphere-2026-2316-RC2 -
AC2: 'Reply on RC2', Bastian Seidl, 26 Aug 2026
Dear Reviewer,
Thank you very much for your careful evaluation of our manuscript, your positive assessment of the relevance of our study, and your constructive comments.
Thank you for drawing our attention to the organismal typology proposed by Jardim et al. (2025). We agree that consistent terminology for coralline algae and the habitats they form is important for facilitating communication across different fields of research. We therefore carefully considered the proposed typology and its implications for our manuscript. In doing so, we also examined the more recent classification of non-geniculate coralline algal growth forms proposed by Maneveldt et al. (2026), which explicitly discusses Jardim et al. (2025).
In the original manuscript, we used “crustose coralline algae” (CCA) as a general term for non-geniculate coralline algae, rather than to distinguish attached from free-living growth forms. This usage is noted by Maneveldt et al. (2026), who state that non-geniculate corallines are commonly referred to as CCA, particularly in ecological studies. This broader use of CCA also continues in recent literature published after Jardim et al. (2025), where rhodoliths are described as being composed or formed by CCA (Vizon et al., 2025; Camacho and Fredericq, 2025; Jung et al., 2026; Schmidt et al., 2026; Johnson, 2026). We therefore do not consider our previous use of “CCA” and “rhodolith” to imply mutually exclusive categories.
Nevertheless, we recognise that using CCA in this broader sense may cause ambiguity, particularly given recent efforts to standardise terminology for coralline algae. Following the terminology used by Maneveldt et al. (2026), we will therefore use “non-geniculate coralline algae” (NGC) throughout the revised manuscript when referring to the algal organism, and “rhodolith” specifically when referring to the free-living growth form.
We agree that the analysis of a single protuberance from a single rhodolith represents an important limitation of our study. As the reviewer points out, this prevents us from assessing intra- and interspecimen variability and limits the extent to which the observed relationships can be generalised beyond the analysed specimen. We acknowledge that this limitation was not sufficiently addressed in the original manuscript.
The analysed protuberance was not selected because it presented the “nicest” banding structure, as suggested by the reviewer. As stated in Sect. 2.2 (Sample preparation), five protuberances were prepared to account for variability in cutting orientation, and the protuberance with the most regular alignment of cells relative to the image plane was selected to minimise measurement bias associated with tilted cells. This selection was based on SEM examination of the prepared sections and followed the methodological considerations outlined by Bracchi et al. (2021), who emphasise the importance of controlling cell orientation when measuring three-dimensional structures in two-dimensional sections of coralline algae. Analysing all five prepared sections irrespective of cell orientation could introduce orientation-related measurement variability, making it difficult to distinguish cutting-orientation effects from biological variability among protuberances. Reliable comparisons among multiple protuberances require consistent control of cell orientation across sections, which was not achieved among the five prepared sections.
We agree with the reviewer that regular growth banding does not demonstrate that bands or cells have been sectioned longitudinally. As described above, cell orientation relative to the image plane was instead assessed from cellular structures visible during SEM examination. To avoid ambiguity, we will make this distinction more explicit in the revised manuscript. Although some variation in cell orientation within the image plane is visible in Fig. 2a, this does not itself indicate the degree of tilt relative to the image plane. However, we acknowledge that our approach cannot ensure that every cell throughout the 38-year record was sectioned perfectly longitudinally, and will therefore explicitly recognise potential deviations from the longitudinal cell axis as a source of measurement uncertainty.
Regarding the position of the measurement transect, we assume that the reviewer is referring to Fig. 1d rather than Fig. 2d, as Fig. 1d shows the longitudinal section of the analysed protuberance and the transect position. We agree that growth-increment thickness can vary spatially within a protuberance and that some increments visible to the right of the transect appear wider. The transect was, however, not positioned to capture maximum growth-increment thickness. Rather, its placement was guided by the same methodological consideration described above: selecting an area with suitable cell orientation for quantitative cell-dimension measurements along a transect perpendicular to the growth increments. To avoid ambiguity, we will make this rationale for transect placement more explicit in the revised manuscript.
We agree that analyses of multiple protuberances within and among rhodoliths would be required to quantify intra- and interspecimen variability and to determine whether the relationships observed in the analysed specimen are representative of S. nodosum more broadly. However, quantifying such variability was not the objective of the present study, which focused on temporal variability in cell dimensions within a multi-decadal record from a single protuberance. The importance of extending this approach to multiple protuberances was already recognised in the original manuscript, where we identified such analyses as necessary for investigating intra- and interspecimen variability and proposed automated image analysis as a means of enabling more extensive sampling in future studies. We therefore agree that additional biological replication is necessary to assess the generality of the observed relationships, but do not consider such replication necessary for conclusions explicitly restricted to the analysed specimen. We also note that Referee #1 identified the analysis of a single protuberance as the principal limitation of the study, while assessing the laboratory methods as rigorous and the statistical analyses as robust, and recommended addressing this limitation through greater caution in the interpretation of the results. In response to the concerns raised by the reviewers, we will therefore add a dedicated limitations paragraph addressing the lack of biological replication and revise the Discussion and Conclusion to restrict our interpretations accordingly. We will explicitly state that the present study cannot determine whether the observed relationships are representative beyond the analysed specimen.
Thank you again for your helpful comments and suggestions.
References:
Bracchi, V. A., Piazza, G., and Basso, D.: A stable ultrastructural pattern despite variable cell size in Lithothamnion corallioides, Biogeosciences, 18, 6061–6076, https://doi.org/10.5194/bg-18-6061-2021, 2021.
Camacho, O. and Fredericq, S.: Diversity of Brown Macroalgae (Phaeophyceae) Emerging from Deepwater Rhodoliths Collected in the Gulf of Mexico, Diversity, 17, 860, https://doi.org/10.3390/d17120860, 2025.
Jardim, V. L., Grall, J., Barros-Barreto, M. B., Bizien, A., Benoit, T., Braga, J. C., Brodie, J., Burel, T., Cabrito, A., Diaz-Pulido, G., Gagnon, P., Hall-Spencer, J. M., Helias, M., Horta, P. A., Joshi, S., Kamenos, N. A., Kolzenburg, R., Krieger, E. C., Legrand, E., Page, T. M., Peña, V., Ragazzola, F., Rasmusson, L. M., Rendina, F., Schubert, N., Silva, J., Tâmega, F. T. S., Tauran, A., and Burdett, H. L.: A Common Terminology to Unify Research and Conservation of Coralline Algae and the Habitats They Create, Aquat. Conserv., 35, e70121, https://doi.org/10.1002/aqc.70121, 2025.
Johnson, M. E.: Rhodoliths as Global Contributors to a Carbonate Ecosystem Dominated by Coralline Red Algae with an Established Fossil Record, J. Mar. Sci. Eng., 14, 169, https://doi.org/10.3390/jmse14020169, 2026.
Jung, J., Wald, T., Foreman, A. D., Bieler, A. L., Janussen, D., Moretti, S., Duprey, N. N., Marconi, D., Pérez-Medina, C., O’Dea, A., Schiebel, R., Halfar, J., Fernández-García, C., Berger, M., Donovan, M. K., Frazier, A. E., Cramer, K., Haug, G. H., Sigman, D. M., and Martínez-García, A.: Crustose coralline algae biomineral-bound nitrogen isotopes provide a baseline to reconstruct coral trophic strategies, Commun. Earth Environ., 7, 438, https://doi.org/10.1038/s43247-026-03459-2, 2026.
Maneveldt, G. W., Brodie, J., Calderon, M. S., Diaz-Pulido, G., Gabrielson, P. W., Kato, A., Le Gall, L., Maridakis, C., Martone, P. T., Moro, I., Mrowicki, R. J., Nelson, W. A., Peña, V., Rindi, F., Rousseau, F., Schils, T., Schwoerbel, J., Tâmega, F. T. S., and Trentin, R.: An updated classification of growth forms in non-geniculate coralline algae (Corallinophycidae, Rhodophyta), J. Phycol., 62, 490–511, https://doi.org/10.1111/jpy.70144, 2026.
Schmidt, W. E., Arakaki, N., Gurgel, C. F. D., Gabriel, D., Sauvage, T., Norris, J. N., and Fredericq, S.: Two New Mesophotic Species of the Red Algal Genus Chrysymenia (Rhodymeniaceae, Rhodymeniales) from the Gulf of Mexico: C. stanlaurelii sp. nov. and C. oliverhardyi sp. nov., Diversity, 18, 320, https://doi.org/10.3390/d18060320, 2026.
Vizon, C., Lagourgue, L., Jorissen, H., Raviglione, D., Payri, C. E., Bonnard, I., and Nugues, M. M.: The metabolome of crustose coralline algae is driven by phylogeny and environmental conditions, Algal Res., 90, 104146, https://doi.org/10.1016/j.algal.2025.104146, 2025.
Citation: https://doi.org/10.5194/egusphere-2026-2316-AC2
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RC3: 'Comment on egusphere-2026-2316', Anonymous Referee #3, 12 Aug 2026
The manuscript presents cell measurements of nearly 3000 cells from the rhodolith-forming coralline algae Sporolithon nodosum, evaluating environmental variables (sea surface radiation (SSR), sea surface temperature (SST), and sea surface CO₂ partial pressure (pCO₂)) as potential drivers of coralline algal cell dimension variability.
The authors aim to motivate multi-site comparison of rhodolith cell dimensions by emphasizing the role of SSR on cell dimensions and its consequences for rhodolith structure and ecosystem function. However, the assumptions required to justify the utilized age model lack sufficient validation. At minimum, the authors must provide evidence from the literature demonstrating that the measured light/dark bands occur on an annual basis in rhodoliths from the same species and same/similar locality. Without this validation of temporal alignment, or clear rationale for the assumptions made, the claims relying on 38 years of continuous annual growth, including temporal alignment of surface-ocean environmental data, cannot be conclusively supported.
With proper reframing, this topic could interest the coralline algal community, which has long maintained that cell length and density may be, at least partially, seasonally controlled. However, the community has also acknowledged potential controls on growth increment formation including burial, tidal patterns, large-scale climate patterns, rolling, or vital effects. The mechanisms controlling calcite density and cell dimension remain unconstrained and appear species- and location-specific, requiring calibration. While rolling is likely not the only driver of growth hiatuses, it is considered by some as one of the primary influences. It is highly unlikely that a rhodolith could remain oriented the same way over ~40 years, a sentiment echoed by the authors themselves (Lines 330–334). If this were the case, the underside should be quantifiably unhealthy after being fixed and unexposed to sunlight for such an extended period. This information, or some explanation, should be added to contextualize assumptions about growth hiatuses. The authors cite Sletten et al., 2017 (Line 44) and other studies (e.g., McCoy and Kamenos, 2014; Freiwald and Heinrich, 1994) in which the unreliability of light/dark cycles of rhodolith banding as annual markers has been explicitly described. The definition that an annual cycle equals one alternating light/dark band, relied upon as the temporal basis for this study, renders the entirety of conclusions regarding environmental regressions fundamentally flawed without validation.
The study is based on a single Sporolithon nodosum rhodolith and one SEM transect (2975 cells), which limits generalizability across individuals and habitats. Given this, the strong temporal and environmental claims seem overextended. Additionally, without confidence in the age model, all resulting correlations with environmental variables (pCO₂, SST, SSR) are unfortunately spurious.
Line by line comments are attached.
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AC3: 'Reply on RC3', Bastian Seidl, 26 Aug 2026
Dear Reviewer,
Thank you very much for your careful evaluation of our manuscript, your positive assessment of the relevance of our study, and your constructive comments.
We agree that a free-living rhodolith is unlikely to remain in the same orientation over several decades. However, a constant orientation is not necessary for analysing cell dimensions through time, as rotation alone does not prevent the preservation of a continuous sequence of cells. Rather, the dieback of downward-facing algal thallus, which may appear as a hiatus after recolonisation, largely depends on the duration of unfavourable conditions and the characteristics of the sediment (Wilson et al., 2004). Importantly, frequent turnover may reduce the duration of unfavourable downward-facing conditions and thereby prevent thallus dieback between turning events (Schlüter et al., 2021). The shallow-water rhodolith habitat at Army Bay is characterised by strong hydrodynamic disturbance, and the rhodoliths frequently show abraded branch tips indicative of regular movement (Teichert et al., 2026). Should a growth hiatus occur, it can be identified by the presence of larger, perpendicularly arranged hypothallial cells that are distinct from the perithallial cells forming the majority of the thallus (Woelkerling, 1988). We acknowledge that temporary reductions or cessation of growth that do not result in thallus mortality would not necessarily produce such a structural discontinuity. However, these do not constitute growth hiatuses and would not interrupt the continuity of the preserved growth record. A growth hiatus, characterised by an eroded surface of the older thallus overlain by hypothallial cells, is visible at the base of the analysed SEM transect in Fig. 3a, marking the youngest hiatus observed in the algal thallus. Accordingly, our analysed transect ends above these hypothallial cells, and the growth increment to which they belonged was excluded from our analysis. Although the hypothallial cells are difficult to discern at the scale of Fig. 3a in the submitted manuscript, they are clearly visible in the full-resolution SEM transect provided in the Zenodo repository.
We appreciate the reviewer’s concern regarding chronological calibration and provide two independent lines of evidence supporting the annual interpretation of the growth increments.
Coralline algae have been shown to produce longer cells during summer and smaller cells during winter (e.g. Chan et al., 2017; Melbourne et al., 2023; Ragazzola et al., 2016). In our study, the chronology was initially established solely from alternating light and dark growth bands visible in the light-microscope section. When these independently assigned growth increments are compared with the SEM-derived cell-length record, cycles of shorter and longer cells show an approximate correspondence with the growth bands (Fig. 3), although some spatial offset occurs along the transect. Because this seasonal pattern was not used to establish the chronology, its correspondence with the independently assigned growth increments provides independent support for the interpretation of the light–dark band couplets as annual growth.
Seasonal Mg/Ca cycles are commonly used to establish coralline algal chronologies, with recurring Mg/Ca minima identified as winter growth markers and used to assign individual years (Gamboa et al., 2010; Halfar et al., 2011; Light et al., 2018). To provide a second independent check on the chronology of our specimen, we performed LA-ICP-MS 24Mg concentration (ppm) mapping on a separate section of the same rhodolith. The spacing of the resulting Mg cycles was similar (~100 µm) to that of the light–dark growth increments identified in the light-microscope section. This close agreement in cycle spacing provides a second independent line of evidence supporting the annual interpretation of the optically identified growth increments.
We will include the chronological justification in the revised manuscript and provide the detailed supporting evidence, including the LA-ICP-MS 24Mg map, in the Supplementary Material.
We agree that the analysis of a single protuberance from a single rhodolith represents an important limitation of our study and was not sufficiently addressed in the original manuscript. This prevents us from assessing intra- and interspecimen variability and limits the extent to which the observed relationships can be generalised beyond the analysed specimen. We will therefore add a dedicated limitations paragraph addressing the lack of biological replication and revise the Discussion and Conclusion to restrict our interpretations to the analysed specimen where appropriate. However, as outlined above, the annual interpretation of the growth increments is independently supported by cell-length cyclicity and Mg cyclicity, and we therefore do not consider the resulting environmental correlations to be spurious.
Our responses to the specific comments included in the reviewer’s PDF are provided below.
Line 19: Constructing an age model based entirely on growth increments is known to be unreliable, and as such I do not have confidence in the described temporal duration of the study.
We appreciate this concern. Based on the two independent lines of evidence provided above, we consider the annual interpretation of the growth increments to be well supported.
Line 34: In the interest of unifying language, the authors may benefit from adhering to the definitions of CCA vs. rhodoliths described in Jardim et al., 2025 (https://doi.org/10.1002/aqc.70121).
Thank you for this suggestion. As noted in our response to Referee #2, we will revise the terminology throughout the manuscript to avoid ambiguity. We will use “non-geniculate coralline algae” (NGC) when referring to the algal organism, and “rhodolith” when referring specifically to the free-living growth form.
Line 35: The authors may correct a typo: "CCA fulfill a wide range…"
Thank you for pointing this out. We use British English throughout the manuscript, for which “fulfil” is the correct spelling, and will therefore retain the current wording.
Line 43: The authors cite Sletten et al., 2017, though they still attempt to construct their age model based on growth increments, despite Sletten et al. explicitly demonstrating the unreliability of this approach.
We acknowledge the concerns raised by Sletten et al. (2017) regarding the use of growth increments alone for chronological assignment. We therefore provide independent support based on cell-length cyclicity and Mg cyclicity, as described above.
Line 48: Primary and secondary pit connections are additionally diagnostic features for taxonomic identification, which the authors may mention.
Thank you for this suggestion. We agree that primary and secondary pit connections are diagnostically important features in coralline algal taxonomy. We will revise the sentence to also mention their taxonomic significance and cite Woelkerling (1988).
Line 69: The authors cite Ragazzola et al., 2012, and as such must be aware of inconsistencies and the capacity for non-temporal controls to be exerted on cell length during anomalous conditions, which could very well have occurred over the last ~40 years. Even if there were a possibility to confirm age model chronological reliability, or sufficient evidence to support the annual nature of growth increments (which has been seen to be widely variable across different localities), these concerns would remain.
We agree that cell length may be influenced by environmental and biological factors beyond those investigated in the present study, including anomalous conditions. Importantly, however, cell length was not used to establish the chronology, but represents the response variable analysed independently of the optical growth-band assignment. Our analyses therefore do not assume that temporal variability in cell length reflects a single environmental control. Indeed, SSR explains only approximately 23.5% of the observed variability. In Sect. 4.3 (uncertainties affecting in situ light availability), we explicitly discuss additional sources of variability that are not captured by the environmental datasets used here, including episodic changes in light availability associated with turbidity, marine darkwaves and shading, as well as rhodolith rotation.
Line 117: Whether or not growth hiatuses are reliably identifiable remains an open question. I have concerns that without partial mortality during normal growth, these hiatuses would not be visible. As such, I would advise the authors not to refer to growth increments above the first visible hiatus as a reliable chronological control.
Thank you for raising this point. A growth hiatus in the strict sense requires thallus dieback followed by recolonisation and therefore represents a discontinuity in the preserved growth record (e.g. Schlüter et al., 2021). Temporary reductions in growth rate, for example when a protuberance is oriented downwards, do not constitute growth hiatuses if the thallus remains viable and growth subsequently continues. Such short-term variability may contribute to natural variability in growth, but does not interrupt the continuity of the preserved sequence or invalidate the interpretation of long-term trends. As clarified above, the youngest observed growth hiatus occurs at the base of the analysed SEM transect, where an eroded surface overlain by characteristic hypothallial cells indicates dieback followed by recolonisation. No comparable evidence of a growth hiatus is present in the analysed sequence above this boundary. To avoid ambiguity, we will replace the term “first observed hiatus” with “youngest observed hiatus” in the revised manuscript. We therefore consider the reference to the sequence above the youngest observed hiatus to be justified and retain the current wording.
Lines 133-140: This section raises the most concern. The fundamental basis of this paper depends on temporal alignment, which makes two flawed assumptions requiring significant further investigation to validate.
We agree that temporal alignment is fundamental to the analyses presented in this study. We will revise Sect. 2.4 (chronology) to clarify that the annual interpretation of the optically identified growth increments is independently supported by seasonal cell-length cyclicity and Mg cyclicity, with the detailed supporting evidence provided in the Supplementary Material.
Thank you again for your helpful comments and suggestions.
References:
Chan, P., Halfar, J., Norley, C. J. D., Pollmann, S. I., Adey, W., and Holdsworth, D. W.: Micro-computed tomography: Applications for high-resolution skeletal density determinations: An example using annually banded crustose coralline algae, Geochem. Geophys. Geosyst., 18, 3542–3553, https://doi.org/10.1002/2017GC006966, 2017.
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Citation: https://doi.org/10.5194/egusphere-2026-2316-AC3
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AC3: 'Reply on RC3', Bastian Seidl, 26 Aug 2026
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General Comments
The paper provides a robust dataset ( ∼ 40 years) of cell measurements from one protuberance of a rhodolith built by Sporolithon nodosum, collected at a water depth of 5 meters on the Whangaparāoa Peninsula in New Zealand.
The study aims at reconstructing cell size variation (cell length and width, annual growth, and number of cells per growth increment) during time and correlate such variation with environmental parameters (SSR, SST, and pCO₂, derived from different sources during the time span of the study) to identify which environmental parameter control size-trend over time.
This topic is of great interest to the scientific community studying coralline algae biomineralization processes. Despite the available literature, much effort is still needed to describe this phenomenon trying to define a descriptive model between environmental parameters and growth trends. Differences exist among species, and the wide bathymetric distribution of CCA implies the need for several specific case studies. Therefore, I believe this paper will contribute to improve our knowledge of the topic.
The authors followed rigorous laboratory methods and provided robust statistics to support their findings. Furthermore, all data and methods are available in an open repository. The only limitation is that the authors provided data from a single protuberance of a single specimen. I suggested therefore to be more cautious in "Discussion" when exposing your interpretation.
Specific comments have been included in the attached PDF.
Best regards.