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)
- RC1: 'Comment on egusphere-2026-2316', Anonymous Referee #1, 03 Jun 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
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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.