the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2025
Species-specific differential dissolution morphology of selected coccolithophore species: an experimental study
Abstract. We conducted a laboratory CaCO3 dissolution experiment to detect differential dissolution morphologies of three selected coccolithophore (abundant marine calcareous phytoplankton) species, Coccolithus braarudii, Helicosphaera carteri, and Scyphosphaera apsteinii. These species were selected because they are ecologically and biogeochemically important (significant contributors to CaCO3 production) and have been less studied than Gephyrocapsa. Muroliths of S. apsteinii dissolve faster than lopadoliths, which in turn dissolve as fast as H. carteri but faster than C. braarudii. Lopadolith R-units dissolve faster than V-units. Comparison with field samples shows that experimental data are helpful when interpreting field samples. For example, we identify dissolution in water and sediment samples reported in the literature. In C. braarudii dissolution reveals a nanostructure on the proximal side of the distal shield, an observation that has implications for coccolith biomineralization models, which do not currently account for the formation of such a structure.
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- RC1: 'Comment on egusphere-2025-1921', Anonymous Referee #1, 11 Aug 2025
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RC2: 'Comment on egusphere-2025-1921', Anonymous Referee #2, 07 Sep 2025
The manuscript describes a study of the effects of short term (11 hour) dissolution experiment on three species of coccolithophores, presenting detailed observations of the progressive morphological changes over the dissolution experiment and comparing and contrasting these in three modern species. In its current form, the manuscript has two challenges. Firstly, in many sections the manuscript still reads like a draft and has not yet been brought into optimal structure or organization, with important and details missing from from text and figures. Secondly, the focus and scope do not seem well matched to the journal Biogeosciences, since there is (aside from one vague sentence in the introduction) no discussion of the biogeoscience/biogeochemical cycle significance of the observations, but rather detailed micropaleontological observations. This suggests that the paper in its current form is better suited to a micropaleontology journal such as Journal of Nannoplankton Research or Marine Micropaleontology. The paper is currently written to be accessible only to a calcareous nannofossil expert(a small community that usually does not overlap with the ocean carbon cycle community), and it does not give sufficient background to the nannolith morphological elements discussed to have relevance to Biogeoscience readership.
Specific suggestions
Abstract
I found the abstract contains a lot of nomenclature which is very specialized and I suggest revising so that readers unfamiliar with V unit/R unit or lopadolith and murolith can understand. Probably in the abstract it is sufficient to describe that the dissolution rate depends on the crystallographic orientation of the crystal. At the same time, the “ nanostructure” on the proximal side of the distal shield in braarudii is vague, and could be described in better detail. I also suggest the abstract start with an introductory sentence or two describing the motivation of the study.
Introduction:
I think this needs to be restructured/reorganized a bit.
The first paragraph describes the motivation – that dissolution is important in the carbon cycle – it needs to go a bit further and explain why, eg what is the effect on upper ocean alkalinity.
Then, the bridge to the next concept is a bit vague – is the goal of characterizing dissolution morphologies to distinguish in upper ocean water column samples whether dissolution has occurred? Is it to distinguish in ocean sediment samples if dissolution has occurred? Is it to use the variance in dissolution morphologies to assess changing causes of dissolution intensity either in the upper water column (as a function of deep export mechanisms or water column chemistry) or changing dissolution intensity experienced at the seafloor? Bring the information in lines 102-108 earlier in the introduction and use it to motivate the study.
Then two aspects of dissolution are mixed – the selective dissolution of different species relative to each other, and the evolution of morphology of a given species with progressive dissolution. Separate these and distinguish them. Comment which have been tested experimentally and which are inferred only from field studies.
You can point out that experimental dissolution studies provide good source of information on the evolution of morphology with dissolution, without confounding factors from field studies such as variance in the primary biomineralization morphology.
Discuss the prior results of experimental dissolution studies together and in a similar way, eg details are given about the findings of dissolution intensity of C. leptoporus but nothing is mentioned of G. huxleyi. Review together what previous experimental studies have been done and what they found, before motivating study on other species which expand knowledge about other species which are a significant part of the carbon budget.
Materials and methods
Lines 154-171 - it is justified that precise omega calcite is not important for the study but it would be helpful to provide some estimates of the uncertainty around the 0.033 value given. Overall this paragraph could be made a bit more concise and less informal.
If the study goal was to distinguish the evolution of coccolith morphology due to post-mortem dissolution in the ocean during sinking, it is unclear why the dissolution experiment was conducted on living cells which where only subject to transient (11 hour) dark dissolution and then later returned to viable conditions. The duration of the dark period is comparable to a typical dark cycle during cell growth when respiration maintains cell metabolism, only the temperature shock may have suspended normal operation. What advantage does this have compared to lysing a cell population and then completing the dissolution experiment over 11 hours on post-mortem material?
Was the decreased temperature during the incubation accounted for in the carbon chemistry calculations?
Results
As background, potentially at the start of results or discussion or potentially as background before the methods, the nannolith or coccolith morphology of the three species needs to be presented (eg contrasting placolith vs nannolith, describing R units, V units, loadoliths, muroliths) .
Figures 6 and 7 reflect a useful presentation of the progressive dissolution features over the course of the experiment.
The discussion of field samples and the Figure 8 caption should indicate the origin of the field samples, the trap depth and omega in the sediment traps, the typical transit time (eg length of time exposed in the water column before capture in trap based on setting velocity and trap depth). This section is still rather underdeveloped. Lines 280 to 284 need some contexts – in today’s photic zone are there really areas with comparable undersaturation that living coccolithophores are trying to survive in? If so then this should be noted and contrasted with the majority of the photic zone which lies in strongly oversaturated waters.
Citation: https://doi.org/10.5194/egusphere-2025-1921-RC2
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- 1
This manuscript presents a carefully designed and well-documented experimental study on species-specific differential dissolution in three large, ecologically important coccolithophore species—Coccolithus braarudii, Helicosphaera carteri, and Scyphosphaera apsteinii—which, despite their biogeochemical relevance, have received less attention than the model taxon Gephyrocapsa huxleyi. By exposing live cultures of all three species simultaneously to strongly undersaturated seawater (Ω_calcite ≈ 0.033) in a controlled dark, low-temperature environment, the authors systematically document dissolution sequences at high temporal resolution (15 sequential time points). The side-by-side design, combined with high-quality SEM imaging, allows for direct interspecific and morphotype-specific comparisons, notably revealing that C. braarudii is the most dissolution-resistant, followed by H. carteri and S. apsteinii, and that within S. apsteinii lopadoliths, R-units dissolve faster than V-units.
The ability to match experimentally documented features with those observed in sediment trap and water-column samples provides strong evidence that the laboratory findings are applicable to natural assemblages and can help resolve long-standing challenges in distinguishing dissolution from malformation in field material. Particularly noteworthy is the identification of a nanostructure on the proximal side of the distal shield in C. braarudii, which is not only morphologically distinctive but also potentially significant for understanding coccolith biomineralization and the role of organic–mineral interactions.
Methodologically, the study is highly reproducible: strain origins, growth media, culture conditions, and carbonate chemistry manipulations are clearly described; measurements and calculations are transparent; and SEM preparation parameters are fully reported. The workflow is appropriate for the study’s aim of determining dissolution sequences. The authors also acknowledge relevant limitations, such as the absence of biological replication and potential influences of the organic coating.
Figures are generally of high quality and well-aligned with the text, but small adjustments in scale bar clarity, annotation, and caption detail would improve accessibility. Likewise, restructuring certain paragraphs for focus, redefining specialist terms at first use, and clarifying methodological points (e.g., morphological assessment approach, rationale for species choice) would further strengthen the manuscript. Overall, the study offers novel insights, a rigorous experimental basis, and clear potential to inform both modern carbonate cycling studies and palaeoceanographic interpretations.
Specific comments:
L50–56 – The transition from global carbon cycle context to coccolithophore dissolution is abrupt. Consider adding a bridging sentence to connect the large-scale biogeochemical role with the specific processes studied here.
L64–81 – This paragraph contains redundancy. Please streamline by consolidating repeated points on the challenges of interpreting field samples, removing the “first task/second task” phrasing, and integrating the sediment observations with the experimental confirmation of shield separation into a single, concise statement.
L82–86 – Separate citations for dissolution studies and for G. huxleyi as a widely used model species. For the latter, the following would suffice: Wheeler, G. L., Sturm, D., & Langer, G. (2023). Journal of Phycology, 59, 1123–1129. https://doi.org/10.1111/jpy.13404.
L87–94 – The rationale for including the three focal species could be made more explicit. Briefly state their ecological/biogeochemical relevance to justify why they are important to study alongside G. huxleyi.
L109–112 – Consider stating an explicit hypothesis and broader scientific context for the work. Explain why these experiments were chosen and how they address the research gap.
L147 – The phrase “present in the same vessel” is unclear. Consider rephrasing to explicitly state that all three species were combined in a single 2.7 L container for simultaneous exposure to identical seawater chemistry: i.e. container, bottle (most direct, since you specify 2.7 L bottle later), incubation container, sample container, test bottle, reaction bottle, experimental flask.
L156 – The term “Ω_calcite” is introduced here without definition. Consider either introducing it earlier in the Introduction or adding a brief explanatory sentence here (e.g., defining it as the saturation state of seawater with respect to calcite and indicating the dissolution/precipitation thresholds).
L170 – Include the manufacturer (and country) of the pH meter here, and ensure all instruments mentioned in the Methods are accompanied by manufacturer details for consistency and traceability.
L192–194 – The description of post-experiment coccolith morphology assessment is vague. Specify how morphology was evaluated (e.g., visually via light microscopy, SEM), whether any images were taken, and if these observations were documented systematically or informally. This would help readers gauge the reliability and reproducibility of the qualitative note on increased malformations.
L200–201 – The two SEM instruments used have markedly different resolution capabilities (Phenom Pro desktop SEM vs. Zeiss Merlin FE-SEM). Consider briefly noting the resolution differences and explaining whether certain morphological features were only discernible in high-resolution Zeiss Merlin images. This would clarify the role of each microscope in capturing fine-scale dissolution features.
L222–233 – The authors appear to assume prior reader familiarity with the terms “V-units” and “R-units.” For clarity and accessibility, briefly redefine these terms at first mention in the Results (e.g., “R-units, the smaller radial crystals…”), as readers may not recall definitions from earlier literature. In addition, introduce these concepts in the Introduction to provide essential context for readers new to coccolith microstructure.
L245–266 (3.2) – This paragraph mixes multiple points without a clear topic sentence. Consider breaking it into two or more paragraphs, each beginning with a sentence that signals the main point (e.g., “Our experimental dissolution sequence can be directly applied to field samples…”). This would improve readability and help the reader follow your argument.
L253–256 – When noting that similar features in the literature have been ascribed to malformation, briefly explain how your experimental approach enables you to differentiate them from dissolution. For example, you could outline practical diagnostic criteria that can help others reliably distinguish dissolution-driven morphologies from true malformations. This guidance would be especially valuable for field sample interpretation.
L281–283 – The statement that all three species need a coccosphere to live could be expanded by incorporating contrasting evidence from Johns et al. (2023, Science Advances). While your data support the idea that coccosphere collapse compromises survival—potentially due to loss of motility, increased vulnerability to grazing, and reduced protection—Johns et al. observed mixed-species coccospheres in natural assemblages, indicating that coccosphere integrity can be re-established or modified through incorporation of foreign coccoliths. This suggests that survival may depend more on maintaining a continuous covering than on the species-specific origin of coccoliths. It would strengthen the discussion to acknowledge that coccosphere collapse might not represent an irreversible endpoint if interspecific coccolith exchange or repair occurs, while also noting that the physiological consequences and protective efficacy of such hybrid coccospheres remain to be tested experimentally.
L294–298 – While the description of proximal vs. distal shield nanostructures is clear, the discussion does not explicitly connect these differences to potential roles in dissolution resistance or susceptibility. Consider adding a short paragraph explaining whether the absence/presence of nanostructure could alter etching rates, mechanical stability, or dissolution onset.
L305–311 – The comparison to extracellular calcifiers is interesting but remains speculative. To strengthen this, relate how the hypothesized organo-mineral composite structure might affect dissolution in C. braarudii—for example, could it slow etching or provide structural reinforcement?
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