the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Nov 2025
Combined effects of low temperature and low light intensity on elemental content and macromolecules of coccolithophores
Abstract. The calcifying coccolithophores Gephyrocapsa oceanica and Emiliania huxleyi can grow preferentially in deep waters (150–200 m), however, their physiological and biochemical strategies for acclimating to the combined constraints of low temperature and low irradiance remain unclear. In this study, we subjected three coccolithophore strains (G. oceanica NIES–1318, E. huxleyi PML B92/11 and RCC1266) to low temperature (9 °C) and low light intensity (15 μmol photons m–2 s–1), and compared their growth rates, particulate inorganic carbon (PIC), particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP) contents, as well as carbohydrate and lipid levels, with those under standard cultivation (21 °C, 150 μmol photons m–2 s–1). The results revealed that low temperature and low light intensity acted synergistically to decrease growth rate, POC contents and the POC : PON and POC : POP ratios, whereas did not significantly affect POP content in any of the strains. While increased light intensity enhanced PIC and PON contents at high temperature, it reduced them at low temperature. Low light intensity was identified as the primary factor leading to reduced carbohydrate and lipid level. Collectively, these findings indicate that to acclimate to low–temperature and low–light conditions, coccolithophores prioritized reducing the metabolic cost of carbohydrate and lipid biosynthesis, thereby allocating more resources to phosphorus metabolism–a physiological adjustment that can significantly influence biogeochemical cycles in the deep ocean.
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- CC1: 'Comment on egusphere-2025-5131', Kunshan Gao, 23 Nov 2025 reply
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RC1: 'Comment on egusphere-2025-5131', Anonymous Referee #1, 10 Feb 2026
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In “Combined effects of low temperature and low light intensity on elemental content and macromolecules of coccolithophores”, the authors grow three strains of the morphospecies Gephyrocapsa huxleyi at two levels each of light and temperature, and measure growth rate, stoichiometry, and various macromolecules.
In all, the dataset is exciting, and clearly reflects a lot of work, but the authors need to do more to synthesize their results and place them within the context of existing research in order to draw biogeochemically-relevant conclusions about their study species, and about the ecological role coccolithophores play at the base of the photic zone in different ecosystems.
To their credit, it seems as if the authors are struggling with a good problem: too much data. It is difficult to construct a concise narrative about three strains over four treatments; however, they could very easily have written twice as long a discussion section. The results section was data-heavy and difficult to parse—I would recommend focusing on the salient, statistically-significant results which will be focused on in the discussion, rather than listing every percentage change of every measurement between every treatment, all of which are visible in the figures and could also be presented in tabular form. Similarly, in many cases, it would be more effective to simply list the statistics, and only include relative increase or decrease when it will be a focus of analysis. Similarly, for future reference in the literature, I would like to see a main text table or supplemental table with all of the measurement values (mean +/- sd), rather than just the ANOVA test statistics.
In their discussion, the authors miss valuable opportunities to place their own measurements with those in the literature. For instance, how do the strains responses under different treatments relate to their ecological niche (where they were isolated)? What implications does this have for their behavior and biogeochemical impact in the open ocean? Furthermore, recent research has shown evidence of mixotrophy/osmotrophy potentially playing an important role in carbon acquisition for coccolithophores living at extremely low light levels—what are the implications of the results from the current study with regards to G. huxleyi metabolism and carbon demand in this light? Several times, the authors make a conjecture, or hypothesis, about the mechanism of one of their measurements (e.g. in the PIC discussion, lines 398-404), then leave the idea there without supporting it with other evidence or data. Additionally, the authors cite papers which do not support (or, at least, are totally tangential to) the author's point - e.g. Shemi et al., Tangunan et al.
Finally, I would recommend the authors consider standardizing the structure of their discussion, and of each paragraph therein. There are several disconnected ideas and/or hypotheses in many paragraphs, which frequently left me confused, and I suggest they take the time to iterate the presentation of each idea with its supporting evidence and context in the literature.
As an aside, to improve readability, I suggest the authors include a native English speaking co-author or use an AI tool to correct grammar errors, misspellings, e.g. add definite articles like “the”, which may not exist in their first language.
- Lines 124-130 – capitalization of culture collections?
- Line 331—I wouldn’t characterize cultures grown at 9°C as “extreme low temperature”—high latitude phytoplankton, for instance, all grow under much lower temperatures.
- Lines 335-337 – The authors say carbohydrates and lipids were primarily light dependent. However, in all strains grown under high light, the lower temperature cultures express significantly lower carbohydrates and lipids than the higher temperature cultures. What physiological changes contribute to this change in expression?
- Line 367 – are these findings inconsistent? 9 and 15C are at very different points along a PAR response curve--perhaps on opposite sides of the apex?
- Line 386 – I don’t believe there is any discussion of secondary metabolites in Shemi et al? The study focused rather on intracellular recycling, no?
- Line 393 – is there any evidence connecting this claim with the citation?
- Lines 411-424 – unnecessary to cite the same paper five sentences in a row
Citation: https://doi.org/10.5194/egusphere-2025-5131-RC1
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- 1
The paper entitled “Combined effects of low temperature and low light intensity on elemental content and macromolecules of coccolithophores” by Shang et al. reports about the growth and elemental compositions of several coccolithorphore strains. It presents a potentially valuable dataset on the physiology of coccolithophores under conditions relevant to the bottom layer of euphotic zone in the oceans. The experimental approach is sound, and the core findings are clear. However, several key aspects require clarification and expansion to fully contextualize the results and strengthen the paper's conclusions before it can be considered for publication.
Major Comments
When describing "growth rate changes", the authors repeat strain-specific data such as "an 81.39% decrease in growth rate for G. oceanica under LTLL and a 63.18% decrease for E. huxleyi PML B92/11" (corresponding to Result 3.1). However, they do not subsequently analyze the reasons for "strain-specific differences in growth rates", making these data mere result restatements without adding new argumentative value.
- Ambiguous Mechanism for the Temperature-Dependent Reversal of Calcification
Regarding the counterintuitive result that "high light reduces PIC content at low temperatures", the discussion proposes two hypotheses: "photoinhibition disrupts ion transport" and "coccoliths act as microlenses for light concentration". However, it fails to clarify the primary-secondary relationship or synergistic effect between these two hypotheses:
The simultaneous proposal of two hypotheses without targeted data support leads to a superficial mechanistic explanation.
Strain-Specific Differences: Inadequate Analysis of Causes and Lack of Universal Discussion
The study involves three coccolithophore strains (G. oceanica NIES–1318, E. huxleyi PML B92/11, RCC1266), and results show strain-specific variations in multiple indicators (e.g., the PIC:POC ratio only increases significantly for G. oceanica under LTLL, Result 3.3). However, the discussion only mentions at the end that "strain diversity helps coccolithophores adapt to different habitats" and does not further analyze the causes of these differences:
Extension of Ecological Significance: Insufficient Specific Linkage to Deep-Sea Biogeochemical Cycles
The discussion proposes that "the adaptation strategies of coccolithophores affect deep-sea biogeochemical cycles" but only generally mentions "enhanced carbon sequestration" and "underestimation of carbonate production", without establishing specific quantitative or process-based connections:
Literature Comparison: When comparing with previous studies, the discussion has gaps in explaining "result differences":
In summary, the manuscript addresses an interesting topic but requires revisions to fully realize its potential. The most critical issues are the interpretation of deep-water populations as "growing" versus "surviving," the discussion of experimental limitations relative to the deep ocean environment, and the crucial temperature extrapolation to 4°C. Addressing these points will greatly improve the manuscript's robustness and ecological relevance.