the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Aug 2025
Contributions of Transparent Exopolymer Particles by Specific Phytoplankton Groups in the Cosmonaut Sea, East Antarctic
Abstract. Transparent exopolymer particles (TEP) play a crucial role in marine carbon cycling. While phytoplankton are known to be the primary contributors to TEP, the impact of changes in phytoplankton community structure on TEP production in natural aquatic environments remains incompletely understood. This study employed multiple linear regression (MLR) modeling to quantify the contributions of two dominant phytoplankton groups, diatoms and haptophytes (primarily Phaeocystis antarctica), to TEP production in the surface waters of the Cosmonaut Sea, antarctica during the austral summer. Results demonstrate that in situ TEP production by each group can be estimated by scaling laboratory-derived theoretical values with an environmentally adjusted correction factor. These factors, primarily governed by phytoplankton community structure, reveal taxon-specific discrepancies between field and laboratory TEP production capacities. Notably, temperature, ammonium, and polysaccharide composition act as secondary modifiers of through indirect physiological effects. This study revealed that when the chlorophyll a concentration (Chl a) of P. antarctica exceeds 0.5 μg/L in the Cosmonaut Sea, its TEP production capacity surpasses that of diatoms at equivalent biomass levels – challenging the paradigm of diatom-dominated TEP contributions. In the research area, P. antarctica contributed 14.6–82.5 % (mean: 48.6 ± 15.4 %) to total TEP production, while diatoms contributed 31.0–112.0 % (mean: 55.1 ± 21.2 %; values >100 % reflect co-occurring group contributions). This highlights the pivotal role of P. antarctica in Southern Ocean carbon cycling and provides mechanistic insights for refining polar carbon budget models.
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RC1: 'Comment on egusphere-2025-3445', Anonymous Referee #1, 27 Oct 2025
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AC1: 'Reply on RC1', Ji Hu, 30 Oct 2025
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Dear Anonymous Reviewer,
Thank you very much for your time and effort in reviewing our manuscript. We sincerely appreciate your positive and constructive feedback.
We are particularly encouraged by your recognition that “the topic is highly relevant” and that “this study is very valuable to Antarctic research.” It is gratifying to know that you found our work to be a meaningful contribution to understanding the potential impacts of climate change on phytoplankton composition and the carbon cycle in the Southern Ocean.
We also note your comment regarding the mathematical approach, specifically that it “relies on many assumptions and has limitations, however the limitations are well addressed in the manuscript.” We thank you for this acknowledgment. We strived to be transparent about the methodological framework, and we are pleased that our discussion of these limitations was found to be adequate.
In light of your supportive comments, we will ensure that the significance of our findings and their implications for Antarctic research are further emphasized in the final version of the manuscript, particularly in the Discussion and Conclusion sections.
Once again, we are grateful for your insightful review and your endorsement of our work.
Sincerely,
Dr. Hu, Ji
On behalf of all co-authors.
Citation: https://doi.org/10.5194/egusphere-2025-3445-AC1
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AC1: 'Reply on RC1', Ji Hu, 30 Oct 2025
reply
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RC2: 'Comment on egusphere-2025-3445', Anonymous Referee #2, 15 Nov 2025
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Review of Manuscript Ref: egusphere-2025-3445
Title: Contributions of Transparent Exopolymer Particles by Specific Phytoplankton Groups in the Cosmonaut Sea, East Antarctic
The manuscript presents information on the contribution phytoplankton production of TEPs by phytoplankton in the Cosmonaut Sea, East Antarctic.
The authors applied multiple linear regression model to quantify the relative contributions of two major groups diatoms and haptophytes (~P. antarctica) during the austral summer. Their approach estimates values of TEP using theoretical values combined with an environmentally adjusted correction factor.
General comments:
The manuscript is clearly written, logically structured, and supported by well-conducted analyses.
The following points require clarification:
The authors mention the use of CHEMTAX (Line 156); however, it is not clear what specific analyses were performed with the software. If CHEMTAX was used, the authors should provide the initial input matrices as well as the final output matrices generated besides the model.
The results show a strong correlation between TEP and all phytoplankton groups. In fact, the findings suggest that the community composition—at least for these major groups—was influenced more by nutrient availability than by other environmental factors. This limitation when applying the same methodology in different conditions should be acknowledged explicitly in the manuscript.
Specific comments
Were chlorophyll b (indicative of chlorophytes) and zeaxanthin also detected?
Additionally, in Line 201, it is unclear how the presence of picoeukaryotes was assessed. If you did not analyze picoeukaryotes, please clarify this sentence accordingly.Line 237 please express salinity in psu units
Line 260, what are other groups? Cryptophytes and chlorophytes? specify
The station distribution or sampling lines should be shown in some of the figures—as dots or markers (e.g., in Fig. 2). This would help readers better visualize the spatial coverage of the sampling with hydrographic conditions etc.
Fig 4. Unclear if the results presented are from the pigment analyses after CHEMTAX or not
Fig. 5. Is the abundance of the phytoplankton groups in absolute abundance here or is relative abundance derived from pigments, unclear.
In Line 333, the absence of a significant relationship between MW% and phytoplankton abundance deserves further discussion. Lasternas & Agustí (2010) reported that the influence of ice-melt waters was negatively associated with diatoms, while P. pouchetii responded differently in the Arctic. A comparison between your results and previous findings from both the Arctic and Antarctic would strengthen the discussion and help with the observed patterns.
Citation: https://doi.org/10.5194/egusphere-2025-3445-RC2 -
AC2: 'Reply on RC2', Ji Hu, 04 Dec 2025
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Dear Anonymous Reviewer,
Thank you very much for your effort in improving the quality of our paper, and we sincerely appreciate your positive and constructive feedback. We positively response to your comments one by one as below:
The authors mention the use of CHEMTAX (Line 156); however, it is not clear what specific analyses were performed with the software. If CHEMTAX was used, the authors should provide the initial input matrices as well as the final output matrices generated besides the model.
We thank the reviewer for this comment. As requested, we have now provided the complete details of our CHEMTAX analysis in the Supporting Information to ensure full transparency and reproducibility.
The results show a strong correlation between TEP and all phytoplankton groups. In fact, the findings suggest that the community composition—at least for these major groups—was influenced more by nutrient availability than by other environmental factors. This limitation when applying the same methodology in different conditions should be acknowledged explicitly in the manuscript.
Thank you for your insightful comments. We greatly appreciate your observations regarding the limitations of the applicability of our research conclusions under different conditions, and we fully agree with your assessment. In response, we have explicitly incorporated a discussion of this limitation into the manuscript. In the discussion section (lines 324-328 in Section 4.1), we have added text to clarify that our conclusions are primarily applicable to the specific environmental conditions of this study. We acknowledge that it should be cautionous to refrain from directly extrapolating our methods and conclusions to other environments, such as oligotrophic waters, as other factors may exert a more significant influence. Additionally, we have emphasized this point in the summary section (lines 506), avoiding overly absolute statements and highlighting the necessity for comparative studies of cross-scale systems in the future.
Were chlorophyll b (indicative of chlorophytes) and zeaxanthin also detected?
We re-checked the pigment composition using CHEMTAX and identified the primary pigments associated with various algal species. We acknowledge that the correspondence of chlorophyll b to chlorophytes was an oversight on our part during the initial writing. This has been corrected in lines 156-161. Additionally, zeaxanthin, classified as an accessory pigment, is considered optional in CHEMTAX and is not included in the matrix if the root mean square (RMS) error is deemed acceptable.
Additionally, in Line 201, it is unclear how the presence of picoeukaryotes was assessed. If you did not analyze picoeukaryotes, please clarify this sentence accordingly.
We thank the reviewer for this important comment. We did not directly measure picoeukaryote abundance in our study. The statement was based on the consistent regional observations reported by Han et al. (2022) and the overall low chlorophyll-a concentrations we observed. To avoid overstatement, we have revised the sentence (lines 201-204) in the manuscript to clarify that the low abundance of picoeukaryotes is an inferred characteristic of the region rather than a direct measurement from our specific cruise.
Line 237 please express salinity in psu units
Thank you for the suggestion. We have revised the manuscript to express salinity in psu throughout the text, figures, and tables.
Line 260, what are other groups? Cryptophytes and chlorophytes? specify
Thank you for pointing out this lack of clarity. We have revised the sentence on Line 260 to explicitly specify that the "other groups" refer to cryptophytes and chlorophytes. The text has been updated accordingly.
The station distribution or sampling lines should be shown in some of the figures—as dots or markers (e.g., in Fig. 2). This would help readers better visualize the spatial coverage of the sampling with hydrographic conditions etc.
Thank you. We agree that adding the sampling stations will greatly improve the clarity and context of the figures. Accordingly, we have now plotted the station locations (as black dots) on Figure 2 and also on Figure 3,6 and 9. These updates allow readers to directly correlate the hydrographic (or other) data with the precise spatial coverage of our sampling, which significantly enhances the figure.
Fig 4. Unclear if the results presented are from the pigment analyses after CHEMTAX or not
Fig. 5. Is the abundance of the phytoplankton groups in absolute abundance here or is relative abundance derived from pigments, unclear.
We thank the reviewer for these important comments regarding the clarity of data presentation in Figures 4 and 5. We have now revised the manuscript to correct these ambiguous statements.
- Figure 4:The results presented in this figure are indeed derived from the CHEMTAX analysis of marker pigments. The proportion of phytoplankton in this figure is relative abundance derived from pigments. It reflects the proportional contribution of each group to the total phytoplankton community structure as calculated by CHEMTAX.We have updated the figure caption and the corresponding results in the text to explicitly state this.
- Figure 5:This figure also presents the absolute abundance (in µg L⁻¹ of Chl a) of phytoplankton groups derived from the CHEMTAX analysis.We have clarified this in the caption of fig. 5.
In addition to these figure-specific amendments, we have also revised the text throughout the Results section to consistently preface community structure findings with phrases.
In Line 333, the absence of a significant relationship between MW% and phytoplankton abundance deserves further discussion. Lasternas & Agustí (2010) reported that the influence of ice-melt waters was negatively associated with diatoms, while P. pouchetii responded differently in the Arctic. A comparison between your results and previous findings from both the Arctic and Antarctic would strengthen the discussion and help with the observed patterns.
Thank you for directing us to the highly relevant study by Lasternas & Agustí (2010). We agree that comparing our findings with this and other polar studies greatly strengthens our discussion. We have now expanded the discussion in Section 4.1 (lines 334-348) to explicitly address the absence of a significant relationship between MW% and phytoplankton abundance. As suggested, we have incorporated a comparison with Lasternas & Agustí (2010), who documented a taxon-specific response to ice-melt in the Arctic, and discussed how the contrasting responses of different phytoplankton groups (e.g., diatoms vs. Phaeocystis) in our study could have led to the observed lack of a clear bulk relationship.
Once again, we are grateful for your insightful review and your endorsement of our work.
Sincerely,
Dr. Hu, Ji
On behalf of all co-authors.
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AC3: 'Reply on AC2-Additional details', Ji Hu, 04 Dec 2025
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The results show a strong correlation between TEP and all phytoplankton groups. In fact, the findings suggest that the community composition—at least for these major groups—was influenced more by nutrient availability than by other environmental factors. This limitation when applying the same methodology in different conditions should be acknowledged explicitly in the manuscript.
Revised version：Additional content in lines 324-328 “It is important to recognize that the observed benefits of nutrient availability as a primary structural factor may be influenced by the specific environmental conditions at our study sites (e.g., high nutrient loading, water stability, etc.). The broader applicability of this relationship to ecosystems that are significantly different, such as oligotrophic open oceans or highly dynamic estuaries, needs further validation.” Additional content in line 506 "Controlled mechanistic experiments to quantify how environmental parameters (temperature, nutrients, light) modulate taxon-specific factors."
Were chlorophyll b (indicative of chlorophytes) and zeaxanthin also detected?
Revised version：Additional content in lines 156-161 "Specifically, the characteristic pigments identified for diatoms, haptophytes, cryptophytes, dinoflagellates, and chlorophytes include fucoxanthin, 19'-hexanoyloxyfucoxanthin, alloxanthin, peridinin, and chlorophyll b, respectively(Wright et al., 2010). The specific initial matrix and final output matrix information could be found in the Supporting Information."
Additionally, in Line 201, it is unclear how the presence of picoeukaryotes was assessed. If you did not analyze picoeukaryotes, please clarify this sentence accordingly.
Revised version：Additional content in lines 201-204 "Based on the observations by Han et al. (2022), the study area may generally be characterized by a low abundance of picoeukaryotes and prokaryotic cells. Considering the homogeneity of environmental forcing and minimal microbial disturbance, the structure of the phytoplankton community is likely to be the primary direct driver of TEP production in the Cosmonaut Sea surface waters during austral summer"
Line 260, what are other groups? Cryptophytes and chlorophytes? specify
Revised version： cryptophytes > chlorophytes.
Fig 4. Unclear if the results presented are from the pigment analyses after CHEMTAX or not
Revised version：Figure 4 Phytoplankton population percentage based on CHEMTAX analysis in the surface layer of the Cosmonaut Sea
Fig. 5. Is the abundance of the phytoplankton groups in absolute abundance here or is relative abundance derived from pigments, unclear
Revised version：Figure 5 Heatmap of correlations among various parameters (The unit corresponding to each parameter is consistent with Table 3)
In Line 333, the absence of a significant relationship between MW% and phytoplankton abundance deserves further discussion. Lasternas & Agustí (2010) reported that the influence of ice-melt waters was negatively associated with diatoms, while P. pouchetii responded differently in the Arctic. A comparison between your results and previous findings from both the Arctic and Antarctic would strengthen the discussion and help with the observed patterns.
Revised version：Additional content in lines lines 334-348 "Additionally, MW% data from the region (Figure 2) reveal that the surface layer of the Cosmonaut Sea is affected by meltwater input (MW% > 2%), which may enhance water column stratification and potentially favor phytoplankton growth (Massolo et al., 2009; Rivaro et al., 2014). However, the significant correlation between MW% and phytoplankton abundance is absent. It can be interpreted as the net result of contrasting, group-specific responses to ice-melt within the community. This is consistent with the findings of Lasternas and Agustí (2010) in the Arctic, where ice-melt negatively affected diatoms but not the dominant Phaeocystis pouchetii. In our study, a similar dynamic may have occurred: meltwater potentially suppressed diatoms or other sensitive groups, while other taxa (such as certain flagellates or P. antarctica) remained unaffected or even benefited. The variation in phytoplankton community composition may obscure the impact of meltwater on diatoms, potentially mitigating this negative effect. The comparison between the Arctic (Lasternas and Agustí, 2010) and our Antarctic study highlights that while the specific dominant species may differ (P. pouchetii vs. P. antarctica), the general principle of group-specific sensitivity to meltwater appears to be a consistent feature across polar ecosystems."
Citation: https://doi.org/10.5194/egusphere-2025-3445-AC3
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AC2: 'Reply on RC2', Ji Hu, 04 Dec 2025
reply
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RC3: 'Comment on egusphere-2025-3445', Anonymous Referee #3, 15 Dec 2025
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The study combines field data, empirical relationships and statistical modelling, in order to quantify the production of TEP by different phytoplankton groups in the Cosmonaut Sea, East Antarctica. The mathematical approach used relies on many assumptions and has limitations, however the limitations are well addressed in the manuscript.
The topic is highly relevant in the waters surrounding Antarctica, in which the phytoplankton composition could be altered by climate change with cascading implications on TEP production and carbon cycle, making this study is very valuable to Antarctic research.