the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2025
Biogeochemical Dynamics of the Sea-Surface Microlayer in a Multidisciplinary Mesocosm Study
Abstract. The sea-surface microlayer (SML) represents the thin uppermost layer of the ocean, typically less than 1,000 µm in thickness. As an interface between the ocean and the atmosphere, the SML plays a key role in marine biogeochemical cycles. Its physical and chemical properties are intrinsically linked to the dynamics of the surface ocean’s biological communities, especially those of phytoplankton and phytoneuston. These properties, in turn, influence air–sea interactions, such as heat and gas exchange, which are modulated by the interaction between organic matter composition and surfactants in the SML and the underlying water (ULW). However, the dynamic coupling of biogeochemical processes between the SML and the ULW remains poorly understood. To contribute to filling this knowledge gap, we conducted a multidisciplinary mesocosm study. In this study, we induced a phytoplankton bloom and observed the subsequent community shift to investigate the effects on the SML biogeochemistry. Samples were collected daily to analyse inorganic nutrients, phytopigments, surfactants, dissolved and particulate organic carbon (DOC, POC), total dissolved and particulate nitrogen (TDN, PN), phytoplankton and bacterial abundances, and bacterial utilisation of organic matter A clear temporal segregation of nutrient samples in the SML and ULW was observed through a self-organising map (SOM) analysis. Phytoplankton bloom progression throughout the mesocosm experiment was classified into three phases: pre-bloom, bloom, and post-bloom based on Chlorophyll-a (Chla) concentration. Chla concentration varied from 1.0 to 11.4 μg L⁻¹. POC and PN followed the Chla trend. Haptophytes, specifically Emiliania huxleyi, dominated the phytoplankton community, followed by diatoms, primarily Cylindrotheca closterium. An enrichment of surfactants and DOC was observed after the bloom. During the bloom, a distinct surface slick with complete surfactant coverage of the air–sea interface created a biofilm-like habitat in the SML, leading to increased bacterial cell abundance. The bacterial community utilised amino acids as the preferred carbon source, followed by carbohydrates in both water layers. Our findings highlight that the SML is a biogeochemical hotspot, playing a crucial role in the production, transformation, and microbial activity of autochthonous organic matter, thus exhibiting the potential to strongly affect air–sea exchange. Incorporating SML dynamics into Earth system models will enhance climate predictions and improve ocean-atmosphere interaction studies on both regional and global scales.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1773', Anonymous Referee #1, 03 Jul 2025
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AC1: 'Reply on RC1', Riaz Bibi, 15 Aug 2025
We sincerely thank the reviewer for their thorough and thoughtful evaluation of our manuscript and for recognizing its scientific merit, novelty, and overall quality. We appreciate the constructive suggestions, which have helped us further improve the clarity and precision of our manuscript.
Comment
Response
I note only a single minor technical adjustment, one formula would benefit from an explicit definition of a parameter to eliminate any residual ambiguity. This is a straightforward revision that the authors can very easily accommodate.
We thank the reviewer for noting this oversight. In the revised manuscript, we have provided an explicit formula for the enrichment factor as EF = CSML / CULW, where CSML and CULW are the concentrations of the variable in the SML and ULW, respectively (line 298-299). All parameters are now clearly defined to avoid ambiguity.
Line 21 – For clarity, please specify where the mesocosm experiment was carried out so that the experimental setting is clear from the abstract alone, without readers needing to consult the full paper.
We have specified the location of the mesocosm experiment, the Center for Marine Sensor Technology (ZfMarS), Institute of Chemistry and Biology of the Marine Environment (ICBM), Wilhelmshaven, Germany, to clarify the experimental setting directly in the abstract.
Line 185 – It is unfortunate that analyses for DOC, PON, TDN, PN, and pigments could not be carried out on the all SML samples; however, this statement contradicts the information in line 223 and again in the Results section (line 490), where it is reported that both SML and ULW samples were analyzed for DOC and TDN (see line 495). This discrepancy is confusing for the reader and should be checked and corrected.
We have corrected the inconsistency regarding DOC and TDN analyses between Line 185 and Lines 223, 490–495. The revised text now consistently states which parameters were analyzed for SML and ULW samples. Specifically, DOC and TDN were analyzed for both SML and ULW samples, whereas POC and PN were analyzed only for ULW samples. This change ensures clarity and consistency across the Materials and Methods and Results sections.
Please note that the TDN dataset is currently undergoing re-evaluation. Any updates resulting from this process will be incorporated into the revised final manuscript but will not affect the overall conclusions of the study.
Line 413 – Minor suggestion: Although readers in this field will understand the context, I recommend adding parentheses around the sum of the N-nutrients so that it is mathematically explicit.
We have revised the expression of the sum of N-nutrients, ((NO3− +NO₂⁻): PO₄³⁻),adding parentheses for mathematical clarity.
Citation: https://doi.org/10.5194/egusphere-2025-1773-AC1
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AC1: 'Reply on RC1', Riaz Bibi, 15 Aug 2025
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RC2: 'Comment on egusphere-2025-1773', Anonymous Referee #2, 11 Jul 2025
This mesocosm study by Bibi et al offers insights into the biogeochemical dynamics of the sea-surface microlayer during phytoplankton bloom succession. While the study addresses an important topic in marine biogeochemistry, the current manuscript requires significant strengthening of evidence, particularly regarding species identification and bacterial enumeration. The authors should either provide additional supporting data or temper their conclusions to match the available evidence. My specific comments are detailed below:
Major Comments.
1. The identification of Emiliania huxleyi as the main bloom former needs stronger evidence. The authors should provide morphological data from SEM analysis confirming the presence of E. huxleyi cells and characteristic coccoliths. Additionally, molecular validation would be helpful. DNA analysis, such as qPCR with primers targeting the genomes of E. huxleyi and Cylindrotheca Closterium, would confirm species abundance. Lastly, why wasn't the FlowCam useful for detecting E. huxleyi as it was for diatoms? Other studies have used it for such analysis before.
It is most important to accurately describe the phytoplankton community, as the organic matter composition in the SML and ULW is expected to differ significantly between algal species, however the current species identification presented seems unconvincing.2. The statement that "coccoliths shed by E. huxleyi during the final stages of the bloom significantly increased water column turbidity and light scattering" lacks supporting evidence for a coccolithophore bloom, as mentioned earlier. Using 3–4 μm size bins in FlowCam as a proxy for E. huxleyi coccoliths is highly speculative, especially without the demonstration of co-occurrence of E. huxleyi cells. This observed particle fraction might most likely be composed of other pico-eukaryotes or cyanobacteria. Authors should provide convincing evidence for the presence of these coccoliths, or otherwise, should temper their somewhat speculative conclusions that phytoplankton blooms contribute to climate-relevant feedback mechanisms through increased turbidity and albedo effects caused by coccolith shedding from E. huxleyi blooms.
3. The bacterial abundance determination through SYBR staining and flow cytometry raises concerns too: the non-selective nature of the SYBR green dye may potentially lead to overestimation of cell concentration. In addition, I couldn't find a description or a chart of the FCM gating plots. This gating strategy should be clearly described, presenting the positive control bacterial cells compared to the bacterial cell counts from the environmental samples analyzed.
Could the author also explain why cells were not counted through CFU plating or molecular analysis (e.g., qPCR)? It is highly recommended to add such analysis to the manuscript as a validation. Otherwise, the limitations of the SYBR green labeling method should be discussed, and FCM bacterial counts should be termed "bacterial-like particle" counts for accuracy.
Another important point that raises questions about the validity of the bacterial count method is the atypical bacterial growth (Figure 7a). Values are expected to be higher by orders of magnitude, as observed in other E. huxleyi microbiome studies.4. Overall, the manuscript would benefit from a description of what has been previously published versus the novel contributions of this study.
Minor Comment. Several figure captions do not accurately describe the content of the figure panels.
Citation: https://doi.org/10.5194/egusphere-2025-1773-RC2 -
AC2: 'Reply on RC2', Riaz Bibi, 16 Aug 2025
We thank the reviewer very much for their constructive comments and valuable feedback on our manuscript. In the following table, we address reviewer’s each comment individually and describe the changes and improvements made to the manuscript.Comment
Response
The identification of Emiliania huxleyi as the main bloom former needs stronger evidence. The authors should provide morphological data from SEM
analysis confirming the presence of E. huxleyi cells and characteristic coccoliths.Morphological confirmation: We now provide SEM images showing coccoliths characteristic of Emiliania huxleyi and intact cells of Cylindrotheca Closterium, as sown in Figure R1 (see attached PDF). FlowCam images collected after removal of the size filter (<10 µm) also reveal numerous small cells consistent with E. huxleyi. Together, these images confirm the presence of these species within the experimental system and have been included in the Supplementary Information (Fig. S6) in the revised manuscript.
Additionally, molecular validation would be helpful. DNA analysis, such as qPCR with primers targeting the genomes of E. huxleyi and Cylindrotheca Closterium, would confirm species abundance.
We acknowledge the value of DNA-based methods (e.g., qPCR) for species-specific quantification. However, such analyses were beyond the scope of this mesocosm experiment, and no remaining sample material is available for retrospective molecular analysis. We agree on the importance of such approaches and will consider integrating them into similar experiments in the future. In the revised manuscript, we (i) clarify the FlowCam acquisition settings and their limitations in the Methods section (line 255), while presenting the associated findings in the Results (Species Identification) section (line 446-451) and (ii) include the SEM and FlowCam images (Supplementary Fig. S6 in). We think these additions and clarifications strengthen our identification of E. huxleyi as the main bloom former in this study
Lastly, why wasn't the FlowCam useful for detecting E. huxleyi as it was for diatoms? Other studies have used it for such analysis before.
Initially, FlowCam imaging was performed with a >10 µm size filter to focus on larger diatoms, as the experimental design aimed to trigger a diatom bloom following nutrient addition. This prevented the early detection of smaller cells such as E. huxleyi. After an increase in turbidity suggested the presence of other taxa, the filter was removed, revealing abundant small cells later confirmed via SEM as E. huxleyi. However, these FlowCam data only cover the latter half of the experiment, limiting their use for full temporal bloom reconstruction.
It is most important to accurately describe the phytoplankton community, as the organic matter composition in the SML and ULW is expected to differ significantly between algal species, however the current species identification presented seems unconvincing.
We have strengthened our species identification by including SEM and FlowCam images confirming the presence of E. huxleyi and C. closterium (provided as Supplementary Fig. S6). We have also clarified the limitations of our initial FlowCam settings in revised manuscript.
The statement that "coccoliths shed by E. huxleyi during the final stages of the bloom significantly increased water column turbidity and light scattering" lacks supporting evidence for a coccolithophore bloom, as mentioned earlier. Using 3–4 μm size bins in FlowCam as a proxy for E. huxleyi coccoliths is highly speculative, especially without the demonstration of co occurrence of E. huxleyi cells. This observed particle fraction might most likely be composed of other pico-eukaryotes or cyanobacteria. Authors should provide convincing evidence for the presence of these coccoliths, or otherwise, should temper their somewhat speculative conclusions that phytoplankton blooms contribute to climate relevant feedback mechanisms through increased turbidity and albedo effects caused by coccolith shedding from E. huxleyi blooms.
We now provide SEM images as evidence for coccolithophore presence, showing both intact Emiliania huxleyi cells and detached coccoliths during the late phase of the bloom (Supplementary Fig. S6).
Contribution to turbidity and optical properties: In our mesocosm experiment, we observed a measurable increase in water column turbidity and albedo during the late bloom phase, coinciding with SEM-confirmed coccolith shedding and an increase in the LISST 3–4 µm particle size fraction (compare Fig. 2b and Fig. 5c). These observations are consistent with the previous established knowledge showing that coccolithophores produce calcite plates (coccoliths) contribute to increased turbidity and a whitish appearance of the water column (Holligan et al., 1983; Beaufort et al., 2008; Perrot et al., 2018) and coccolithophore blooms increase backscattering and enhance albedo due to their high refractive index and strong light-scattering properties (Balch et al., 1999, 2005, 2011; Tyrrell et al., 1999; Frouin & Lacobellis, 2002; Gordon et al., 2009; Tyrrell & Merico, 2004; Fournier & Neukermans, 2017) on regional to global scales. We acknowledge that other particles (e.g., C. closterium cells and detrital material) may also have contributed to turbidity; our SEM evidence and particle size distribution data indicate that coccoliths played a substantial role in the observed turbidity change.
Revised climate-relevant conclusions: While we observed increased turbidity and albedo (Fig. 2b and 3c) coinciding with coccolith presence (Fig. 5c), we agree that attributing global-scale climate feedback mechanisms solely to coccolith shedding in this mesocosm experiment exceeds the scope of our study. We have therefore revised the discussion (line 648-650; line 696-699) and conclusion paragraph (line 809-812) to emphasize that our findings represent local experimental conditions and are consistent with previously documented optical effects of coccolithophores.
The bacterial abundance determination through SYBR staining and flow cytometry raises concerns too: the non selective nature of the SYBR green dye may potentially lead to overestimation of cell concentration. In addition, I couldn't find a description or a chart of the FCM gating plots. This gating strategy should be clearly described, presenting the positive control bacterial cells compared to the bacterial cell counts from the environmental samples analyzed.
We understand the reviewers' concern about accurate cell counts; however, the method we used for determining bacterial abundance is standard in microbial ecology (Marie et al., 1999; Brussaard et al., 2010). To ensure accurate quantification, the flow rate was calibrated using fluorescent bead standards. Though SYBR Green can also bind to non-DNA particles, leading to inaccurate results, especially in complex samples like soil. This nonspecific binding can cause overestimation of DNA or reduced accuracy in quantification. However, in our case, samples were filtered through a 5 µm filter to remove most larger particles and non-bacterial debris, thereby minimizing the likelihood of overestimation.
Could the author also explain why cells were not counted through CFU plating or molecular analysis (e.g., qPCR)? It is highly recommended to add such analysis to the manuscript as a validation. Otherwise, the limitations of the SYBR green labeling method should be discussed, and FCM bacterial counts should be termed "bacterial-like particle" counts for accuracy.
While CFU counts are valuable for estimating viable bacteria, they rely on the ability of cells to grow on a given culture medium. In marine environmental samples, typically only 1–10% of bacteria are culturable under laboratory conditions, meaning CFU counts would significantly underestimate total bacterial abundance and therefore would not be directly comparable to flow cytometry results. Similarly, qPCR can help quantify specific bacterial taxa; however, it is not optimal for estimating total bacterial abundance in diverse marine samples. This is because marine bacteria vary greatly in the number of ribosomal RNA operon copies per genome, which can bias total abundance estimates.
Flow cytometry with SYBR Green staining is a widely used, established method in marine microbial ecology for estimating total bacterial abundance, and results are directly comparable to other studies using similar methods. In light of the reviewer’s suggestion, we will adopt the term “bacterial-like particles” when referring to our flow cytometry counts. However, this terminology is not used in marine microbial ecology.
Another important point that raises questions about the validity of the bacterial count method is the atypical bacterial growth (Figure 7a). Values are expected to be higher by orders of magnitude, as observed in other E. huxleyi microbiome studies.
The primary objective of the mesocosm study was to obtain a comprehensive understanding of the synergistic and antagonistic interactions among multifaceted biogeochemical processes in the SML and ULW during the development and decline of an induced phytoplankton bloom. Accordingly, our study was not designed as a microbiome study of Emiliania huxleyi, although this species was blooming during the experiment. Variability in bacterial cell counts likely reflects the influence of multiple environmental and biogeochemical factors beyond E. huxleyi abundance, which may have contributed to the observed lower cell numbers.
Overall, the manuscript would benefit from a description of what has been previously published versus the novel contributions of this study.
We have revised the Discussion (line 629-640; line 704-711) to explicitly compare our findings with previously published studies, most of which were conducted in field settings and focused on individual aspects of the SML, rather than broader interactions such as SML–ULW coupling and the interplay between physical, chemical, and biological processes. We now clearly highlight the novel contributions of our study, which include presenting the first comprehensive and integrated measurements of biological (phytoplankton biomass and community composition, bacterial abundance and metabolic profiles), chemical (nutrients and surfactants), and physical (turbidity and solar irradiance) parameters from paired SML and ULW samples. In addition, our study captures the coupling of complex, multifaceted biogeochemical processes between the SML and ULW over the full course of a phytoplankton bloom succession within a controlled mesocosm setting.
Please note that Referee #1 considered the novelty of our work clear and highlighted that our approach and findings represent a valuable addition to the existing literature.
Minor Comment. Several figure captions do not accurately describe the content of the figure panels.
We have carefully and thoroughly reviewed and revised all figure captions and ensured that each accurately and clearly describes the content of its corresponding panels, including all symbols, abbreviations, and units where applicable. We believe these changes will improve clarity and consistency across all figures.
References:
Holligan, P. M., Viollier, M., Harbour, D. S., Camus, P., and Champagne-Philippe, M.: Satellite and ship studies of coccolithophore production along a continental shelf edge, Nature, 304, 339 342, 1983.
Beaufort, L., Couapel, M., Buchet, N., Claustre, H., and Goyet, C.: Calcite production by coccolithophores in the south east Pacific Ocean, Biogeosciences, 5, 1101–1117, 2008.
Perrot, L., Gohin, F., Ruiz-Pino, D., Lampert, L., Huret, M., Dessier, A., and Bourriau, P.: Coccolith-derived turbidity and hydrological conditions in May in the Bay of Biscay, Prog. Oceanogr., 166, 41–53, 2018.
Balch, W. M., Drapeau, D. T., Cucci, T. L., Vaillancourt, R. D., Kilpatrick, K. A., and Fritz, J. J.: Optical backscattering by calcifying algae—separating the contribution by particulate inorganic and organic carbon fractions, J. Geophys. Res., 104, 1541–1558, 1999.
Balch, W. M., Gordon, H. R., Bowler, B. C., Drapeau, D. T., and Booth, E. S.: Calcium carbonate budgets in the surface global ocean based on MODIS data, J. Geophys. Res., 110, C07001, 2005.
Balch, W. M., Drapeau, D. T., Bowler, B. C., Lyczskowski, E., Booth, E. S., and Alley, D.: The contribution of coccolithophores to the optical and inorganic carbon budgets during the Southern Ocean Gas Exchange Experiment: New evidence in support of the “Great Calcite Belt” hypothesis, J. Geophys. Res.-Oceans, 116, C04004, 2011.
Brussaard, C. P. D., J. P. Payet, C. Winter, and M. G. Weinbauer.: Quantification of aquatic viruses by flow cytometry. Pages 102–109 in S. W. Wilhelm, M. G. Weinbauer, and C. A. Suttle, editors. Manual of aquatic viral ecology. ASLO, Waco, Texas, USA. 2010.
Tyrrell, T., Holligan, P. M., and Mobley, C. D.: Optical impacts of oceanic coccolithophore blooms, J. Geophys. Res., 104, 3223–3241, 1999.
Frouin, R. and Iacobellis, S. F.: Influence of phytoplankton on the global radiation budget, J. Geophys. Res. Atmos., 107, ACL-5, 2002.
Gordon, H. R., Smyth, T. J., Balch, W. M., Boynton, G. C., and Tarran, G. A.: Light scattering by coccoliths detached from Emiliania huxleyi, Appl. Optics, 48, 6059–6073, 2009.
Fournier, G. and Neukermans, G.: An analytical model for light backscattering by coccoliths and coccospheres of Emiliania huxleyi, Opt. Express, 25, 14996–15012, 2017.
Marie, D., F. Partensky, D. Vaulot, and C. P. D. Brussaard.: Enumeration of phytoplankton, bacteria, and viruses in marine samples. Pages 11.11.1–11.11.15 in J. P. Robinson, Z. Darzynkiewicz, P. N. Dean, A. Orfao, P. S. Rabinovitch, C. C. Stewart, H. J. Tanke, and L. L. Wheeless, editors. Current protocols in cytometry. John Wiley & Sons Inc, New York, New York, USA. Mei, M. L., and R. Danovaro. 2004. Virus pro Brussaard, C. P. D., J. P. Payet, C. Winter, and M. G. Weinbauer. 2010. Quantification of aquatic viruses by flow cytometry. Pages 102–109 in S. W. Wilhelm, M. G. Weinbauer, and C. A. Suttle, editors. Manual of aquatic viral ecology. ASLO, Waco, Texas, USA. 1999.
Tyrrell, T. and Merico, A.: Emiliania huxleyi: Bloom observations and the conditions that induce them, in: Coccolithophores: from molecular processes to global impact, Springer, Berlin, Heidelberg, 75–97, 2004.
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AC2: 'Reply on RC2', Riaz Bibi, 16 Aug 2025
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I have completed my review of the manuscript and am pleased to report that it squarely addresses key scientific questions within the remit of Biogeosciences. The study offers novel data and concepts, extending current understanding in an area of clear relevance to the journal’s readers. The conclusions drawn are both substantial and well substantiated by the results presented.
The authors’ experimental design, underlying assumptions, and analytical methods are described in sufficient detail to ensure full reproducibility. The data clearly support the authors’ conclusions.
With respect to context, the manuscript gives appropriate credit to prior work while delineating its own original contribution. The title accurately reflects the study’s scope, and the abstract delivers a concise yet complete summary of objectives, methods, results, and implications.
The overall presentation is well structured, logically organized, and written in fluent, precise English. Mathematical symbols, units, and abbreviations are defined correctly throughout. I note only a single minor technical adjustment—one formula would benefit from an explicit definition of a parameter to eliminate any residual ambiguity. This is a straightforward revision that the authors can very easily accommodate.
No sections, tables, or figures require clarification, reduction, or removal. The reference list is balanced and up to date, and the distribution of material between the main text and the Supplementary Information is appropriate, providing readers with convenient access to extended datasets and methodological detail.
In sum, the manuscript represents a significant and well-executed contribution to the field. I recommend acceptance pending the minor technical correction noted above.
Suggestions:
Abstract:
Line 21 Suggestion: For clarity, please specify where the mesocosm experiment was carried out so that the experimental setting is clear from the abstract alone, without readers needing to consult the full paper.
Introduction
The Introduction is exceptionally well written. It summarizes all key information on the sea-surface microlayer (SML) and cites the most relevant literature, providing a broad yet coherent overview of the SML’s chemical properties, dominant processes, and unique position as the interface between two major environmental compartments. The authors give particular emphasis to organic matter—its formation, transformation, and cycling at the air–sea boundary—which is critical for the questions addressed in this study. The authors present a clear, specific working hypothesis that directly links the marine and atmospheric compartments and stresses their reciprocal interaction and interdependence.
Materials and methods
The experimental set-up is thoroughly described, with all relevant technical details. The authors highlight how the SURF system’s design features allow the experiment to simulate in-situ conditions as faithfully as possible. The study period is sufficiently long to capture the natural temporal evolution of environmental processes. Although mesocosms can never reproduce the full complexity of the open ocean, the authors explicitly acknowledge this limitation emphasizing that their approach is well suited for studies of selected biogeochemical processes within the sea surface. They also document how specific technical challenges were addressed—optimizing pump operation, replacing sampled water with fresh inflow, and adding nutrients at discrete time points.
Line 185 – It is unfortunate that analyses for DOC, PON, TDN, PN, and pigments could not be carried out on the all SML samples; however, this statement contradicts the information in line 223 and again in the Results section (line 490), where it is reported that both SML and ULW samples were analyzed for DOC and TDN (see line 495). This discrepancy is confusing for the reader and should be checked and corrected.
All experimental procedures are described in detail. The authors clearly note any deviations from published protocols, and the methods are presented with sufficient information to ensure full reproducibility.
Results
Given the volume of data collected, the authors have made sound editorial choices about what to place in the main manuscript and what to relegate to the Supplement, thereby avoiding an overload of detail in the core text.
Line 413 – Minor suggestion: Although readers in this field will understand the context, I recommend adding parentheses around the sum of the N-nutrients so that it is mathematically explicit.
All measured values and concentration data for the various parameters, including the SOM analysis, are presented and explained clearly and concisely.
Discussion
The authors convincingly position this study as the first multidisciplinary mesocosm experiment to track biogeochemical interactions in both the sea-surface microlayer and the underlying water throughout an entire phytoplankton-bloom succession. They show that biogeochemical variability is consistently greater in the SML than in the ULW, emphasizing the SML’s highly dynamic nature stemming from its direct exposure to the atmosphere.
Key mechanisms governing organic-matter cycling and biological activity in the SML are clearly articulated, together with their implications for air–sea exchange (as further explored in the accompanying special-issue papers). The data support the authors’ working hypothesis, and the observed relationships among the measured biogeochemical parameters are interpreted in a manner that is both rigorous and well anchored in the existing literature.
The study demonstrates that the SML acts as a biogeochemical reactor: it accumulates surface-active compounds and promotes DOM transformation via formation of an OM-rich biofilm. By highlighting the central roles of phytoplankton and microbial communities in modulating surface-ocean carbon cycling, the authors make a valuable contribution to our understanding of processes within the SML and its variability. Such insights are essential for improving climate models and refining air–sea exchange estimates.