the Creative Commons Attribution-NonCommercial 4.0 Deed License.
the Creative Commons Attribution-NonCommercial 4.0 Deed License.
Fucoidan carbon is stored in coastal vegetated ecosystems
Abstract. Coastal vegetated ecosystems are key-nature based solutions for climate change mitigation. Mangroves, seagrass meadows and saltmarshes contribute to carbon sequestration not only through their photosynthetic activity but also by anchoring sediments with their extensive root systems. By modulating flow coastal vegetation creates a low energy environment for sediment that includes carbon to accumulate. These roots physically stabilize the sediment, prevent erosion and enhance long-term retention of organic carbon. Hence, we hypothesized marine, algae derived organic matter may especially accumulate in plant vegetated ecosystems. We used algal and plant glycans as carbon sequestration proxy to trace the input and stabilization from source to sink and found those molecules in 93 sediment cores across different coastal vegetated ecosystems from temperate to tropical regions. Specific monoclonal antibodies showed algal-derived fucoidans were present in sediments of coastal vegetated ecosystems. Our findings suggest that the restoration of plant ecosystems that fix carbon dioxide, protect coasts and enhance biodiversity should also be enumerated for the stored carbon from distant donors. Conclusively, carbon sequestration is a synergistic outcome of photosynthetic contributors acting in concert across different ecosystems.
Status: open (until 07 Nov 2025)
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RC1: 'Comment on egusphere-2025-4715', Morgan Raven, 02 Oct 2025
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AC1: 'Reply on RC1', Inga Hellige, 20 Oct 2025
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Thank you very much for your constructive and valuable feedback. We are currently working on incorporating your suggestions and preparing a detailed point-by-point response.
Citation: https://doi.org/10.5194/egusphere-2025-4715-AC1
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AC1: 'Reply on RC1', Inga Hellige, 20 Oct 2025
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RC2: 'Comment on egusphere-2025-4715', Anonymous Referee #2, 14 Oct 2025
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The manuscript by Hellige et al. entitled “Fucoidan carbon is stored in coastal vegetated ecosystems” examines the storage of polysaccharides in sediments of coastal ecosystems. A protocol was set up to analyse nearly a hundred samples of different sources, through immunodetection procedures and chemical assays of monosaccharide contents. While the study is thoroughly performed and well-written, I have the feeling that more could have been said or shown.
My main concern is regarding the novelty, as original papers have already been published in the field with rather similar conclusions, some from the same group (Vidal-Melgosa et al. 2022, https://doi.org/10.1101/2022.03.04.483023) and from others with the same methods (Salmeán et al. 2022, https://doi.org/10.3389/fpls.2022.785902) and combined with temporal dating of the sediments. The only difference would be the sampling location. While these new samples can probably drive interesting conclusions on coastal ecosystems, the discussion on their different properties is only partial, giving weak conclusions. Progress is limited in this study. Hence, in its current form, I cannot recommend the manuscript for publication. But I would encourage a submission of a more consistent story, including additional data that authors may have and/or a heavily reworked manuscript.
Main comments
- Authors have analyzed the monosaccharide composition of their 93 samples but a table giving the full inventory to these compositions is not included. This would be useful to evaluate authors’ statement on the fact that “fucose, galactose, glucose, mannose, xylose and galacturonic acid were found to be shared between all ecosystems and depths”. In addition to this, the picture gained by the antibodies can only be shaped by the available probes. So far probes targeting sulfated galactans for instance (derived from red algae) are missing, thus the monosaccharide content is a necessary information to be displayed.
- The paper later focuses on the abundance of fucoidan, seen through antibody detection (BAM1) correlating a fucose content. Is there any reason why the BAM1 detection is not apparent in Fig. S2 in contrast to other antibodies? As it seems to be apparent only in Fig. 3 and beyond.
- In contrast, the BAM7, JIM13 and to some extent LM27 antibodies seem to give nice signals on the arrays, Fig. S2 (as discussed in the results section) but the use of these antibodies, and their most likely corresponding monosaccharides, are not included afterwards in a more comprehensive study. For instance, how are mannuronic and guluronic acid (as a proxy for alginates) correlating BAM7 recognition? A similar question can be raised for galactose/galacturonic acid and JM13, and xylose for LM27.
- The results indicate a possible alginate detection across all ecosystems. Alginates being a rather simple and linear polysaccharide, easily degraded by microorganisms, one would not expect this polysaccharide to be that abundant relative to other polysaccharides. Can authors comment on this? Or could it be a cross-detection of pectins, as somehow suggested? In any case, a comparison with the uronic acid content (galacturonic, mannuronic and/or guluronic acids) would be appreciated.
- Page 7 lines 182-185. Three sub-sampling are discussed for the porewater samples in the saltmarsh zone. This is not clear to me if similar sub-samplings have been obtained for the sediment’s parts, and if so why they are not discussed? Overall, I feel that the manuscript would benefit, and get stronger conclusions, by having more information on the properties of the sampled areas (salt, oxygen levels, vegetation type, or any type of relevant information).
Minor comments
Salmeán et al. 2022 indicated the high content of mixed linkage glucans in their Fjord sediments, and to some extent cellulose and xylan. Is there any reason why the antibody labelling targeting such components are not included in this study (especially knowing that glucose seems to be detected at high concentrations in some cases)? -or is the screening giving no detection?
Page 3 lines 90-95. Please indicate proper antibody sources as some are not included in the referenced paper, Vidal-Melgosa et al. 2022. In particular for Fig. S2: what LM6-M and LM6 stand for, are they different antibodies?
Page 4 line 105. Could authors explain why “the relative abundance of acidic sugars of over 50 % were excluded from calculations”?
Citation: https://doi.org/10.5194/egusphere-2025-4715-RC2 -
AC2: 'Reply on RC2', Inga Hellige, 20 Oct 2025
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Thank you for taking the time to share your valuable feedback. We are in the process of addressing your comments and will provide a point-by-point response.
Citation: https://doi.org/10.5194/egusphere-2025-4715-AC2
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- 1
This manuscript compares abundances of polysaccharides in coastal blue carbon ecosystems from endogenous and exogenous sources. Surprisingly, the authors find that monosaccharide abundances do not show major first-order patterns across environmental categories. One major style of variation appears to be spatial patterns in the relative abundance of fucoidan in porewater (at the North Sea site), and similar relationships with sedimentary fucose are suggestive of a global pattern. The authors interpret these data to indicate that polysaccharides from marine algae are transported to, and trapped within, coastal blue carbon ecosystems in significant quantities. These data provides a new level of insight into these ecosystems that complements other analytical approaches and has potentially impactful implications for coastal management. However, there are a few issues that need attention before publication. I have included some additional minor notes below.
Perhaps most importantly, this study integrates data from a wide diversity of global sites categorized as mangroves, salt marshes, seagrasses and unvegetated areas. Most of this data apparently showed relatively constant compositions and is lumped together into the statistical treatment on Figure 1. However, I had a hard time finding key aspects of the description of the sampling sites, especially was was covered within the umbrella of “unvegetated.” When I got to the Supplement, I was very surprised to learn that a single sampling site could contained all four types of system – I did not expect “salt marsh” and “mangrove” to be a few hundred meters apart from what I read in the text. This left me with a ton of questions about the elevations of transects relative to tides, plant types and coverage, spatial relationships within sites (is the mangrove upstream of the marsh? Is the unvegetated site a former seagrass bed or subaerial?) etc etc etc… and I was left unsatisfied with only the lat/long data and one “example” of a sampling site map. More precise and thorough site descriptions are needed to support the conclusions from Figs 1+2, which are highly dependent on site classification choices.
Given all of these potential axes of variability within the site, I would also hope to find a complete dataset for the raw monosaccharide abundances, to support replication / followup work. I didn’t see these data provided, where are they? Figure 1 only shows a derived / statistical tool, so I wasn’t able to actually see the data. The raw data need to be shown or at least provided in tables.
Finally, the processes invoked to interpret the results hinge on the transfer of polysaccharides between the dissolved, weakly sorbed, and particulate pools. The paper would be strengthend by a careful clarification of which fraction of the polysaccharide pool is relevant for each data type, and an expanded discussion of these POM-DOM relationships (more below).
Detailed / minor notes:
Several of the key observations and results in the text don’t come through as well as they could in the abstract. Specifically, the gradients observed in salt marshes are some of the most convincing evidence for an exogenous source; clearly stating what you observed will help focus your reader on your new contributions.
Line 35 – A bit more detail in this comment would help put your new results in context. What key data types were used in the Kennedy paper (etc) to quantify algal carbon in those environments? Is this d13C and C:N, or are there molecular tracers involved?
Because you are later going to reference different regions of the salt marsh at different tidal elevations, this would also be a useful framing for the background data presented. Do these previously published salt marsh data correspond with the upper or lower marsh?
Line 44 – More detail would be helpful (here or in the discussion) to lay out your working model for how fucoidan is physically transported in coastal environments. How is fucoidan (and other polysaccharides) distributed among the dissolved, sorbed, particulate (etc) phases? What do we know about how it can move in and out of the particle-associated phase?
Line 127 – The axes in Figure 1B/C are derived from statistical treatments that readers may not be familiar with. Given the emergent variables x1 and x2 show this relationship between glucans and fucans, how does it appear in the “raw” dataset? Please also show the direct relationship between the relative abundances of these compounds, at least as a supplemental figure. (Does the slope / form of this relationship between concentrations reveal anything useful?)
Line 143 – Somewhere it would be useful to at least mention how “total carbohydrates” by your digestion method relates to carbohydrate-derived carbon in proto-kerogens, humics, and other geopolymers that have experienced recombination. Is there any trend observed in extractable carbohydrates with depth or effective age that could suggest aging affects recovery?
Line 152 – “Spearman rank correlation of mean depth” is unclear, explain exactly what is being shown? Please also show a less derived version of this relationship, i.e., between depth and the ratio itself. If the relationship is only present in the “Spearman rank” version please discuss the implications of that.
Unvegetated areas – what are these? Water depth? Tidal exposure? Nearby or upsteam C sources? Grain size or sediment type? How are the unvegetated sites related to nearby vegetated sites? I imagine that a great deal of work was done to look for relationships between the data and essential these essential site variables even if strong reportable relationships weren’t found. Nonetheless it would be valuable to expand the discussion of the approach that was taken (what relationships were tested for?).
Line 174 – The relationships in Figure 3 show EDTA extracts while Figure 4 shows porewater extracts. What do the solid extracts show (or not show) spatially, equivalent to the DOM? Is there evidence that the DOM source is actually deposited into the sediments? What is the model for its delivery to the high marsh? Please also clarify which sites are included in Fig 4 in the art or its caption.
The sample type /method and its implications for which portion of the fucoidan pool is being sampled should be clarified throughout.
Line 212 – The “glycan continuum” is an important conclusion and deserves greater explanation for your readers. What other evidence has been levied for this idea in other systems? How does it work? How could it be tested? What are its implications for coastal polysaccharides?
Line 228 – Explain the proposed relationships between DOM production and POM production.
Line 242 – The authors propose that fucoidan might be a tracer for exogenous polysaccharides for management purposes. So, to what extent can your dataset here give quantitative constraints on the size of this pool? Can it be compared with literature data on the net storage of C in this system? Comments on d13C implications? Is there any way to compare the accumulated flux to the source - tidal flushing volumes x reported concentrations in seawater?