The manuscripts presents a study on the development of a method adapted to the determination of alginate in sediments.

The interest of this modified protocol is well justified, both from a scientific and from practical approaches. The work planning is well presented and the results are clearly presented and discussed.The authors used the protocol to characterize samples from different locations.

Only minor comments are suggested:

Unify the format of units, w/w or w·w-1

Why yield is expressed per 300 g?

Latin names in References should be in italics

This is a useful paper, its main contribution is methodological. The authors show that the original food-chemistry protocol performs poorly in sediment, and that their modified protocol improves recovery substantially. That is the strongest result in the paper, and it should be the main message.

My main concern is overstatement. The paper often sounds as if it has cleanly measured brown-algal carbon in sediment. But the evidence presented is more limited. The method measures uronic acids. The authors themselves show some remaining non-alginate signal, and the enzyme experiment only supports that at least part of the measured material is alginate. The paper is much stronger if framed as a promising sediment method with a first field application, not as definitive proof of brown-algal carbon sequestration.

Main comments

The paper should center its novelty more clearly. The real novelty is not the broad blue-carbon context. It is that the authors adapted an existing alginate method to a much harder matrix, marine sediment, and identified two practical changes that matter: adding EDTA and removing ethanol precipitation.

The claims about specificity should be softened. The phytoplankton test still gives a residual signal equivalent to 23% of the sediment signal, which is not trivial. The enzyme test also leaves a large residual signal, and the authors conclude only that alginate accounts for at least one-third of the uronic acids detected. That supports the presence of alginate, but not a clean one-to-one conversion from color signal to alginate alone. The wording should reflect that more carefully throughout.

The conclusion goes a bit too far. The manuscript currently says the method provides “quantitative and direct evidence” of brown-algal carbon in ocean sediments, and later links the findings to coastal carbon sequestration. That feels stronger than the data support. The study shows alginate-like material in surface sediments. It does not directly show long-term burial or sequestration. I would recommend more cautious language here.

The introduction is longer than needed. It spends too much time on general uronic-acid chemistry before getting to the actual gap. I would shorten that part and move faster to the practical problem: no established method exists for alginate in marine sediment, where cations, low concentrations, and other uronic-acid sources complicate the analysis.

There is also a wording inconsistency that should be fixed. The abstract calls alginate “unique to brown algae,” but the introduction later states that alginate is also produced by some bacteria. The authors should use one careful formulation throughout.

The paper has a real contribution and the method improvement seems meaningful. The main issue is in the framing, not the core idea. With tighter wording, clearer explanations, and more cautious claims, the manuscript would become stronger and more convincing.