The authors add hydrated lime to filter-sterilized seawater and track the changes in DOC concentration and optical parameters over a 24 hour period. The resulting conclusions provide a preliminary view into how dissolved organic carbon would change immediately following the addition of calcium hydroxide to seawater, work that is relevant for attempts to sequester carbon from the atmosphere. The conclusions from this work are limited, but could be a valid first step towards understanding this portion of the impact of liming on DOC.

The abstract discusses ‘more pronounced effects’, but the manuscript appears to have no statistical tests applied to support the conclusions presented by the authors.

I am confused a little about how pH was considered as a variable. I recognize that two different conditions were set up (pH = 9 and pH = 10). However, the CDOM/FDOM samples were then brought to neutral pH before analysis. Given that one key parameter under consideration is pH, I don’t understand the justification to remove the impact of pH on the CDOM/FDOM measurement.

The authors indicate that pH was measured, but the only pH data are presented for the initial conditions. Did the change in pH over the course of the incubation correspond to any of the changes in DOC/CDOM/FDOM? Given the importance of pH on the solubility of carbon in water this is an important parameter to consider over the time of the incubation.

Abbreviating Mediterranean Sea and Baltic Sea seems unnecessary.

Additional comments:

Line 33: ‘These effects…’

Line 52: ‘sharp increase in pH of about 1 unit, becoming lower than 0.2 units, 1400 – 1600 m far from the discharge site…’ this is somewhat awkward and not clear. Please reword this sentence.

Line 102: I would alter the order information is presented in this paragraph. The first sentence indicates the bottles were mixed, and then the exception is given. This would be clearer if you start by indicating first that supernatant was collected from each bottle, then the bottles were mixed, and then an additional sample was collected.

Line 106: the methods section here indicates the particles were removed by filtration, but line 166 indicates no mixed samples were collected. Please clarify.

Line 147: correct to Kruskal Wallis (only one L in Kruskal)

Line 178: correct to tryptophan

Line 272: Considering the rather low salinity of the Baltic Sea sample, the statements about the increased terrestrial DOM can be attributed to the fact that the sample was not a truly marine sample. Given the large salinity differences between the two samples, the conclusions about Mediterranean vs. Baltic might be a stretch as they could also be solely due to differences in salinity, as the authors elude to in this paragraph.

Line 293: Extrapolating conclusions to the importance of seasons is a bit of a stretch here, two samples collected one month apart do not allow any sort inference about the relevance of season. There are many additional parameters that would be needed before details on OAE would be clear, seasonality is but one.

Figure 4: are there replicates for all the samples? If yes, please note that the error bars are smaller than the symbol.

Supplemental Table S1 – What is an ‘EU representative product’? Is the product specification the desired set of parameters or the actual set of parameters? What is UniCalce (2021)? It is not in the list of references provided.

Table S2 – tryptophan (no e at the end)

The manuscript by Santinelli et al. tests how ocean alkalinity enhancement (OAE) affects DOM composition and concentration. They conducted an experiment with two seawater samples, in which Ca(OH)2 was added to pH of 9 or 10, and they found that in both samples, there was removal of DOC and of CDOM / FDOM at the highest pH treatment. Possible reasons are discussed, and adsorption to secondary carbonate minerals is identified as the most likely explanation.

Overall, the scope of the manuscript is a bit limited in only having experimented with two different water samples. However, I think that the novelty and timeliness of the study are both very high, and the two water samples cover two extremes in terms of DOM source and composition, so I think that the results are likely to be generally representative of effects from lime addition. I think that the experiments are fundamentally robust, even though clearly more research is needed to understand the underlying mechanisms and work out the implications. But I support publishing this manuscript already, because that will motivate further work in this direction. I have a number of relatively minor suggestions for revisions as listed below.

Line 23: you say that DOM is “the largest … mixture of organic molecules on Earth”. I suppose it depends a bit on how one wants to define individual pools, but collectively the soil organic carbon pool on land is at least twice as large as the DOC pool, so perhaps “one of the largest” would be better?

Lines 52 and 74-75: Initially you talk about enhancement by +1 pH unit and release of 10 kg/s, but then the experimental treatments are +1 and +2 pH units based on release of up to 25 kg/s, but the same reference is given for both. Maybe explain this a bit more clearly in the introduction so it doesn’t come across as being inconsistent?

Line 132: “The EEMs were elaborated” – better to say “processed” or “analysed”.

Line 138-139: I don’t understand well how the sample sizes come about, and especially why the sample size would be different, given that the experimental design is the same. I suggest explaining this here. Based on Section 2.1, I would expect a sample size of (2 pH treatments + 1 control) x (4 time points) x (3 replicates) = 36 samples for the CDOM/FDOM analysis.

Lines 247–263: This is an interesting discussion. Another study that might be relevant here is Kaushal et al. (2020), who conducted aragonite precipitation experiments with seawater and different DOM sources to investigate humic substance incorporation into coral skeletons (for transparency: I am a co-author on that study). In that case, the evidence suggested preferential incorporation of terrestrial humic substances, which I think is consistent with the results here, and more generally helps explain incorporation of FDOM into coral skeletons. The literature cited here at the moment seems to be mostly on freshwater lakes/ponds.

Lines 186-292: It’s clear that in the Baltic sample, a smaller percentage of the initial DOC pool was removed, but I think it’s important to also recognise that this represents a much larger absolute quantity of DOC removed. I didn’t see data presented anywhere on the amount of CaCO3 being formed, but the first sentence here suggests that there was less CaCO3 formed in the Baltic experiment (please clarify whether you have data to show that less carbonate was formed, or whether this is an assumption). If the amount of CaCO3 that was produced in the Baltic samples was equal to or less than the amount in the Mediterranean, then that means that per mass of CaCO3 the Baltic DOM is being strongly preferentially removed compared to the DOM in the Mediterranean. This is consistent with the conclusion in Kaushal et al. (2020) that CaCO3 preferentially removes terrestrial FDOM.

Line 298: “any hypothesis of liming-based OAE” is unclear. Do you mean “any proposal” or “any plans for”?

Section 4.3: This section might be improved with a bit more detailed discussion. I appreciate that this will necessarily be a bit speculative, but the main change seems to be in the humic and fulvic fractions, which typically not the most highly labile parts of the DOM pool. At the same time, if the DOM is sorbed onto CaCO3 particles, that in itself might alter the bioavailability, regardless of the inherent properties of the DOM. You could therefore discuss in a few sentences which of these scenarios you think is the more probable one. Perhaps even more important would be if you could provide some thoughts on the design of experiments to test this, as it would be important to avoid artefacts from direct pH impacts on the microbial community.

Kaushal et al. (2020). Sub-annual fluorescence measurements of coral skeleton: relationship between skeletal luminescence and terrestrial humic-like substances. Coral Reefs 39:1257–1272. https://doi.org/10.1007/s00338-020-01959-x