The manuscript of Medhavi Pandey and coauthors on ‘Sedimentary organic matter signature hints at the phytoplankton-driven Biological Carbon Pump in the Central Arabian Sea’ presents new and significant data on a topic of general interest. However, as the second paper on the same samples and along a similar scientific avenue following the paper of Pandey et al. (2023, Environmental Monitoring and Assessment, 195/1), I would suggest a broader appreciation of the first paper, i.e., to build on the first paper and not just reusing the same data. By doing so, a clearer distinction of the two papers as stand-alone contributions would be possible. By a clearer distinction, the two papers may receive more visibility, and may be better cited in the upcoming literature. Following a clearer distinction of the two papers, the Abstract and Conclusions may need to be reorganized.

I believe that an improvement of syntax and language, as well as the figures, would enhance the overall quality of the paper. Overall, I would suggest to use the present / present perfect tense when presenting the results, which allows a more dynamic reading. Finally, I would suggest to accept the paper for final publication following major improvements. To support revisions of the manuscript, I provide an annotated pdf file.

Finally, the figures and tables should be improved. For example, the panel a of the Fig. 1 does not show the water-depth related differences discussed in the paper, because all of the sampling sites are shown in the same kind of blue indicating the water depth from 2000-4000 m (the Fig. 1 is better in the 2023 paper). Please find detailed suggestions in the annotated pdf file. 

The manuscript ‘Sedimentary organic matter signature hints at the phytoplankton-driven biological carbon pump in the Central Arabian Sea’ by Pandey et al. looks at the composition of top sediment cores along a productivity gradient in the central Arabian Sea with the aim to evaluate the relative contribution of diatoms, flagellates, coccolithophores and zooplankton to carbon sequestration. This is an interesting study and Figure 5 nicely illustrates how the regional differences in the physical and biogeochemical environment lead to a different community structure of primary producers, different grazer abundances and different export. The manuscript builds on a previous study by the same lead author (‘Interlinking diatom frustule diversity from the abyss of the central Arabian Sea to surface processes: physical forcing and oxygen minimum zone, Environmental Monitoring and Assessment, 195(1), 161, 823 https://doi.org/10.1007/s10661-022-10749-7, 2023), but introduces lipid biomarker results as a novelty. The authors’ main conclusion is that dinoflagellates rather than diatoms or coccolithophores contribute most to the sedimentary flux. This conclusion derives from higher amounts of dinosterol (used for dinoflagellates) compared to brassicasterol (used for diatoms) and alkenone (used for coccolithophores) per unit organic matter in the sediment.  

                However, brassicasterol is not a reliable marker for diatoms. Rampen et al. (2010, ‘A comprehensive study of sterols in marine diatoms…’ ) analysed the sterol composition of > 100 marine diatoms species and in regard to brassicasterol they wrote: ’As this sterol is only the fifth most common sterol and absent in all radial centric diatoms and some important groups of bi(multi)polar centrics, our data support the statement by Barrett et al. (1995) that 24- methylcholesta-5,22E-dien-3b-ol should not be considered a general biomarker for diatoms. Furthermore, this sterol has also been found in many other groups of algae like Haptophyceae, Cryptophyceae, Chrysophyceae, Bangio[1]phyceae, and in a number of dinoflagellates (Volkman 2003)’. In line with Rampen (2010), I found more brassicasterol in Emiliania huxleyi than in 30 polar diatom species. The occurrence of brassicasterol in coccolithophores has also been described by others (e.g. Ding et al. 2019, ‘Lipid biomarker production …’). Looking at the 8 diatom species that the authors mentioned in Fig. 4 (Coscinodiscus, Thalassiosira etc..), none of them has been found producing brassicasterol in  Rampen et al. (2010).

                The second point, I would like to bring across: Sterols and other lipid biomarker such as fatty acids have rarely a fixed ratio to carbon or biomass. The production of these components can be highly sensitive to environmental conditions, e.g. light levels, nutrient supply, pH etc. Therefore, even though sediment cores contain more dinosterol than brassicasterol, this does not allow extrapolation to algae cell numbers or biomass. Ratios of two sterols have some potential for regional comparisons of relative abundance or presence vs absence, but not to quantify biomass.

If the authors would like to move forward with sterol biomarkers, I would suggest they analyse the sterol composition of their two main diatom species (Coscinodiscus and Thalassiosira) – either picking sufficient live cells from the sediment or water column, or culture them and grow sufficient biomass. Based on those findings, the samples from top sediment cores could be re-analysed for the ‘right’ sterols. Rampen et al. (2010) found chalinasterol (24- methylcholesta-5,24(28)-dien-3β–ol; m/z 470) in both Coscinodiscus and Thalassiosira (but likely different species). The same should be dome with their common platted dinoflagellates and coccolithophores. This will help to correctly interpret the sterol composition of the sediment cores. For a further read on sterols in microalgae (including the production of dinosterol) I would recommend Volkman (2017, 10.1007/978-3-319-24945-2_19  )and for phyto-vs-zoosterol ratios (Kohlbach et al. 2021, https://doi.org/10.3389/fmars.2020.610248 )