Comment on bg-2021-232 Anonymous Referee # 2 Referee comment on " Intact polar lipids in the hadal seabed of the Atacama Trench point to lateral sediment transport and in situ production as key sources of labile organic matter

Flores et al. provide a comprehensive survey of intact polar lipids from ocean floor sediments collected along the rim and within the deepest portion of the Atacama Trench. The diversity and abundance of the benthic lipidomes were further contrasted with that of the overlying 0-700m water column (with results published by Cantarero et al., 2020). This environmental baseline survey is then used to make inferences about the source and transport of lipids into the trench. This paper is generally well written (more so for the results and discussion). The data represent an important contribution.


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The potential temperature-salinity-dissolved oxygen (θ-s-O2) diagrams revealed an oxygenated and well-mixed 235 water mass occupying the deeper parts of the AT (Fig. S1)

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An overview of the most important IPLs contributing to dissimilarity between samples ( Fig. 3) (Table   273 2). Additionally, this cluster exhibited a large diversity of PC molecules, with lower relative abundance (Fig. 3).

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The difference in the total number of acyl carbon atoms in both alkyl chains, rather than in individual fatty acids,

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To test whether IPLs detected in the Atacama Trench (AT) exhibit signals unique to this environment, we  Ext-AR and PI-OH-AR), and one ornithine lipid (OL-37:6). While unique to hadal sediments, their total 392 concentration was low (~53.32 ng g -1 sediment) and they contributed ~0.00012% of the total IPL pool (Fig. 6a).

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We then performed a cluster analysis to compare IPLs in deep-sea surface sediments against IPLs reported in the

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However, their biological origin is highly diverse and also includes heterotrophic bacteria (Oliver and Colwell,

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The hierarchical cluster analysis on variations in the relative abundance of PGs suggests that several compounds 446 maintained a similar proportion in bathyal and hadal sediments, which differs from the water column (Fig. 7a).

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Most PGs in the bathyal and hadal sediments have long acyl carbon chains (C34-C41), and they show odd-and 448 even-numbered polyunsaturated fatty acids (Fig. 7a). The average chain-lengths of even-numbered n-C18, n-C20

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PEs showed a similar distribution in bathyal and hadal sediments (Fig. 7b), where they are dominated by long-

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The most abundant DGDGs in hadal and bathyal sediments of the AT were DGDG-34:2, which has been including various groups of diatoms, brown and green algal chloroplast membranes (Harwood, 1998), and 569 cyanobacteria (Siegenthaler, 1998;Wada and Murata, 1998). SQDGs have also been found in bacteria from the 570 α-and γ-proteobacterial lineages (Benning, 1998). In the overlying water column of the Atacama Trench,

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SQDGs showed a consistent distribution in bathyal and hadal sediments, where they are dominated by long-chain 575 (C30-42) fatty acids (Fig. 8c). This is contrasting to their distribution in the overlying water column where they are waters of our study area and SQDG-38:4 has been correlated with NO3 - (Cantarero et al., 2020).

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The observed differences in the distribution of SQDGs in deep sediments compared to the water column suggests 583 an in situ production of previously poorly characterized compounds, and/or lateral transport in addition to at least 584 some export from surface waters.

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The most abundant IPL from the entire data set of Bathyal and hadal sediments is DGCC-42:6 (Fig. 9c). This is  Fig. 10a), the NMDS (Fig. 10b), and the SIMPER analysis 681 (Cluster 1 in Table S1). The deep-sea surface sediments showed weak clustering with the IPLs reported in the completely absent in the overlying water column (Fig. 11). This reinforces the idea that these IPLs most likely 744 originate from in situ microbial production in sediments.

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The most abundant IPL in sediments, DGCC-42:6, was not present in cluster 1, which only contains hadal 747 sediments (Figs. 2 and 3). Instead, this compound is prominent in clusters 3, 4, and 5, which contain both hadal  (Table S1). Circle ratios are proportional to the relative abundance of IPL 763 compounds per sample. Samples are organized along the Y axis and shown in colors that match the hierarchical cluster 764 analysis in Fig. 10  The authors declare that they have no conflict of interest.