the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2025
Bacteriohopanepolyols track past environmental transitions in the Black Sea
Abstract. Bacteriohopanepolyols (BHPs) are structurally diverse compounds produced by a wide range of bacteria making them ideal candidates as chemotaxonomic biomarkers and indicators of bacterially-driven biogeochemical processes in the geological record. In this study, we characterize changes in the BHP distribution in the Black Sea over the past 20 thousand years (ka), as the basin underwent three distinct environmental phases: (i) an oxic lacustrine phase where the Black Sea was disconnected from the global ocean; (ii) a transition period marked by the initial influx of marine water into the basin; and (iii) a marine phase where the basin was permanently euxinic. During the lacustrine phase we observe a high abundance and diversity of nucleoside BHPs (Nu-BHPs) that are likely derived from elevated terrigenous inputs as well as production of Nu-BHPs in the brackish-to-fresh water column. The transition phase is marked by a decrease in the abundance of most Nu-BHPs and an increase in the abundance of methoxylated-BHPs as well as BHPs such as aminobacteriohopanetriol which are ubiquitous across a wide range of environments including soils as well as marine and freshwater settings. The euxinic marine phase (7.2 ka-present) can be divided into two stages based on changes in BHP composition. The early stage is characterised by a high abundance of aminobacteriohopanetetrol and aminobacteriohopanepentol, which were likely produced by methanotrophs at the oxycline. A shallow oxycline likely allowed for increased transport of these BHPs to the sediment. The later marine phase is characterised by a decline in these BHPs, likely due to a deepening of the oxycline and reduced transport of BHPs from the oxycline to the sediment. The changes in BHP distributions throughout the record may either be attributed to shifts in the bacterial communities or physiological adaptations of bacteria to the changing environment. Throughout the record, diagenetic products of BHPs (e.g., anhydrous-bacteriohopanetetrol) were detected. These degradation products, however, remain a small proportion of the overall BHP composition, indicating excellent preservation conditions throughout the record. This study offers new insights into changes in microbial communities and biogeochemical processes that occurred in the Black Sea during the Last Deglaciation and Holocene in response to substantial shifts in the hydrology and oxygen conditions of the basin.
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Status: open (until 11 Jul 2025)
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RC1: 'Comment on egusphere-2025-1796', Anonymous Referee #1, 27 May 2025
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In the study "Bacteriohopanepolyols track past environmental transitions in the Black Sea", the authors Cutmore et al use specific bacterial lipids from a sediment core to make statements about changes in the palaeoenvironment over the last 20,000 years. The lipids are precursor molecules of the most widely distributed (in soils, sediments, petroleum) and most widely used biomarkers or chemofossils: hopanes. The portfolio of bacteriohopanepolyols (BHP) used by the authors is large and not always easy to keep track of, even for experts. The method employed uses extraction and analysis with high resolution of the underivatised BHPs. The object of the study, the Black Sea after the last major glaciation, is well chosen as the authors can draw on existing work and interpretations. In a combination of improving the understanding of known processes and transitions in the palaeoenvironment and testing previously insufficiently understood BHP sources, the authors arrive at interesting results. Statistical methods are used to help with this. In addition to partially known and refined interpretations, the study provides new biomarkers for salinity changes over time. This development is well known for the Black Sea, but the biomarker ("methoxy-BHT") can be very helpful for the interpretation of other settings.
The manuscript is generally well written, the length is appropriate, but concluding statements are sometimes missing. Some of the illustrations should be more instructive in order to better inform readers who are not experts in the field (e.g. also by adding semi-quantitative data/illustrations). I also see room for improvement in the presentation of the BHPs and their names. There are some discrepancies in the naming, which makes it unnecessarily difficult to follow the text given the abundance of proven BHP structures. Moreover, the limitation of the identification of structures by mass spectrometry should be mentioned here and there. Further, it should not be forgotten and should be mentioned at appropriate points in the text that BHP can only be used to analyse part of the microorganisms (bacteria) and even only part of them. For example, when discussing the relevance of methane-oxidising bacteria compared to methane-oxidising archaea in the Black Sea, your own data can only provide information on the first aspect. This is not always clear, especially to readers who have little expertise in the use of BHP.
General points:
First, the determination of BHP concentrations is complicated by the certainly very different responses of individual BHP during ionisation and decay during mass spectrometric analysis. The authors correctly describe this. However, they withdraw from this and write that they cannot make any (semi-)quantitative statements. In practice, however, they do it indirectly themselves by adding up the peak responses (in relation to TOC) in Figure 3, for example. If no quantitative comparisons are permitted, a presentation that totals up to 100 % is also out of the question. I think it should be possible to use this data and also make a semi-quantitative statement on the relative concentration of at least the majority of BHPs versus time/depth (with mentioning of the restriction). I would find such a curve helpful as insert to Figure 3 (as a or b). Of course, it would also be interesting to see whether this is consistent with other studies that have analysed (fewer) BHPs, but where there is a large overlap in the BHP biomarkers used and most common?
Second, there seem to be only a few errors or ambiguities here, but the authors should check this again very carefully. For example, the term "anhydrous-" BHT is sometimes used and sometimes "anhydro-" BHT. Furthermore, it remains unclear to me what "BHT" means in Figure 3, for example? In S4, the course of BHT-22S is shown. Is the former a sum of the different BHT isomers? Please do not take this as a request to show all isomers. This would complicate the manuscript even more. However, how certain is the structural elucidation of "BHT-22S" really? Are there, for example, co-elutions with BHP extracts from reference organisms? Me-adenosylhopaneHG-diMe exists twice in Figure 4, for example (in addition to 2-Me and 3-Me). Are these isomers? Which ones are meant in the text when the compound is mentioned? Please double-check everything again. Further, a correct "...may be..." to a structure proposal in a cited study ("BHT-22S") does not seem to be discussed here and the structure or interpretation of the structure is simply adopted (text and Fig. S4). Of course, it is not possible to simply repeat the very extensive structural elucidations that colleagues from Strasbourg in particular carried out on individual BHPs in the 1980s and 1990s. Nevertheless, the limitations of structure elucidation with MS-(MS) should not be forgotten and uncertainties should be described to remind readers to this limitation.
Third, as a reader, I miss take-home messages in some places in the sub-chapters. In some cases, relatively long explanations with pros/cons/exceptions are described (please utilise the potential for shortening here). However, a quintessence is sometimes missing. One example is the long block of text on "BHPs associated with the N-cycle". As a reader, I would expect a clear categorisation of whether the data fully correspond to the interpretation in Cutmore et al. (2025), whether they are helpful and supportive for the other paper or whether they contradict aspects of it. What is new and different in this manuscript compared to Cutmore et al 2025?
Fourth, illustrations are also a matter of taste. Nevertheless, I find it unfortunate and urge the authors to reconsider whether a form of representation can be found that shows the temporal change in a uniform way. In other words: In Figures 1 and 3, time is represented from top to bottom (similar to the core). A form of representation that I welcome and find understandable. In Figure 4 and 5 (and in the supplement), on the other hand, time is shown from left to right. As I said, it is also a matter of taste, but a change in the basic presentation of data versus depth/time within a study makes it unnecessarily difficult for readers.
Finally, in general, the authors demonstrate a great deal of expertise and generally cite existing studies that have worked on similar issues (including with BHP, albeit with fewer structures) in a good way. Nevertheless, at one point or another I would expect categorisations as to where there is clear support from the new data and where there is not.
Specific points
- Line 44: I think early studies by Helen Talbot should be mentioned in these citations of important papers (from 2003 or similar). Her work was methodologically groundbreaking, but also contributed to the understanding of source organisms and biogeochemical processes.
- Line 88: Please replace "compounds" with "BHPs".
- Line 97: mbsl must be explained.
- Line 130ff: See general note. This is understandable, but if you take a very strict view, then BHPs can also not be shown in a comparative graph (e.g. Fig. 3). However, this greatly reduces the informative value, as only the progressions of individual connections can then be considered. I suggest looking for a way to show at least the total BHPs over time in a semi-quantitative way.
- Line 173: See above. Is it really BHT-22S? The illustration only says BHT. It seems to me that the latter is better documented.
- Line 197: See above. This statement refers to "absolute abundances of BHP", but this is not shown at all. Please find a way and discuss the result comparatively in this chapter.
- Line 211: Please change to "...suggesting rapid early diagenetic modifications of BHPs."
- Line 229ff: This is a good example of a summarising sentence at the end of a discussion. However, what is missing here is that diagenetic changes in the BHP play a subordinate role (as this appears the opinion of the authors!?). It would be interesting to look at the ratio of individual unsaturated to saturated BHPs assuming that some are diagenetic products of the other. At least any diagenetic-looking change would hint at respective relationships. Does this support the statement?
- Line 237: "later"
- Line 279ff: "As future studies discover more..." sounds odd. Please check and consider rewriting.
- Lines 334ff and 380ff: I think it is very unlikely that a relevant amount of BHP comes from sulphate-reducing bacteria. For the lacustrine phase, this can easily be explained by the general absence of relevant sulphate in the water. It would be good if the authors could find a way a shorter way to have this component-specific discussion of why the one is somewhat unlikely, though not impossible, and so on. Similar to the authors, I think that neither the transport of biomass from the sulphidic zone nor relevant biomass in the sediment plays a major role. The text and a (shorter) length should express that.
- Line 356: Aminotriol seems so unspecific to me that it could be produced by all kinds of bacteria.
- Lines 359-366: It is not entirely clear what is meant here. Is there now a peak for methanotrophic bacteria in the transition phase? Does this show aminotetrol and pentol? The study cannot contribute anything to the anaerobic oxidation of methane and the topic should therefore only be mentioned briefly.
- Line 366: The data does not show a "distinct increase" for me. Please describe a little better. Figure 5 shows a peak before the transition phase and, in comparison, slightly increased values in marine phase II.
- Line 441: This statement is difficult to understand based on the data. Aminotetrol and -pentol appear to fluctuate strongly (Figure 5).
- Figure 1: Please write the approximate ages (in ka) next to the models for the three phases.
- Figure 3: Many BHP structures are colour-coded in the figure. It is sometimes difficult to make an assignment and it is even impossible without colour (black/white). Perhaps the authors could write the abbreviations of the BHP on the main horizontal bars to at least make these clearly comprehensible?
- BHP figures (Figure 5, but also in the supplement). Please insert a 0-line (y) for each BHP so that it is easier to see when data points differ from 0
Citation: https://doi.org/10.5194/egusphere-2025-1796-RC1
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