the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2026
Enhanced Carbon Sequestration in Alkaline Lakes through Recycled CO2 Utilization by Diatoms
Abstract. Recycling of microbial respiration-derived CO2(aq) plays a crucial role in regulating carbon sequestration and emissions in alkaline lakes, yet the extent to be assimilated by primary producers remains poorly constrained. In this study, a total of 35 surface sediments were collected across Lake Haixihai and organic molecular as well as carbon isotopic geochemical methods were employed to investigated the efficiency of diatom uptake of respiration-derived CO2(aq). The results show that organic matter in surface sediments is dominated by aquatic macrophytes and terrigenous inputs, with minor contributions from algae and bacteria. The diatom-sourced C25 highly branched isoprenoids (HBIs) were detected in all sediments with C25:2 HBI as the most abundant HBI. The δ13C values of C25:2 HBI strongly correlates with δ13Corg (r = 0.946, p < 0.001) and the extent of microbial reworking as indicated by concentrations of C30ββ-hopane + hopenes and lignin-derived P/(V+S) ratios (r = -0.92 ~ -0.93, p < 0.001), suggesting that diatoms can substantially assimilate respiration-derived CO2(aq). A two-endmember mixing model reveals that the respiration-derived CO2(aq) utilized by diatoms accounts for an estimated 31 % ± 8 % of diatom carbon uptake. Comparisons across oceans, lakes, and wetlands show that inland waters sustain substantial recycling of respiration-derived CO2(aq), which is efficiently assimilated by diatoms and reintegrated into the biological carbon pump. These findings suggest that lakes should not be recognized easily as conduits for terrestrial carbon loss. They are active systems in which biological re-fixation of respiration-derived CO2(aq) can substantially enhance long-term carbon sequestration and mitigate CO2 emissions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2998', Anonymous Referee #1, 23 Jul 2026
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RC2: 'Comment on egusphere-2026-2998', Anonymous Referee #2, 09 Sep 2026
General comments
This manuscript presents molecular and isotopic signatures of 35 sediment samples collected across Lake Haixihai to investigate diatom recycling of microbially respired organic carbon, as a potential mechanism of carbon sequestration in lake sediments. Given the growing interest in freshwater "biological pumps”, this study, employing a novel combination of advanced molecular and isotopic analyses, can appeal to the broad readership of Biogeosciences. Overall, the manuscript is well written and clearly focused on the central argument: the biological recycling of CO2 as a key pathway for carbon sequestration in alkaline lakes. While this argument is based on intriguing findings (e.g., strong correlations in molecular and isotopic signatures between sediment organic matter and carbonates), these biogeochemical proxies provide primarily indirect evidence for the central argument. Below, I suggest several major revisions to help the authors sharpen the logic of the main hypothesis and better ground their conclusions in their empirical findings.
1. CO2 recycling vs. C sequestration
In my view, the presented biogeochemical proxies are interesting per se, but provide primarily indirect evidence for the proposed mechanisms. First, the core hypothesis is conceptually weak: CO2 recycling by diatoms, which can continue to fuel microbial decomposition without a bypass to sequestered DIC, does not inherently ensure long-term C sequestration (burial) in sediments. The authors need to clearly define these two key concepts (CO2 recycling vs. carbon sequestration) and refine the theoretical link between them, supported by relevant literature. Second, the empirical findings, as detailed in my specific comments below, are presented inconsistently regarding the primary carbon source for sediment bicarbonates. The entire Results and Discussion sections require a thorough revision to resolve these internal inconsistencies and address the points outlined below.
- L 265: “Collectively, sedimentary organic matter in Lake Haixihai is mainly derived from aquatic macrophytes and terrigenous sources, with minor contributions from algae and bacterial input.” --> Then, how can you say that algal-derived C (including recycled C) can play a crucial role in C sequestration?
- L 270-272 (Fig. 4) “In deep water sediments of Lake Haixihai, δ13Ccarb exhibits a significant positive correlation with δ13Corg (r = 0.748, p < 0.001; Fig. 270 4a) and a strong negative correlation with TOC (r = -0.852, p < 0.001; Fig. 4b). This indicates that carbonate precipitation is strongly influenced by respiration-derived DIC (Sun et al., 2019).” --> Please explain the mechanisms, considering complex processes occurring in both water column and bottom sediments. For example, the observed correlation might be affected by separate processes such as CO2 evasion or phytoplankton CO2 uptake leaving heavies isotopes in water, which can occur irrespective of organic matter transformations in bottom sediments.
- L 285-288 (& 5) “The remaining samples show a strong positive correlation between δ13CHBI and δ13Corg (r = 0.946, p < 0.001; Fig. 5a). Generally, this relationship can be readily interpreted as diatoms being the main contributor of organic matter. However, biomarker and geochemical evidence shows that organic matter in Lake Haixihai is mainly derived from aquatic macrophytes and terrestrial inputs, and minor from algae and bacteria inputs as discussed in Section 4.1. Thus, the synchronous isotopic variations reflect a close link between DIC utilized by diatoms and respiration-derived DIC within Lake Haixihai.” --> Contradictory descriptions against the preceding one and within the same paragraph.
- L 360-364 “This means that diatoms efficiently utilize respiration-derived DIC, returning it to the biological carbon pump. This process likely account for the high organic carbon sequestration flux observed in Lake Haixihai despite its oligotrophic state (Wang et al., 2019). Combined with CaCO3 precipitation, these pathways capture respired-derived DIC that would otherwise escape to the atmosphere, thereby enhancing internal carbon cycling and stabilizing carbon storage.” --> Please explain how microbial-derived CO2 ends up sequestered in sediments.
- L 410-412 “Lakes, particular alkaline lakes, should not be recognized easily as conduits for terrestrial carbon loss (Tranvik & Jansson, 410 2002). They are active systems in which biological re-fixation of respired carbon can substantially enhance long-term carbon sequestration and mitigate CO2 emissions.” --> Is this a quantitatively significant sink?
2. Data presentation
Sections 3.1 & 3.2 (Tables 1 & 2) require more efficient data presentation, supported by rigorous statistical analyses. For example, in Section 3.1, if depth-dependent trends were observed in the measured variables, these would be far more effectively illustrated using statistical relationships or summary figures rather than relying solely on Table 1. Furthermore, any comparisons between shallow and deep sites should be backed by appropriate statistical tests (e.g., t-tests or non-parametric equivalents).
3. Interpreting algal contributions to isotope signatures
In this study, source tracing relies on the simplified assumption that terrestrial plants and algae possess distinct isotopic signatures. However, given the overlap in d13C between plants and algae, as well as lower C/N ratios of soil organic matter (an important terrestrial source) relative to plant biomass that approach algal values, the simple dichotomy (algae-dominated deep water OM vs. plant-dominated shallow water OM) requires a more refined explanation as well as additional information for algal isotope signatures from the study site or other lakes.
- L 237-239: “In Lake Haixihai, δ13Corg values (-30.7‰ ~ -22.9‰) and TOC/TN ratios (6.16 ~ 12.55) indicate that shallow water sediments contain a mixture of algal and terrestrial organic matter, whereas deep water sediments are mainly dominated by algal input (Fig. 3a).”
Specific comments
- Line (L) 13: “investigate”
- L 16: “correlate”
- L 22-23 “lakes should not be recognized easily as conduits for terrestrial carbon loss.”: Lakes are actually well recognized as sources of terrestrial carbon loss. Please reformulate the sentence to clarify what you want to emphasize.
- L 48 “autogenic carbonate”: Do you mean “authigenic carbonate”?
- L 94 (throughout the manuscript): not “u”g L-1 but “μ(micro)”
- L 114: Vienna Peedee Belemnite (VPDB) as a reference material?
- L 167: characters or characteristics?
- L 170: “show”
- L 180 (&142) “carbon preference index”: You need to define this earlier (L 142) and use the acronym consistently.
- L 185 (& 242-244) “TAR”: The same goes for this term. How can we evaluate the values ranging from 1.14 to 12.01? Are any reference values available?
- L 248-256: Interpretations of the two indicators seem contradictory. “lignin parameters (S/V vs. C/V) show that most terrestrial organic matter is derived from non-woody angiosperms” vs. “The aquatic plant index (Paq), an indicator reflecting the proportion of aquatic macrophytes based on the proportion of mid-chain to long-chain n-alkanes (Mead et al., 2005), is slightly higher in deep water sediments (0.37 ± 0.04) than in shallow water sediments” vs. “aquatic macrophytes and terrigenous inputs both contribute to sedimentary organic matter. Despite the small surface area of Lake 255 Haixihai (3 km2), nearshore sediments are strongly influenced by terrigenous input.”
- L 262: Can this indirect evidence be translated to “strong microbial reworking of organic matter”?
- L 373-374: This is a simplification of complex causal relationships. For example, how can you exclude the possibility that freshwater diatoms use 13C-depleted CO2 from other sources (not from reworking)?
- Fig. 5 legend: Please provide more details about the indicators. Otherwise, the figure is not self-sufficient.
Citation: https://doi.org/10.5194/egusphere-2026-2998-RC2
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This study uses diatom-specific C25 HBI biomarkers and compound-specific carbon isotopes to demonstrate that diatoms in Lake Haixihai reassimilate microbially respired DIC. The method is innovative and the core finding advances understanding of inland water carbon cycling. The writing is clear, figures and tables are all well documented. However, the key assumptions and interpretations require further validation, minor revision of this manuscript is recommended.
Here are specific comments:
C25 HBI biomarkers are the only indices for tracking the CO2 sequestration? Any errors can be created for this biomarker?