the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Aug 2025
Hydrothermal activity indirectly influences ice nuclei particles in seawater and nascent sea spray of the Subtropical Pacific Ocean
Abstract. Particles of marine origin may act as ice nuclei when clouds form and therefore influence cloud properties and lifetime. Here we investigate the abundance of Ice Nuclei Particles in bulk seawater (INPSW) collected in natural seawater of the Western Tropical South Pacific and in sea spray aerosol (INPSSA) artificially generated from the surface seawater. The study area was separated into two oligotrophic zones (the Melanesian Basin and the Western South Pacific Gyre), and a mesotrophic one (the Lau basin), characterized by high plankton biomass due iron fertilization by underwater hydrothermal activity of the Tonga volcanic arc. Our results show that INPSW were on average 80 % heat labile, strongly suggesting a biological origin. INPSW concentrations were two-fold higher in the Lau basin as compared to both oligotrophic areas at all freezing temperatures. This trend is consistent with a higher abundance of planktonic microorganisms, pigments and particulate organic carbon (POC) concentrations in the Lau basin. Over the whole cruise transect, medium to strong correlations were found between INPSW concentrations and pigments (notably with bacteriochlorophyll-a and carotene), bacterial abundance and POC. The heat stable fraction of INPSW exhibited correlations with Dissolved Organic Carbon (DOC) concentrations and were not as variable as the heat labile INPSW. In the nascent sea spray, INPSSA were also mostly heat labile in coherence with the INPSW. INPSSA were predominantly (60 %), submicron in size (presumed originating from film drops), but the supermicron INPSSA constituted 40 % of the INPSSA and were all heat labile (presumably originating from jet drops). Supermicron INPSSA were between 60 to 80 % heat stable with a high variability between samples, indicating different nature of the two fractions of INPs. Supermicron INPSSA were generally more abundant in the Lau basin, while submicron INPSSA did not exhibit any significant difference between the three regions. We report a transfer function of seawater INPs to SSA INPs of 1.70 m-2.LSW and 3.3 m-2.LSW for heat stable INPs, hinting that heat stable INPs were more efficiently transferred to the SSA. Our results suggest that hydrothermal activity indirectly enhances the INP concentration of surface waters, through boosting the biological activity, which results in increases of the ice forming ability of supermicron sea spray particle. Given the extent of hydrothermal activity throughout the global Ocean, its impact on cloud properties should be considered in future ocean-atmosphere interaction studies.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3580', Anonymous Referee #1, 02 Oct 2025
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RC2: 'Comment on egusphere-2025-3580', Anonymous Referee #2, 22 Oct 2025
GENERAL COMMENTS
Indirect effects (i.e. those related to cloud properties) remain a major source of uncertainty in the Earth’s radiative budget and climate models. Processes related to the formation of ice in clouds are a particular source of uncertainty. There is considerable interest in ice nucleating particles (INP) from the ocean in terms of sources, their abundance in the atmosphere, and how their properties affect the formation and properties of mixed phase clouds. Bars et al. focused their research on ice nucleating particles from the ocean, both in bulk seawater and in artificially generated sea spray aerosol (SSA). Their work took place in the oligotrophic Pacific Ocean, which was a good location as the subtropical gyres (large areas of oligotrophic ocean) cover about a third of the Earth’s surface. In addition, INP from marine sources are more likely to be important in these remote locations that are a great distance from continental sources of aerosol.
In line with previous work, Bras et al. concluded that the INP in sea spray and bulk seawater were biogenic in origin. Correlations between the concentrations of INP from seawater and indicators of biomass (pigments, bacteria concentrations, and particulate organic carbon) further indicate a connection between biological processes in the ocean and INP, and recent organic matter rather than the background pool of recalcitrant carbon. The authors try and make a connection between hydrothermal activity in the ocean to the formation and abundance of INP.
This work adds to measurements of INP associated with marine sources (which are limited), but I do not think it increases our conceptual or mechanistic understanding of INP from the ocean.
The authors try and link INP concentrations with hydrothermal activity. While the title indicates that this connection is ‘indirect’ it seems very tenuous. It may be that some of the relevant information is missing as it is presented in other papers from the TONGA project. However, I feel that if the authors want to make a case linking INP with hydrothermal activity, then this should be explicitly made in the manuscript. At present the only data making this connection is presented in an appendix rather than the main paper. Is it clear that the high biomass is supported by hydrothermal addition of trace elements, rather than other sources, such as upwelling of deep water or aeolian deposition?
SSA were generated using a technique described by Sellergi et al. (2005). While this is a published method, I would like to see some evidence that the size distribution of generated aerosol is representative of natural aerosol. Perhaps this could be added as supplementary material.
The freezing temperature of the blanks was relatively warm, which, as the authors acknowledge means that a proportion of INP cannot be detected by this method. Perhaps what can’t be detected is not as important as INP that freeze at relatively warm temperatures, but I think that the authors should discuss the implications of not being able to detect INP affecting freezing at temperatures below -18 degrees Centigrade.
SPECIFIC POINTS
Abstract
Line 25 – The word ‘coherence’ is probably the wrong word choice here.
Line 32 – 35: Is the connection to hydrothermal activity the most significant conclusion? I would argue that there is still debate over whether SSA and INP can be linked to recent production in the water or whether they are driven by the high concentration background of older, relatively recalcitrant carbon in the dissolved organic carbon (DOC) pool. The connection with recent biological activity, whatever the source of nutrients supporting it, is the more interesting result.
Introduction
Line 44: Replace ‘uplifted’ with ‘lofted’.
Line 50: warmer temperature than what? Inorganic INP? Not clear.
Line 66-67: I think the wording is misleading as it suggests that hydrothermal activity is the source of the INP. This is not the case, the INP are biogenic in origin, but hydrothermal inputs are providing nutrients to support that biological activity.
Methods
Figure 1 – The map is difficult to read.
Lines 121-122: Does freezing and thawing the samples change the surface tension measurements?
Line 125: Larger cells will be excluded from flow cytometry measurements. Perhaps this bias is not an issue as oligotrophic areas of the open ocean tend to be dominated by small phytoplankton. The authors should comment on this.
Line 129: Dissolved Organic Matter (DOM) does NOT include ‘dissolved oxygen’, though many organic molecules contain oxygen atoms as part of their structure.
Section 2.3: Should there be a diagram (e.g. in supplementary material) of the setup described in this section?
Lines 153-158: More methodological information needed. For example, what was the filtration rate?
Line 176: I think something is missing from this sentence. The word ‘either’ implies an alternative.
Line 186: Delete ‘and hypothesis’
Results and discussion
Line 191: Should this be changed to ‘Figure 2’?
Figure 2 – Salinity is a dimensionless unit as it is based on the ratios of conductivity measurements. Delete ‘PSU’ as it is not an accepted unit used by oceanographers.
Line 204: Explain ‘the second passing of LAU’. Not clear from the text that the sites or regions were visited more than once. Make sure that this is clear on Figure 1?
Lines 206-207: Delete ‘PSU” as salinity is a dimensionless unit.
Lines 214-215: change ‘nifHgene’ to ‘nifH gene’.
Lines 230-234: Heating the aerosol to denature protein INP is likely to underestimate the numbers and relative significance of proteins as INP. Some proteins are only partially denatured at temperatures around 100 degrees centigrade and some proteins will reform their structure on cooling (e.g. Alsante et al. 2023; Communications Earth and Environment 4:51 https://doi.org/10.1038/s43247-023-00707-7).
Line 248-249: ‘which are known to be active at warmer temperature….’ Add a reference to support this statement.
Figure 4 – Would it be helpful to add the locations of the hydrothermal inputs on this map?
Line 282: ‘Note that relationships to the surface seawater biogeochemistry……’ I found this sentence too vague.
Line 298: Lots of organisms, in addition to Prochlorococcus, contain alpha-carotenes.
Line 306: I don’t understand what a ‘heat sensitive organisms’ is. All organisms are heat-sensitive outside their temperature range.
Line 330: Change ‘refractory’ to ‘recalcitrant.’ Recalcitrant is the more applicable terms and it is widely used in the literature. Recalcitrant emphasizes resistance to biological degradation, which is more appropriate for DOM in the ocean. Refractory compounds are resistant to degradation from heat, chemical oxidation etc.
Figure 7a – Is there really a difference between heated and unheated aerosol?
Line 394-395: The fact that there was only a boost in SSA INP activity near one of the two undersea volcanoes could be used as an argument that hydrothermal activity was not a significant factor in determining INP activity.
Figure 9 - Could a parametric or even a non-parametric statistical test be used to test the hypothesis that there was a significant difference in ice nucleation between the different zones for different categories of aerosol?
Conclusion
Lines 410-446: I suggest shortening the conclusion. Much of it was redundant as it was repetitive of the previous text.
Citation: https://doi.org/10.5194/egusphere-2025-3580-RC2
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- 1
General comments
This is a very interesting and relevant manuscript investigating ice nucleating particles (INPs) in the Subtropical Pacific Ocean and the influence of hydrothermal activity on marine INPs. This study includes seawater samples collected during the TONGA campaign and sea spray aerosol samples generated from sea spray tank experiments during the campaign. They find that the hydrothermal activity indirectly influences INPs in seawater (as described by the title) through its stimulation of biological activity in the area. The studied regions are areas with little measurement data and the investigation of hydrothermal activity is a topic unexplored with respect to INPs. Therefore, the dataset is of high value to the scientific ice nucleation community.
The manuscript is well written and presented. However, the manuscript would benefit from added discussion/interpretation of the presented results and figures. In addition, the methods description is missing important details. See comments below.
Specific comments:
P1L30: This sentence needs to be reformatted. Is it correct understood that the first number is the total INP transfer and the second is the heat stable INP transfer? If yes, please state more clearly.
In the “hint” statement, please write what you are comparing to. “,hinting that heat stable INPs were more efficiently transferred to the SSA” compared to ...
P4L98: Description of the SSA experiments is lacking in detail. The authors reference papers of similar studies, however, the Sellegri et al. 2005 paper is possibly the same chamber (not clear) but run in that study without water or jets. I would remove this reference as it causes confusion. The tank described in Schwier et al. 2015 seems to be the same as described in the current study.
I suggest adding a few more details to the current description of the sea spray tank. The authors write “jets” in plural – are there several or is it a single plunging jet? If there are several please describe this as not many tanks have several. What flow rate of water through the jet was used? What flow rate of particle filtered air was used? Was the tank temperature regulated?
P6L151: Please check that ‘Sellegri et al 2005’ is the correct reference based on previous possible mix-up.
P6L156: Specify how blank samples were made. Were they also placed in the stage for ~24hours?
P6L159: Samples were collected in November 2019. How long were they stored frozen before INP measurement?
P6L165: The volume of liquid added to the Eppendorf tubes is different for SW and SSA measurements (200µL and 400µL, respectively). The Vali equation takes into account the volume difference when calculating the concentration of INPs. This should be specified in the text, so that the reader understands that the to dataset can be compared regardless of the difference in volume.
P6L168: What does it mean the samples were “being agitated”?
P6L168: The process of filling half the tubes and heating is described twice in the same section. I suggest keeping the following text (start line 172) and rewriting the text starting in line 168.
“Half of the Eppendorf© tubes were then filled with the untreated sample. The seawater samples were then subjected to the same heat treatment as the filter samples, and the second half of the Eppendorf© tubes was filled using the heated samples (Fig. 2).”
P6L170: Comment on the heat treatment study by Daily et al. 2022, where they find that wet INP heat tests at (> 90°C) have the potential to produce false positives. Why did the authors select 100°C?
Daily, Martin I., et al. "An evaluation of the heat test for the ice-nucleating ability of minerals and biological material." Atmospheric Measurement Techniques 15.8 (2022): 2635-2665.
P7L174: What is the limit of detection?
It would also be nice with an idea of the experimental uncertainty e.g. as a number of degrees Celsius. This would give the reader a better understanding of how significant differences between samples freezing temperatures are.
P7L178: Show a figure of the blank sample INP results in the appendix.
State what the “lower temperature that can be reached” is.
P7L180: It is interesting that the heat treatment increased the IN activity. Add a comment on whether this has been seen in other studies (see comment above about paper by Daily et al. 2022).
Section 3.1: It is some really nice data on the seawater characteristics, and the text describes a comparison of the three waters as is viewed in Figure 2. However, I am missing some discussion/interpretation of the comparisons. E.g. What does it mean that the different planktonic microorganisms are in higher abundance in the LAU surface water? Is this expected?
P9L205: The difference in salinity and surface tension between the three waters seems very small (very close to the standard deviation stated). Why is this important to mention?
P9L223: Elaborate on this sentence “The MEL were oligotrophic waters, and WGY were ultraoligotrophic waters.”. How does this fit with the seawater characteristics in Figure 2.
P10L228: It is confusing for the reader to have just read a section where the water types are separated into categories: MEL, LAU, WGY, and now in this section the INP samples are separated into new categories: INP_sw, INP_tot, INP_HS, INP_HL.
It needs to be clearly stated in the start of this section (3.2.1) what the new categories mean and from which waters these INP samples come from. This should also be added to the methods section – a specification of where the INP water samples and air samples where taken (If at all stations, then state that). Also state earlier in the text of section 3.2.1 that the comparison of water types was not possible.
What is the authors definition of heat stable INP (e.g. a specific decrease in IN activity or)?
P15L313: Elaborate on the strong correlation with dissolved iron.
P16L344: I suggest simply writing INP/aerosol instead of shortening to INP/aer as this is likely to confuse readers. I do not believe that “Aer.” is a commonly used abbreviation for units. Same for Figure A3.
Figure 7. In text above figure n_s = surface site density, but in figure caption n_s = nucleation site density. Select one name for the symbol.
Several different labels are written in the caption referring to the same thing. Ns_super = Unheated SUPM?? Use the same label for both legend, caption and text describing the figures.
What do the error bars represent?
P17L357: Split sentence after parenthesis. Elaborate on Córdoba2025 comment, if possible, give n_s values.
P17L375: Elaborate on how this observation contrasts that of McCluskey2018b.
P19L404: It is not clear to me what this transfer function means. Please explain in further detail why the equation ns/INP_sw is used and explain the resulting number in words. In addition, it would be nice to add a reference to other work that has determined a transfer function in a similar way and compare numbers.
The authors could also determine a flux of INPs from water to air using the sea spray tank experimental conditions.
Technical comments
P1L17: Change “study” to “studied”.
P1L18: add “to” after “due”.
P1L22: Consistently use capitol letters. Change to “Particulate Organic Carbon (POC)”.
P2L43: Remove s from “drops”.
P2L44: Consistently use capitol letters. Change to “Cloud Condensation Nuclei (CCNs)”.
P2L50: warmer temperatures compared to what?
P2L59: Split sentence between “coastal sites,” and “and heat labile”. It is confusing that the sentence starts by introducing a specific study and then later mentions other studies.
P3L66: missing parenthesis after references.
P3L73: Remove “the” in front of “bulk seawater” and “SSA”.
P3L91: I suggest replacing “West-East-West with” with “West-East-West reaching”.
Figure 1: I suggest increasing the font size in this figure, especially the legend is difficult to read.
Figure 1: Add to either figure or the caption which stations correspond to MEL, LAU and WGY. This is very helpful for the reader.
P4L100: Change “than” to “to”.
P5L119: Exchange link with details about instrument. The link is just to the campaign website.
P5L120: Add details on the dynotester instrument, e.g. company name.
P6L159: Remove “on land.”
P6L162: Specify the cooling rate for the experiments.
P7L176: Remove “either” or add another type of normalization. This sentence does not make sense.
Figure 2. What is the black square? I assume average/mean? Add this to caption along with a note that the y-axis’ are different and some are in log-scale. This is important when reader is comparing values from the three sites.
P9L220: Add to text how many sampling points for LAU?
P10L235: Add to caption what type of seawater SSW corresponds to.
P13L264: Should read Figure 5?
P13L275: The legend in figure 5. The mean datapoints are squares in the figure but not in the legend for red and blue.
P13L275: Use MEL and LAU as written in text not new abbreviations (Mel. B.).
P13L279: Pearsons’s test – missing an r.
P18L389: Should read Fig. B3 ?
P20L431: Specify what temperature? E.g. sea surface temperature, nucleation/freezing temperature.