the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Dec 2025
Biogenic and nonliving labile particulate iron in the subtropical North Pacific Ocean
Abstract. Biogenic and authigenic particulate iron (pFe) are key components of the marine iron cycle, influencing the fate of Fe in the upper ocean. However, their relative contributions to the total pFe pool are challenging to quantify. The chemical leach commonly used to operationally define ‘labile’ pFe is thought to extract both the biogenic and authigenic phases. To independently determine biogenic pFe, we conducted Fe uptake experiments in the surface mixed layer on 12 cruises in the subtropical North Pacific Ocean. Bulk Fe uptake rates varied ~2.5-fold throughout the year, increasing with increasing Prochlorococcus and picoeukaryotes abundances. We used particulate carbon and phosphorus as biomass estimates in combination with Fe:C uptake ratios, finding that both led to overestimations of the biogenic pFe pool (>200% of labile pFe in the surface mixed layer). Using the nucleotide adenosine-5’-triphosphate (ATP) as an alternate estimate of living biomass instead suggested that biogenic pFe comprised ~60% of labile pFe in the mixed layer. The remainder of the labile pFe pool, defined as ‘nonliving’, dominated the labile pFe pool below the euphotic zone, capturing contributions from detrital organic matter, authigenic minerals, and dust. A comparison of Fe phases between Station ALOHA and the subtropical North Atlantic revealed similar concentrations of dissolved Fe and biogenic pFe, but higher nonliving labile pFe concentrations in the North Atlantic, likely reflecting greater dust deposition. The greater role of biogenic pFe in the North Pacific may enable high efficiency Fe recycling, which rivals that observed in Fe-limited ecosystems.
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Status: open (until 11 Feb 2026)
- RC1: 'Comment on egusphere-2025-6068', Anonymous Referee #1, 14 Jan 2026 reply
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CC1: 'Comment on egusphere-2025-6068', Yang Xiang, 15 Jan 2026
reply
Comments to the authors:
Bates and Hawco present a well-written manuscript demonstrating the partition between different particulate iron (pFe) pools, especially biogenic and nonliving pFe, during seasonal cruises at Station ALOHA. Such work is of great significance given the increasing attention on the role of authigenic mineral phases in the overall surface Fe cycling. The authors have done a good job presenting and interpreting the seasonal variations of particulate Fe data. The use of ATP to estimate biogenic is novel and quite interesting. The manuscript is clear, thorough, and nearly free of typos. However, I do have some substantive comments and editorial remarks as listed below. Overall, I recommend publication with major revisions.
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Major interpretation points
I have two major comments.
Firstly, the authors made many assumptions in their estimations of biogenic pFe, many of which were not mentioned in the text. For example, the Fe:C uptake ratio used in Equations 3-5 does not necessarily equal the cellular Fe:C quotas (Fe:C) used in Sofen et al. (2023). The authors need to acknowledge this and be clear about their assumptions. Additionally, Sofen et al. (2023) used PPLabile in their calculations, while the authors used bulk PP in Equation 4. The authors should have the leachable PP data since they did the Berger leach. Al-Hashem et al. (2022; 10.1029/2022GB007453) found up to 60% of PP that cannot be accessed with the Berger leach, for example. Since the authors are comparing their results with Sofen’s, it’s important to make sure the calculations have been conducted similarly. Otherwise, those results may not be directly comparable.
Secondly, some of the calculations are flawed. The use of PC in Equation 3 to estimate biogenic Fe is likely an overestimation. While phytoplankton production (uptake of dissolved inorganic carbon) is the dominant source of new organic matter in the open ocean, the standing stock of POC (here the authors use PC since those are the only data available) includes significant contributions from other sources. What about heterotrophic and allochthonous sources, for example? Based on the authors’ definition of biogenic pFe, the authors should possibly use phytoplankton carbon rather than total POC in Equation 3. Please refer to Graff et al. (2015; 10.1016/j.dsr.2015.04.006) for the nuances, with the former values much smaller.
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Interpretation points, by line #
Lines 14-16: Do the authors have data or literature to show how much of a fraction is labile pFe in dust deposition at Station ALOHA? I assume that it's a relatively small fraction, right?
Lines 103-104: Are there any specific reasonings for using dFe at 25 m rather than the mean mixed layer concentrations? How different are dFe concentrations at 25 m and within the mixed layer?
Line 117: Are sinking particulate Fe data from sediment traps at Station ALOHA? I assume that data were also extracted from the HOT-DOGS. If so, please add the data source to section 2.3.
Lines 174-179: If carbon uptake rates (GPP) are underestimated by the 14C incubation experiments (values between GPP and NPP), the Fe:C uptake ratios will be overestimated. This could help decrease the pFeBio.
Lines 189-191: Assuming the rest of POC are phytoplankton carbon for simplicity, the actual pFeBio_PC will decrease by 58-74%, which will make pFeBio_PC much more similar to pFelabile and pFeBio_ATP.
Lines 191-193: The way the Fe:C ratio is calculated is more like an average ratio with respect to the bulk pool of particles though. I don't think a lower Fe:C stoichiometry in detritus, a specific pool of particles, makes any difference to the overall calculation.
Lines 194-198: The authors could possibly quantify the effects from variations in phytoplankton community composition, since the C:P of living cells should range between 109 and 195 at Station ALOHA. With the minimum C:Pphyto we have, the pFebio_pp is likely overestimated by up to 50%, which will result in values more similar to pFelabile, but not for all the data.
Lines 198-199: The exact reason will make equation 3 an overestimation of pFebio.
Line 242: If the authors name the remaining pool as nonliving, this implies what has been subtracted is "living". The pFebio is not necessarily pFeliving, right? Naming it as "non biogenic" is also not perfect, but it may be a better term. I agree that a more accurate term to name this pool of pFe is difficult…
Lines 247-258: The authors used a lot of text to explain how different their nonliving pFe is from the more common term used in the field, authigenic pFe. Their explanations rely on the fact that the operationally-defined Berger leach gets the pFe pool that is fully labile. What if the Berger leach also accesses some of the relatively reactive lithogenic pFe? Is that a possibility that could account for the differences and the similarity between pFenonliving and pFelitho? Please clarify.
Lines 250-251: If dFe adsorb onto or somehow get incorporated into nonliving organic particles, they likely exist as authigenic Fe minerals, right?
Lines 272-274: This conclusion is likely not to hold. What about adsorption, disaggregation, and redox?
Lines 291-293: This could also account for the similarity between pFeBio and pFelitho at Lines 272-274.
Lines 311-313: The summer-time dFe concentrations at BATS are much higher. I am not sure why such features were not discussed in the range of dFe.
Lines 316-320: Why not present the comparisons of labile pFe between BATS and Station ALOHA? It’s more meaningful than the derived parameters, which are prone to large uncertainties.
Lines 329-331: What if the authors conduct similar calculations as Sofen et al. (2023) by using their average Fe:C quota at Station ALOHA and labile PP, how will the values of biogenic and nonliving pFe change? Will this affect the conclusions? From what I can tell, Station ALOHA will possibly still have higher biogenic pFe within the mixed layer, but the values below 125 m will be potentially more comparable between these two sites.
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Editorial remarks (by line #):
Lines 138-142: Figure 1a- It seems like the data point from August 2022 is the only one without any error bars. Why is that?
Line 260: I assume that the pFeBio hereafter is pFeBio_ATP. The authors should consider either pointing it out or just using pFeBio_ATP to avoid confusion.
Line 275: Figure 3a- How variable are the labile and lithogenic pFe? It may be helpful to plot the standard deviation at each depth with error bars.
Lines 282-284: The authors should consider plotting this data point on Figure 4c. Additionally, to exclude a data point as an outlier, a statistical method will be preferred.
Line 285: Figure 4b- Is this data point at (0,0) real or of bad quality? I can only see one bar with nonliving pFe as 0 in Figure 4a. When is the other zero nonliving pFe observed?
Line 285- Figure 4d- Is this correlation significant?
Lines 316: Figure 5g-i: The way biogenic pFe was calculated is different between this study (ATP method) and Sofen's method. The authors should consider labeling such differences clearly in the legend or captions.
Lines 355-357: For some reason, I cannot open any of these links. However, I can find the data page on BCO-DMO. It's weird, but the authors should make sure that all of these links work during revision.
Citation: https://doi.org/10.5194/egusphere-2025-6068-CC1
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- 1
Review of egusphere-2025-6068 “Biogenic and nonliving labile particulate iron in the subtropical North Pacific Ocean by Bates and Hawco.
Summary
This manuscript explores the composition and dynamics of labile particulate iron (pFe) in the subtropical North Pacific, focusing on the split between biogenic and nonliving fractions. To achieve this, the authors undertook iron (Fe) uptake experiments (using the Fe isotope double spike method) and carbon (C) uptake to estimate how much Fe and C are incorporated into cells. They then use Fe:C uptake ratios in combination with particulate organic carbon (POC), particulate organic phosphorus (PP) and ATPase to estimate biogenic pFe. To estimate labile iron, Bates and Hawco used the chemical leach method of Berger et al. (2008) to estimate easily mobilised Fe from biogenic and non-living material. Finally, they connect the two and determine that biogenic Fe accounts for approximately 60% of labile Fe in the mixed layer, with the rest being associated with nonliving matter.
Overall, the manuscript is generally well written. One bugbear is that the manuscript keeps directing the reader to other papers for DFe and associated data (see examples below). I realise that the data has been published, but it would be useful to include plots in the supporting materials; otherwise, the reader has to sift through articles to check the claims.
Specific comments
Line 96: Are you saying that at the end of the incubation, there was ~a 50:50 split of the FeDS between the dissolved and particulate phases? Or is this total - probably the total as you added ~ 50 pM.
Line 119-120: Where is the evidence to support this? Please reference a figure or table here - as a reader, I really don't want to have to search through other references for the data. It’s your data (Bates and Hawco, 2025), so this should be easy to generate. Perhaps you could add an extra couple of panels to Fig 1 showing the iron data or add a new figure. Or you could add a figure to the supplementary information showing the DFe data and reference it came from Bates and Hawco, 2025.
Line 122-123: Again, please don't make me read other papers to see the primary data you are referring to - show it here and then reference where it came from.
Line 129: What about Synechococcus? was that measured? It can also be an important player in tropical and subtropical waters. Certainly, it is often found at shallower depths than Prochlorococcus (Flombaum et al., 2013).
Line 134: Table S1 only show correlation data; perhaps you could show the population data for Prochlorococcus, Synechococcus, and picoeukaryotes at 25 m. That will allow the reader to check the data and the points about abundance made in the text.
Line 134: How about presenting the 14C data. It would be nice to see how it varied temporally. We only have the Fe:C ratio data.
Figure 1. Because you say that Prochlorococcus and picoeukaryotes dominate, is it possible to normalise the iron uptake to cell number to get an idea of uptake per cell? As you show in panels c-e, the strong coupling between uptake and Prochlorococcus and picoeukaryotes abundance indicates that it is driven by cell abundance, which is likely to vary seasonally. Based on the comments about the 14C data on lines 133 to 134, I assume that primary production (14C) and cell abundance are not coupled? It might be worth showing this as well.
Line 175: The assumption here is that the Berger method is getting all of the biogenic Fe - did you check the Fe/Al ratio for the labile and total pools to see if they jive with each other? Also, perhaps it should be mentioned that the Berger method was designed to look at labile iron from the Columbia River plume and coastal waters off the West Coast of the US. The values in that study were in the high nanomolar range for iron, whereas concentrations in the present work are subnanomolar. Since most of the iron in the present work is likely within organic molecules, dead and alive, it is possible that the Berger leach does not access this as molecules may need to be oxidised (noting the Berger leach is reducing) to break them down before iron can be accessed. Just a thought.
Figure 3, panel b. The unit nM needs to be removed as this is a fraction calculation.
Figure 4 caption. “…authigenic (navy)..” is mentioned, but the figure key is “Labile nonliving pFe”.
Line 303 – I like this, it's always good to compare to other regions.
References
Bates, E.S., Hawco, N.J., 2025. Dissolved Iron Seasonal Cycle and Residence Time in the North Pacific Subtropical Gyre. Geophysical Research Letters, 52(21): e2025GL118095.
Berger, C.J.M., Lippiatt, S.M., Lawrence, M.G., Bruland, K.W., 2008. Application of a chemical leach technique for estimating labile particulate aluminum, iron, and manganese in the Columbia River plume and coastal waters off Oregon and Washington. Journal of Geophysical Research-Oceans, 113.
Flombaum, P. et al., 2013. Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus. Proceedings of the National Academy of Sciences, 110(24): 9824-9829.