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.

Review of egusphere-2025-6068, “Biogenic and nonliving labile particulate iron in the subtropical North Pacific Ocean“ (Bates and Hawco)

(General Comments)

Estimation of biogenic and authigenic labile particulate Fe is important but challenging work. In previous studies, biogenic Fe fraction can be estimated based on Fe quotas and particulate organic carbon or phosphorus but the last two analytical fractions (C and N) contain organic detritus (e.g., dead cells, fecal pellets). According to the present authors’ idea on “biogenic” Fe (see below on terminology “biogenic”), the above approach can lead to overestimation of biogenic Fe. Instead, the authors introduce a new approach using particulate nucleotide adenosine-5’-triphosphate (ATP) to estimate “living organic carbon” to derive biogenic Fe. Labile particulate Fe minus this biogenic Fe was defined as “nonliving”, which was main labile pFe pool below the euphotic zone. These attempts are really fascinating. The authors also conducted Fe uptake experiments in the surface mixed layer on 12 cruises at Station ALOHA in the subtropical North Pacific for 3-year period coverage. The bulk Fe uptake rates increased with increasing Prochlorococcus and picoeukaryotes abundances. These data are important to understand the Fe availability for primary production in the subtropical North Pacific. Dataset is of good quality, mostly discussion is orderly, and the manuscript provides new findings and approach. Therefore, I highly evaluate these works and think the manuscript deserves to be published.

However, there are several points of concern including the terminology in this study. In this manuscript, the authors defined their “biogenic” Fe using particulate ATP as “living” because of the rapid hydrolysis of ATP in dead biomass. However, in previous studies (e.g. Sofen et al., 2023; Tagliabue et al., 2023), the “biogenic” particles reflect “biological uptake and accumulation in living biomass and differential remineralization from “dead biomass” (Sofen et al., 2023). I’m not so sure which definition is widely accepted, but I prefer to include dead biomass in the “biogenic fraction”. For example, I think Fe in freshly produced fecal pellets is “biogenic”. In the ocean, the term “biotic Fe” has sometime been used (e.g. Boyd et al., 2015), which might be better for this case (“living” biotic Fe). Thus, the authors should discuss the definition of “biogenic” and clarify the difference from the term in previous studies, like Sofen et al. (2023). Including the treatment of the term “biogenic”, several minor comments/questions to be addressed are given below:

1. Page 2, Line 36, “biogenic pFe pool”: Actually, the term “biotic” was used in Boyd et al. (2015). Please make sure of it.
2. Page 3, Line 64, “the biogenic and nonliving labile pFe pools”: Previous studies (Sofen et al., 2023) divided labile pFe to biogenic and authigenic fractions to understand the marine Fe cycles. Considering the “authigenic” fraction, I understand that the biogenic fraction includes fresh organic detrital materials. Why do the authors newly define “the living and nonliving labile pFe pools” in this study? Please clarify the reason here.
3. Page 5, Line 126 – 127, “uptake rates were similar in May for both 2021 and 2023”: How about cell counts of Prochlorococcus and picoeukaryote?
4. Page 5, Line 131 – 132, “Prochlorococcus and picoeukaryotes together make up the bulk of picophytoplankton biomass and primary production at Station ALOHA (Rii et al., 2016).”: According to Bates et al. (2015), particulate biogenic Si inventories in the upper 150 m showed marked increase in spring 2021. Did this reflect the diatom bloom? If so, was any feature found in Fe uptake ratio?
5. Page 8, Line 174 – 175, “The PC and PP approaches consistently overestimated pFe_Bio, which almost always exceeded measurements of pFe_Labile”: What will happen if the Redfield ratio was applied for this calculation? I’m not sure whether “biogenic Fe” should include organic detrital material or not. If the organic detrital material were included, the authors should consider wider ranges of the C:P ratio.
6. Page 9, Line 198, can incorporating heterotrophic bacteria lead to increase or decrease C:P ratio or negligible contribution? Although relevant data is given as spreadsheet, please clarify this.
7. Pages 7–9, Since several previous studies also estimate biogenic Fe including detrital organic matter (e.g., Sofen et al., 2023) and the present authors’ estimate is based on living organic carbon, so I think the use or definition of new terminology is necessary; e.g., “living biogenic Fe/ living biotic Fe” may be better than “biogenic Fe”. I think the term “biogenic” can include not only living but also dead material and fecal pellets etc. In my opinion, “biogenic Fe” should not be confined to what is obtained by the use of ATP.
8. Page 9, Line 217 – 219, “Recent work based on shipboard experiments and Prochlorococcus measurements supports the use of a 250 C:ATP ratio at Station ALOHA (Karl et al., 2022), in agreement with a variety of other field and laboratory studies of marine organisms”: In Howco et al. (2022), the contributions of both Prochlorococcus and heterotrophic bacteria were considered for Fe’ uptake in seawater. However, the C:ATP ratio in heterotrophic bacteria was not discussed here. The authors should mention the uncertainty of C:ATP ratio in heterotrophic bacteria.
9. Page 10, Line 240, “Overall, the ATP approach provides the most reasonable pFe_Bio estimates of the three approaches assessed.”: I’m not sure whether some biogenic fraction including fresh organic detrital materials should be categorized to “nonliving labile pFe” or not. Please explain the merit to consider fresh organic detrital materials and authigenic fraction together as “nonliving labile pFe”.
10. Page 11, Line 262, “euphotic zone”: Please indicate the approximate depths of euphotic zone.
11. Page 11, Line 272, “majority nonliving below the DCM”: Isn’t the heterotrophic bacteria included in this fraction?
12. Page 15, Line 324, Given comparable values of particulate biogenic Fe ranging 0.03–0.10 nM for ALOHA and 0.03–0.08 nM for BATS, and overestimates by 2023Sofen’s method including detrital organic materials, this leads to higher biogenic Fe at ALOHA than BATS. Is it possible to give a likely explanation for the higher biogenic Fe at ALOHA since this is an intriguing feature?
13. In Figure 5 and the caption, and LINE 325, “HOT” had better be replaced by “ALOHA” as I believe the former is the program/cruise name and the latter is the station name.
14. Page 15, Line 330, the description seems to state “overestimation comes from synchrotron X-ray fluorescence”. I believe it is ascribed to the use of organic P, not to X-ray fluorescence.