the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Dec 2024
Nitrogen Fixation in Arctic Coastal Waters (Qeqertarsuaq, West Greenland): Influence of Glacial Melt on Diazotrophs, Nutrient Availability, and Seasonal Blooms
Abstract. The Arctic Ocean is undergoing rapid transformation due to climate change, with decreasing sea ice contributing to a predicted increase in primary productivity. A critical factor determining future productivity in this region is the availability of nitrogen, a key nutrient that often limits biological growth in Arctic waters. The fixation of dinitrogen (N2) gas, a biological process mediated by diazotrophs, not only supplies new nitrogen to the ecosystem but also plays a central role in shaping the biological productivity of the Arctic. Historically it was believed to be limited to oligotrophic tropical and subtropical oceans, Arctic N2 fixation has only garnered significant attention over the past decade, leaving a gap in our understanding of its magnitude, the diazotrophic community, and potential environmental drivers. In this study, we investigated N2 fixation rates and the diazotrophic community in Arctic coastal waters, using a combination of isotope labeling, genetic analyses and biogeochemical profiling, in order to explore its response to glacial meltwater, nutrient availability and its impact on primary productivity. Here we show, N2 fixation rates, ranging from 0.16 to 2.71 nmol N L-1 d-1, to be notably higher than those observed in many other oceanic regions, suggesting a previously unrecognized significance of N2 fixation in these high-latitude waters. The diazotrophic community is predominantly composed of UCYN-A. We found highest N2 fixation rates co-occurring with maximum chlorophyll a concentrations and primary production rates at a station in the Vaigat Strait close impacted by glacier meltwater inflow, possibly providing otherwise limiting nutrients. Our findings illustrate the importance of N2 fixation in an environment previously not considered important for this process and provide insights into its response to the projected melting of the polar ice cover.
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Status: open (until 24 Jan 2025)
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RC1: 'Comment on egusphere-2024-3680', Anonymous Referee #1, 20 Dec 2024
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This study investigated diazotrophy in the coastal area of Baffin Bay in the Arctic Ocean. To my knowledge, three previous studies have already explored diazotrophy in Baffin Bay (Farnelid et al., 2011; Blais et al., 2012; Robicheau et al., 2023, FEMS Microbiology Ecology). Compared to these earlier studies, the novelty of this research is unclear. Additionally, the manuscript appears to be at a very immature stage, and I identified several issues with the study's approach.
First, I believe the authors have a fundamental misunderstanding about nitrogen fixation in the Arctic. The Arctic Ocean is unique in that it has nutrient-poor water masses despite being located at high latitudes. However, unlike the permanently nutrient-depleted conditions in subtropical oligotrophic oceans, these nutrient-poor water masses in the Arctic are not permanent. Due to mixing driven by low temperatures, nutrients from deeper layers are readily supplied to the surface. Therefore, the authors' claim that “The fixation of dinitrogen… plays a central role in shaping the biological productivity” is incorrect. This misunderstanding permeates the entire manuscript, leading me to disagree with its overall content and conclusions.
I also have concerns regarding the authors' approach to addressing their research objectives. Nitrogen fixation activity was measured at only three out of ten stations, which the authors describe as "strategically chosen." However, is this claim valid? How did the authors determine in advance that these stations represented different water masses before conducting observations?
There is also an issue with the method used to measure nitrogen fixation. Seawater samples were collected from three depths—0, 25, and 50 m. The light environments at these depths are likely to differ significantly. Did the authors assess the light environment in the water column? How did they simulate the in situ light conditions (L117-118) without investigating the actual light environment?
Furthermore, the rationale for conducting metagenomic analysis is unclear. This study focuses on nitrogen fixation, so why did the authors not employ an approach specifically targeting the nifH gene? The authors state, “Additional molecular approaches would be necessary to enhance our understanding and show a more detailed picture of the diazotrophic community.” If they acknowledge this, why did they not include such approaches in their study? Targeting the nifH gene would have been more straightforward and relevant than performing metagenomic analysis.
Furthermore, the structure of the manuscript presents an issue. The authors have combined the Results and Discussion sections, which hinders the clarity and comprehension of the content. As detailed below, many of the interpretations of the results are unconvincing and lack sufficient support.
Specific
L43-50 I find it unclear how the authors have linked environmental changes in the Arctic to nitrogen fixation.
L56-58 While it is true that primary production in the Arctic Ocean is often limited by nitrogen availability, the contribution of nitrogen fixation to new production is typically minimal.
L60 To effectively investigate diazotroph diversity, the authors should focus on targeting the nifH gene..
L75-77 Should show the detection limits.
L91-94 It is unclear why filtration was necessary for collecting samples for DIC. Additionally, DIC is not mentioned or analyzed in the results.
L99-101 This part should be included at the beginning of "Seawater sampling."
L102-120 As I wrote above, it is not possible to conduct incubation simulating the light environment without first measuring the actual light conditions.
L134-154 It is unclear why the authors chose to conduct metagenomic analysis.
L157-165 The content written here is not the result. Please show the location of the West Greenland Current and Jakobshavn Isbrae on the map.
L168-169 It is unclear how the authors strategically chosed the stations.
L170-171 What can be said to indicate that there was an influence from the freshwater input?
L172- It is unclear what the author is attempting to convey or the basis on which their statements are made.
L180-185 To my eyes, Station 10 also appears to be stratified near the surface.
L188- There seems to be a lack of objective evidence provided here. What were the characteristics of upwelling?
L203 I think this title is incorrect.
L221 “relatively high N2 fixation rates observed may play a role in bloom dynamics”
I do not agree with this description.
L222-229 I don't understand what the authors are saying.
L235-244 I don't understand what the authors are discussing.
L253-255 This is a story about the subtropics. I don't think the Arctic is the same.
L260-300 This chapter relies too heavily on arbitrary interpretations, making it difficult to follow the argument. While it is indeed interesting that the nif gene of UCYN-A was detected in the metagenome analysis, it is unclear why psbA and rbcL are discussed in relation to nitrogen fixation. These genes are not directly associated with nitrogen fixation processes. Furthermore, the statement at L264-265 is evidently incorrect and requires revision or clarification.
L301-330 The authors state that there was no δ15N signal of nitrogen fixation in the particulate organic carbon (POC). This indicates that the contribution of nitrogen fixation to new production was minimal. Consequently, this finding contradicts the authors' claim of a significant contribution of nitrogen fixation to primary production.
L332-367 The conclusion is overly detailed. The key findings of this study can be summarized in 2-3 sentences, focusing on the main highlights. Additionally, the implications of the results and potential future research directions should be stated concisely.
Citation: https://doi.org/10.5194/egusphere-2024-3680-RC1 -
RC2: 'Comment on egusphere-2024-3680', Anonymous Referee #2, 08 Jan 2025
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General Comments
The authors present an interesting study on nitrogen fixation related processes in the coastal waters of Qeqertarsuaq, West Greenland. Included in this study are new nitrogen fixation rate measurements and also observations that UCYN-A dominated diazotrophic-related gene signatures within metagenomic samples obtained for this region. The latter is particularly interesting given that diazotrophs are typically hard to detect in metagenomic samples that do not specifically target diazotrophs, further highlighting this organism’s importance in the Arctic region studied. The authors have presented their findings in light of other primary productivity dynamics at play in the region, providing further context for microbiological processes occurring in the region in relation to nitrogen and carbon cycling. As the authors point out, nitrogen fixation rates and diazotroph biodiversity data are greatly needed for the Arctic and this study meaningfully contributes to this broader ocean research goal.
Specific Comments
Figure 2: The authors should strongly consider also plotting PAR and Fluorescence that would have been obtained from the CTD sensors in Fig 2. Fluorescence would be especially useful to plot given the high chlorophyll value obtained for station 7 around 25 m depth.
Lines 209-210: The authors should also draw comparisons to nitrogen fixation rates given in Sipler et al. (2017), especially since some of their nitrogen fixation rates for coastal Alaskan Arctic are higher than those given by the authors. I.e., the authors should also report their rates in relation to overall maximum Arctic rates observed thus far in the literature.
Line 247: In figure 3 (left panel) there are PON values plotted near PON = 0 uM and POC = 0.5 uM, meanwhile, PON range is reported in the main text as 0.5 - 4.0 uM. This discrepancy should be addressed.
Line 251: The authors should provide further information and/or citations for inferring the DCM between 15-30m given that they state that they identified it between such depths. Perhaps this was inferred from CTD fluorescence data?
Lines 254-255: The discussion point on nitrogen fixation rates correlating to chl a concentration is interesting, however, the authors should also for greater context point out that their nitrogen fixation rates versus chl a concentration correlation was not significant overall (see Fig A2 & A3). As the authors go on to explain, Station 7 at 25m was a more unique case.
Lines 256-257: It is difficult to orient the images in Figure A1 to the maps in Fig 1. It would be useful if the sampling station(s) were labelled in Figure A1 and/or if an inset diagram was provided similar to what was given by the first two panels in Figure 1.
Line 262: Although I agree that the UCYN-A was detected mainly at 0-5m depths, the authors should also provide some hypotheses or context as to why relatively high UCYN-A nifK values were detected at S3_100m. Have UCYN-A been detected below the euphotic zone in the Arctic previously?
Line 261: The claim that UCYN-A dominated the sequence pool of diazotrophs requires further context and data presentation before readers can fully assess the conclusion given. For instance, was the UCYN-A signature >50% of each of the nif genes? Were there many other nif detected (i.e., what was the proportion of UCYN-A vs. other diazotrophs)?
Line 261: In regard to the broader statement ‘sequences related to UCYN-A’, it is known that there are many ecotypes of UCYN-A which can co-exist in coastal regions (e.g., Robicheau et al. (2023a) showed this in the not-too-distant coastal Northwest Atlantic). Overall, it would be very useful to UCYN-A researchers if the authors were to give some sort of assessment for which ecotype(s) they have observed at their study sites.
Lines 260-265: It is also interesting that UCYN-A dominated the metagenomic nif community given that Robicheau et al. (2023b) showed very low nifH copy numbers in the nearby Baffin Bay, further underscoring this organism’s presence in Arctic surface coastal areas under certain environmental conditions.
Lines 266-267: Regarding the status of UCYN-A as a Nitroplast, also relevant is the cell size ratio between the nitroplast and the total algal cell as outlined in Cornejo-Castillo et al. (2024).
Lines 273-276: The authors should clarify if any other diazotroph sequences were found. At present the results provide a comprehensive assessment of UCYN-A but give little information about any other diazotrophs that may have been detected in the metagenomes.
287-290: Regarding the high Bacillariophyta values and diatom-diazotroph symbiosis concepts, although this is an interesting hypothesis, if the blooming diatom had such a large psbA and rbcL signature would it not also logically be expected to have a relatively high nif signature if each individual diatom cell within the bloom was indeed involved in symbiosis with a diazotroph? The authors should clarify if the general principal of diazotrophs being poorly sequenced in metagenomes is enough to completely overshadow a strong diatom-diazotroph bloom. It would also likely be informative if the authors provided some sort of assessment for the identity of the diatom beyond the ‘Bacillariophyta’ taxonomy name given. For instance, if the genus can be identified, is the group known to contain diatom-diazotroph symbiotic species? Also, the authors should give more information about all the nifH, nifK, and nifD genes identified (not just for UCYN-A) for S7_25m. It would also be relevant to examine the results in Pierella Karlusich et al. (2021) who showed diazotroph metagenomics data for the Arctic Ocean as part of their global analysis.
Section 3.3. title: Without further information regarding the entire diazotroph community derived from metagenomics it is hard to infer that UCYN-A might contribute to the nitrogen fixation during the diatom bloom observed. The authors themselves also suggest that maybe a diatom-diazotroph symbiosis might be at play, therefore it seems that this title is a bit misleading with respect to the limitations of the results.
Technical Corrections
Line 35: The text “in those waters” is too imprecise.
Line 139: Change “x” letter to a multiplication symbol “×”.
Line 151: Change “, nifH” to “and nifH”.
Line 151-153: Authors should include references for NCBI accession codes cited.
Line 231: Typographical error; change “3indicates” to “indicates”.
Line 355: Typographical error; change “?” to a citation or delete question mark.
References cited in Review:
Sipler, R. E., et al. Preliminary estimates of the contribution of Arctic nitrogen fixation to the global nitrogen budget, Limnology and Oceanography Letters, 2, 159–166, 2017.
Robicheau, B. M., et al. Microevolutionary patterns in ecotypes of the symbiotic cyanobacterium UCYN-A revealed from a Northwest Atlantic coastal time series, Science Advances, 9, 2023a.
Robicheau, B. M., et al. Marine nitrogen-fixers in the Canadian Arctic Gateway are dominated by biogeographically distinct noncyanobacterial communities, FEMS Microbiology Ecology, 99, 2023b.
Cornejo-Castillo, F.M. et al. Metabolic trade-offs constrain the cell size ratio in a nitrogen-fixing symbiosis, Cell, 187, 2024.
Pierella Karlusich, J.J. et al. Global distribution patterns of marine nitrogen-fixers by imaging and molecular methods, Nature Communications, 12, 2021.
Citation: https://doi.org/10.5194/egusphere-2024-3680-RC2 -
RC3: 'Comment on egusphere-2024-3680', Anonymous Referee #3, 10 Jan 2025
reply
The authors present new data on nitrogen fixation, primary production, and diazotrophic communities in Arctic coastal waters. Their major finding—that the highest N2fixation rates coincide with maximum chlorophyll-a concentrations and primary production rates at a station influenced by glacier meltwater inflow—is intriguing. Below are my suggestions for further improvement.
Major Comments:
- Introduction:
The introduction lacks clarity, especially given the numerous recent studies in the Arctic. What are the specific research gaps in this region? The objectives of this study should be explicitly defined in the context of these gaps. - Data Analysis:
The statistical analysis requires more robustness. Considering the low rates reported, the authors should calculate the minimum quantifiable rates for N2 fixation using standard error propagation methods based on observed variability between replicates, as demonstrated by Gradoville et al. (2017) and the studies afterwards. - Nutrient Ratios Discussion:
The discussion on nutrient ratios is misleading, as most of the NOx and phosphate data appear to be below detection limits. The N:P ratios of 0/0 should be labeled as "undetermined" rather than zero in the table. Regardless, it is recommended to avoid calculating or discussing N:P ratios under these circumstances. - Section 3.4 "δ15N signatures in particulate organic nitrogen (PON) does not provide clear evidence of nitrogen fixation" Since N2 fixation rates have already been measured, this section seems redundant. Consider repurposing the δ15N data for other relevant analyses if needed.
Specific Comments:
Line 205: How were the detection limits defined? Please clarify the method.
Line 277: “The contribution of N2 fixation to carbon fixation (as percent of PP) is relatively low, but may increase with a further onset of bloom periods.” Such sentences throughout the manuscript are pure conjectures and have not substantiated to be convicting. Either remove these or substantiate them.
Line 236: Replace N ratio with N:P ratio.
Line 231: “3”, typo?
Line 231: How does the Redfield or non-Redfield ratio indicate that the particulate organic matter (POM) is freshly derived from an ongoing bloom? Provide a more detailed explanation.
Line 355: Replace “?”
Figure 3(a): The red line representing the Redfield slope appears to have a higher slope than described. Ensure the figure and text are consistent.
References:
Gradoville, M., Bombar, D., Crump, B., Letelier, R., Zehr, J., & White, A. (2017). Diversity and activity of nitrogen‐fixing communities across ocean basins. Limnology and Oceanography, 62, 1895–1909. https://doi.org/10.1002/lno.10542
Citation: https://doi.org/10.5194/egusphere-2024-3680-RC3 - Introduction:
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