the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Sep 2026
Ideas and Perspectives: Value and efficiency of ocean sample return and laboratory analysis: Key analytes are orders of magnitude rarer than a needle in a haystack
Abstract. There is a pressing need to understand how the oceans are responding to human perturbations, yet their vastness poses a considerable challenge. The development and deployment of biologically and ecologically relevant in situ sensors are frequently invoked as a means to improve this understanding; however, there are still relatively few sensors for these parameters due to the analytical challenges involved in their measurement. Specifically, many analytes of interest are orders of magnitude rarer by mass than a needle in a haystack (e.g., dissolved iron and eDNA are 104 to 108 times rarer), and this scarcity creates challenges involving preconcentration, interfering elements or compounds, and analytical methods difficult to perform underwater. While sensor development is important, the role of sample collection and return is also an important, yet oft overlooked, complement for studying ocean change and is ripe for additional development. Laboratory analyses of returned samples can be high throughput for challenging analytes like micronutrients and omics parameters, and these techniques are operational now. Moreover, sample return allows parallel analyses and archiving through splitting of sample material. Similar to human health diagnostics, laboratory analyses can provide ocean health diagnostic capabilities showing how ecosystems are responding, complementing sensor-based vital sign measurements indicating environmental change. Together, these properties make sample collection and return a critical component for achieving the global ocean coverage needed to capture the environmental changes well underway today, and this approach should continue to be factored into ocean observing strategies.
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Status: open (until 30 Oct 2026)
- RC1: 'Comment on egusphere-2026-5382', Anonymous Referee #1, 19 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5382', Anonymous Referee #2, 21 Oct 2026
reply
General comments
The manuscript highlights the continued need to improve sample collection and preservation strategies for subsequent laboratory-based analyses. Although the development of robust in situ sensors remains a major goal for the oceanographic community, enabling the acquisition of large datasets for comprehensive ecosystem characterization, the authors effectively explain that many important biogeochemical variables cannot yet be adequately measured by sensors. This limitation is particularly relevant for compounds that occur at trace concentrations in seawater or are analytically challenging to quantify. It also emphasizes the advantages of sample-based analyses over sensor-based measurements, particularly the possibility of preserving samples for further examination and generating an analytical record that can be revisited as methodologies improve.
While the manuscript does not introduce fundamentally new concepts or technological developments, it provides a valuable perspective on the challenges associated with the in situ analysis of biogeochemical variables present at extremely low concentrations in the marine environment. The analogy comparing trace concentrations to finding a needle-in-a-haystack is an effective and accessible way to illustrate these analytical difficulties. Nevertheless, I consider that a broader discussion of existing analytical approaches, recent advances in shipboard and underway measurement systems, and the limitations associated with sample preservation would increase the significance of the goal of this manuscript and the message would reach a broader audience.
Specific comments
The manuscript appears to be strongly oriented toward the limitations of in situ analyses and sensor-based approaches. The authors should explicitly state in the conclusions that the discussion is primarily focused on trace metals, -omics, and organic compounds, as these are the principal examples used throughout the text. This clarification would help define the scope of the article and avoid overgeneralization.
The discussion of existing methods and sensor technologies is currently insufficient. Line 73 cites numerous studies but provides little information on the analytes, methodologies, and analytical performance addressed in those works. Consequently, readers are required to consult each reference individually to obtain information that should be summarized within the manuscript itself. A table compiling the principal analytes, methodologies, detection limits, advantages, and limitations of the different approaches would greatly strengthen the paper and provide a much more useful overview of the current state of the art.
In addition, the manuscript does not acknowledge the significant advances achieved in ship-based continuous analytical systems. Instruments running with input from seasoar or from underway seawater systems aboard research vessels, ferries, and other observational platforms are increasingly capable of generating high-frequency measurements while maintaining detection limits lower than those achieved by many in situ sensors. Examples include continuous analyses of macronutrients, as well as measurements of pCO₂, total alkalinity, and dissolved inorganic carbon (DIC). I suggest the authors to include these developments to provide a more balanced view of the current state of marine biogeochemical observations.
Another important aspect that deserves further consideration in the article is the challenge of sample preservation. The authors present laboratory analysis as the preferred alternative to in situ measurements, but the preservation and transport of samples to onshore laboratories can itself introduce significant sources of uncertainty. The magnitude of preservation-related artifacts is highly variable depending on the parameter being measured. In some cases, chemical or biological changes during storage may substantially alter sample composition, potentially compromising data quality. Consequently, onboard analyses may represent the alternative for certain variables. I would therefore encourage the authors to discuss the uncertainties associated with sample preservation and transport, including toxicity (e.g. by using HgCl2), where preservation effects can be substantial.
The manuscript could benefit from a more nuanced discussion of analytical requirements to avoid, again, overgeneralization. Depending on the objectives of a given study, exhaustive characterization of all chemical species or fractions may not be necessary. For example, detailed determination of particulate, colloidal, and dissolved iron fractions may not always be required to address specific research questions (line 171). In such cases, targeted sensor-based measurements or onboard analytical systems focused on the variable of interest may provide the most efficient and scientifically appropriate approach. This perspective would help contextualize the trade-offs between laboratory-based analyses and in situ or onboard measurement strategies.
Figure 1 appears somewhat biased in its current form, particularly regarding the representation of nutrient concentrations. The figure gives the impression that trace metals, organic compounds, and omics-related targets are uniquely challenging because of their low concentrations, whereas other analytes are comparatively less problematic. However, as acknowledged by the authors elsewhere in the manuscript, the determination of nutrient concentrations in oligotrophic surface waters using sensors remains analytically challenging due to their extremely low abundances. To provide a more balanced perspective, I suggest including nutrient concentrations representative of surface waters, or concentrations approaching current analytical detection limits, in addition to the deep-water values currently shown. For example, using a concentration of 0.1 µM would correspond to a "needle-in-a-haystack" analogy that is several times more challenging than the scenarios currently depicted (Supplementary Table S2).
Figure 2 appears to have limited relevance to the main narrative of the manuscript and does not seem essential for supporting the principal arguments developed in the text. I therefore suggest moving this figure to the Supplementary Material, where it can remain available for interested readers without disrupting the flow of the main article.
Citation: https://doi.org/10.5194/egusphere-2026-5382-RC2
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General comments:
The manuscript entitled, ‘Value and efficiency of ocean sample return and laboratory analysis: Key analytes are orders of magnitude rarer than a needle in a haystack” by Saito et al., presents the continued need for field sample collection and return as a critical component for achieving the global ocean coverage. Given ongoing changes in the ocean, these diagnostics are ultimately essential for validating and improving biogeochemical general circulation model performance and forecasting capacity. By using a mass to mass comparison based on finding a needle in a haystack, the authors make the importance of analytes analysis easy to understand. Overall, this is a very nicely reasoned and succinctly written perspective paper, and is worthy of prompt publication. Minor comments are summarized by line below.
Specific comments:
Line 26: To what do ‘these parameters’ refer? Please clarify.
Line 30: I assume this is also influenced by the need for consistent and stable measurements in the highly complex marine environment.
Line 38: Also in a high precision way.
Line 135: Please add a definition of ‘dissolved concentration’, e.g. <0.2 µm.
Line 136: Does this concentration refer only to surface waters or to the full depth mean? This should be clarified.
Line 175: Please add the relevant references.
Line 247: Change to ‘omics data’.
Line 379-383: I suggest that this is also because "high precision" is an abstract concept and error bars are not particularly exciting topics, as is likewise the case in other areas of space and planetary science.
Figure 1. Why the selected metals differ between surface and deep waters? How was this defined?
Figure 2. Please add the n value for each size class.
Supplementary: Table S1: Why the measurements differ among the needle groups?