Reproducible determination of dissolved organic matter photosensitivity

Armstrong, Alec W.; Powers, Leanne; Gonsior, Michael

doi:https://doi.org/10.5194/bg-18-3367-2021

Articles | Volume 18, issue 11

https://doi.org/10.5194/bg-18-3367-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-18-3367-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 11

Research article

|

07 Jun 2021

Research article |

| 07 Jun 2021

Reproducible determination of dissolved organic matter photosensitivity

Alec W. Armstrong, Leanne Powers, and Michael Gonsior

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Final revised paper (published on 07 Jun 2021)
Preprint (discussion started on 02 Jul 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Comments', Patrick Neale, 30 Jul 2020
- AC1: 'Reply to comments', Alec Armstrong, 01 Oct 2020
RC2: 'Review: “Reproducible determination of dissolved organic matter photosensitivity” by Alec W. Armstrong et al', Anonymous Referee #2, 10 Sep 2020
- AC2: 'Reply to comments', Alec Armstrong, 01 Oct 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (08 Oct 2020) by Clare Woulds

AR by Alec Armstrong on behalf of the Authors (12 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Nov 2020) by Clare Woulds

RR by Andrew Wozniak (10 Dec 2020)

Suggestions for revision or reasons for rejection

In “Reproducible determination of dissolved organic matter photosensitivity”, Armstrong et al. present a new methodology for obtaining reproducible results for photodegradation experiments of DOM SPE extracts. In the end, I believe that this paper is likely publishable, but I am recommending further revision. At its heart, this paper is a methods paper, and it is unclear to me whether the method is superior to less arduous methodologies. Comparisons of this technique to alternative approaches that clearly show why this approach should/could be adapted or clearer demonstrations of how inferences cannot be obtained without this technique are lacking or not (yet) convincingly explained. Below, I elaborate in more detail.
The authors have presented a technique for collecting a “reproducible and high resolution time series of fluorescent measurements during experimental irradiation of a single sample.” By requiring optically-thin conditions, a solid phase extraction step, and setting pH at 3, the technique removes inorganic constituents that can affect photochemistry and fluorescence spectra and dilutes the sample to avoid photochemical self-shading effects. In Fig. 6, the authors show that the SPE DOM and whole water samples differed in apparent composition. I would have liked to have seen more discussion for how this would affect ecological inferences. Does the apparent enhanced reproducibility warrant removal of the sample matrix (and loss of environmental conditions/relevance)? Also, did the authors compare the various kinetic parameterizations for the 2 user comparison in Fig. 5? Is the SRNOM PPL data in Fig. 6 the same as presented in Fig. 5? If no, how do the authors explain the variability in Fig. 5 (and 6) and is the affect of the different users on the modeling greater than that of the sample prep?
The sample manipulations (PPL, pH=3, etc.) also have the effect of removing the sample matrix which the authors suggest they can exploit to isolate the DOM. This, of course, removes environmental conditions, but the authors argue they can exploit this to measure potential DOM photodegradation or DOM “photosensitivity”. By monitoring fluorescence spectra from photochemical time-series incubations, the authors can model the loss of FDOM components into labile and semi-labile components. In section 3.3.2, the authors present a (credible) argument using 2 wetland samples for how the kinetics of loss of these labile and semi-labile components can be used to inform FDOM composition, source, and previous light exposure histories. It’s not clear to me whether these conclusions require the approach used here. I would have liked to have seen a comparison to the alternative (some combination of fewer datapoints, no manipulation of pH, whole water, etc. that is currently being used in the literature) to demonstrate the superiority of this method or how using an alternate method gives misleading information. (Alternatively, I missed the explanation, and the manuscript would benefit from a clearer statement of methodological benefits.) They then suggest a series of speculative hypotheticals for how photosensitivity differences might have implications for other ecosystem processes. These remain hypotheticals at this stage, but if the method is demonstrated to be superior/necessary, answers to many of the questions that are posed would be quite valuable.
To their credit, the authors acknowledge that their approach represents only “one step in the direction of disentangling” the role of photodegradation of DOM in surface water ecosystems. I agree. DOM photodegradation is a complex problem that requires many approaches. This may be an invaluable approach, and I am curious to hear the authors’ thoughts on my concerns.
Further comments:
- In the Introduction, line 70, the authors state that the goal is “to compare the photosensitivity of DOM sources….” But this is misleading. That may be the ultimate goal, but it is not a major focus in this paper which seems to be a proof of concept for the methodology – as noted by the title of the manuscript. I recommend revising the stated goal to reflect the actual work.
- I found the storage comparisons to be useful but distracting from the main body of the manuscript. I recommend they be moved to supplemental information.
- The term DOM photosensitivity adds to the already confusing dictionary of photochemical jargon. -why not use DOM potential photodegradation which is the definition provided by the authors for photosensitivity?
- In the paper and in the responses to one of the reviewers, the authors note that their approach may not be suitable for every situation and that researchers will have to make that decision for themselves. But ultimately, the authors are writing this paper because they hope the method will be adopted in the future. I suggest the authors make specific recommendations for when using this approach makes sense (and maybe suggest when it does not).
- “optically thin” is mentioned in line 135 but not defined until line 315. Move its definition to first mention.
- line 172, are there any effects of adding Cl with HCl? Removing halides was mentioned as a benefit of the PPL step.
-line 290 – it would be beneficial to state what the two types of reactions are.
- In Fig. 4, the results of the stir/no stir comparison are not discussed. Stirring does not appear to have much effect on the PARAFAC results (less effect than is seen for Fig. 5), is that correct?
- In Fig. 8 caption, what are DF, FN, and QB?
- nitrite actinometry is an important recommendation from this study but is not described in detail in the methods. Please add these details so researchers can replicate your work.
- line 439, a sentence was started but never finished.

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RR by Anonymous Referee #4 (01 Feb 2021)

Suggestions for revision or reasons for rejection

The authors present a method how to continuously measure photodegradation of DOM with a high reproducibility. By applying their method to variety of different environmental DOM sources they were able to show differences in photosensitivity between these sources.
I only have some minor points that I would like to have clarification on.

Line 117: Can the authors be more specific what “longer storage“ means? In the cited Murphy et al. (2018) study I could not find anything on the effect storage has on bulk DOM vs PPL extracted DOM. Can they provide an estimate how long bulk DOM can be stored (I assume frozen) for fluorescence analysis as opposed to PPL extracts?

Line 119: If the samples are frozen, microbial activity should not play role. How do the authors store the samples prior to analysis?

Line 146: It would be nice to see a picture of this set-up or some schematic drawing. This information could be provided in the suppl. material.

Line 166: The authors explained in the response to the reviewers why this pH was chosen. Nevertheless, I would appreciate a comment here how a pH of 3 corresponds to the expected pH of the samples that were analyzed in this study.

Fig. 1 I would mention in the figure caption that solid lines represent emission spectra and dotted lines excitation spectra.

Line 258 onwards: Why aren’t the components 3 and 4 referred to respective F520 and F450 from here on in the manuscript? This would make it easier to compare to the previous Murphy et al. (2018) study.

I find the structure of the Results and discussion a bit confusing. I would have expected the authors to first start with the method evaluation and comparison, i.e. comparing bulk DOM and PPL extracts, etc. instead of directly starting with the photodegradation experiments. Before the results of the experiments can be discussed I would first want to see that the method is robust. Also, from the wide number of experimental set-ups described in the methods, it was hard for me to directly understand which experiment the authors are discussing first. The results and discussion would benefit from a more structured presentation. If they want to keep the current order an introductory paragraph explaining what will be described in the following sections would be helpful.

Line 281: why is Fig. 10 cited here, when next in line of figures should be Fig. 3?

Section 3.1.4: Can the authors refer to how their results relate to other studies that compare PPL and bulk DOM extracts?

Fig. 7: Please define what is meant with SL (= semi labile) and L (= labile), it is hard to find this information in the text and it would thus be helpful if this abbreviation is defined in the figure caption.

Fig. 8: Please clarify what DF, FN and QB are referring to. This figure only shows the time series on the whole water samples, did the authors also do this experiment for the PPL samples? Do they change if they are stored over longer periods of time?

Lines 431 onwards: Mention and evaluate again in this paragraph the potential for the (quantitative) loss of compounds during PPL extraction and how this may affect the outcome and interpretation of photodegradation experiments. It does seem like this loss of compounds does not affect the PARAFAC model, but do the authors think that this is universal for all different types of samples from different environments or may the loss of compounds have more profound effects on variations in FDOM?

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ED: Publish subject to minor revisions (review by editor) (23 Feb 2021) by Clare Woulds

AR by Alec Armstrong on behalf of the Authors (24 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Apr 2021) by Clare Woulds

Short summary

Living things decay into organic matter, which can dissolve into water (like tea brewing). Tea receives its color by absorbing light. Similarly, this material absorbs light, which can then cause chemical reactions that change it. By measuring changes in these optical properties, we found that materials from some places are more sensitive to light than others. Comparing sensitivity to light helps us understand where these materials come from and what happens as they move through water.