the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Oct 2023
Picoplanktonic methane production in eutrophic surface waters
Sandy E. Tenorio
Laura Farías
Abstract. In the last decade, there have been several research articles on the methane paradox (aerobic CH4 production) first described in the 1960s. In this study, we present observations of CH4 supersaturation in the surface layer in the central Chile upwelling zone (36° S, 73° W) throughout two seasonal cycles (2018–2021). Additionally, CH4 cycling experiments were performed using plankton fractions (natural planktonic community, <150, <3 and <0.2 µm) in a seasonal phytoplankton succession. Our findings highlight the significant role of picoplankton (<3 μm) in CH4 production on the ocean surface, contrasting with the limited involvement of larger organisms (<150 μm). Incubations with methylated substrates such as methylphosphonic acid (MPn) and trimethylamine (TMA) stimulated CH4 production in the picoplankton fraction during both upwelling (austral spring-summer) and non-upwelling (winter) seasons, being particularly relevant in the later period when Synechococcus contributed with high relative abundance. Long-term microcosm experiments underscore the importance of heterotrophic bacteria and cyanobacteria in methylotrophic methanogenesis, enhancing CH4 regeneration, mediated by dissolved organic matter (DOM) recycling. In conclusion, picoplankton emerges as a key factor in both production and metabolization of methylated substrates, being responsible for maintaining CH4 supersaturation. These findings provide valuable insights into the biogeochemical processes driving CH4 dynamics in highly productive upwelling waters.
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Sandy E. Tenorio and Laura Farías
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-185', Anonymous Referee #1, 13 Oct 2023
The study by Tenorio Farías inspects the production of methane under oxic conditions over a time period of four years in the surface waters of the Chilean continental shelf. The authors conclude that a mixture of processes contributes to the process including photosynthesis-associated methane production and demethylation of methylphosphonates and trimethylamines.
The data is overall very important as it strengthens the significance of oxic methane production in the marine environment, a process that went largely overlooked until recent years. The study is also important in showing that multiple processes are involved under different environmental conditions, unlike other papers calling for the sole significance of one process or the other.
Nevertheless, the paper in its current state is not yet suitable for publication. I have made my comments on a PDF version of the preprint. Here are just a few main points:
1) The writing can be improved for clarity. In some cases, the thread of the main message is lost and the contribution of a paragraph or a section of it to the main story is not clear. Additionally, some choice of words is inaccurate or incorrect.
2) The methane profile in Fig. 2 does not clearly highlight temporal hotspots of oxic methane production. I am confident that oxic methane production occurs and that it plays a significant role, however, graphically, it appears as in most cases (not all) the surface methane results from the upwelling of methane-rich water. Hence, I encourage the authors to reconsider their presentation of that panel. highlighting the consumption of bottom-derived methane and the production of new methane in the surface layer. With this respect, please also review the units for methane concentration - is it nM as expected for oceanic environments and mentioned in the text or is it µM as in Fig. 2 and table 2. ?
3) In some cases the view of the authors is somewhat too simplistic. For example, methylphosphonates can be derived from many sources, while the authors suggest N maritimus as a sole source (see also typo in name in Fig 7). Similarily, TMA is one example of methylated amines, but there are papers discussing Mono and Di-methyl amines. Hence the discussion in the paper should present TMA as an example while stating that other similar compounds are likely present, and their degradation results in methane emission.
4) The experimental results are in some cases confusing. Why does a fraction of the community produce more methane than the entire community (Fig. 3a)? Other questions regarding the experiments and their presentations are in the PDF.
5) Font size is not uniform across the text, especially for references within the text. References are also not uniformly formatted.
As mentioned, in addition to these comments, please pay attention to all suggestions / comments / queries in the attached PDF.
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RC2: 'Comment on bg-2023-185', Anonymous Referee #2, 28 Nov 2023
In the manuscript by Tenorio and Farías, the methane saturation in the depth profile was monitored together with other parameters such as oxygen, chlorophyll and nutrient concentrations in the upwelling area off Chile over seasonal cycles. In addition, oxic methane formation experiments were conducted with the precursor compounds TMA and MPn, which had already been identified as precursors for oxic methane formation in previous studies. Overall, I think that the data are interesting for the readership of Biogeoscience. However, I also have some criticism that should be addressed.
The introduction contains many sentences that either need to be deleted or clarified as they are too general. This concerns Line 38-39, 56-59, 61-65, 70- 72, 69-70
Line 69 “ however, since picoplankton are small in size and biomass, it is difficult to observe this relationship.“ not necessary to mention
Line 39-43 Oceanic methane biogeochemistry is oversimplified. Add a few sentences, e.g. what typical methane depth profiles look like and why. I also recommend moving the information about the studied upwelling region from the Results and Discussion chapter to the Introduction. The selected literature references are also not always well chosen, e.g. the publication by Weber et al. 2019, which deals with an improved estimation of global oceanic methane fluxes into the atmosphere, is incorrectly referenced here.
In addition, the previous state of research on the pathways of oxic methane formation needs to be revised in the introduction. Overall, the section is too short and the studies cited are not presented in a differentiated manner. This aspect is important as potential methane precursor compounds were also investigated in this study. Methane formation in the gut of zooplankton or in anoxic micro size of particles is not mentioned in the introduction, but should be discussed in the manuscript when investigating TMA as a potential methane precursor.
Line 70- 72 It would be better to formulate hypotheses or objectives of the present studies at this point. It would also help the reader to provide a brief overview of what was done to answer the hypothesis or achieve the objectives.
Material and methods
It would be good to have a geographical map of the study area in which the sampling site is marked.
Line 73. A brief overview of this chapter and moderation of the text would also improve the reader's understanding here.
Chapter 2.4 it would be better to add subchapters.
Explain the Brunt-Vaisala frequency and how you measured/calculated it
Line 239-252 It would be good if some of the information were included in the introduction.
Line 253-257 I suggest marking upwelling and non-upwelling periods in the Figure 2
Line 270 Instead of mentioning euphotic, it would be more precise to refer to the chlorophyll concentration
Line 270-277 This discussion is quite speculative and subjective. Can the lack of correlation be statistically proven with the data? e.g. by correlating upwelling parameters and methane concentration? While in-situ production of methane is thinkable, a lateral influx of methanogenic methane from the coast is conceivable. Ultimately, a mass balance, such as that undertaken by Hartmann et al, 2020, is required to determine whether methane was produced aerobically in situ. This should be included in the discussion.
Figure 2 C shows the Brunt-Vaisala frequency but has not been discussed in the text. How does this frequency relate to the methane profile, what can be deduced from Figure 2 C?
Line 274 ff Explain how the lack of a seasonal correlation indicates inflow from a river. Here it would be good to have a geographical map, as already mentioned.
Line 276 Methane from sediments could also be introduced by the river input, this should also be taken into account / included in the discussion.
Line 296 This could be proven by a correlation analysis.
Line 316-322, This reasoning is a bit confusing to me, since you found no correlation between chlorophyll and methane formation, right? Even if methane formation by phytoplankton is possible in principle and is rightly discussed here, the point that there was no correlation between methane and chlorophyll should be taken up again here.
Line 410- 412. The reference for TMA is missing. Delete “because they have a methyl radical (-CH3), a potential precursor for CH4 formation in oxygenated environments”
Citation: https://doi.org/10.5194/bg-2023-185-RC2
Sandy E. Tenorio and Laura Farías
Sandy E. Tenorio and Laura Farías
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