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The “methane paradox”: Mechanisms of CH4 production in oxygenated lake waters

English title The “methane paradox”: Mechanisms of CH4 production in oxygenated lake waters
Applicant Lehmann Moritz
Number 192327
Funding scheme Project funding (Div. I-III)
Research institution Institut für Umweltgeowissenschaften Universität Basel
Institution of higher education University of Basel - BS
Main discipline Hydrology, Limnology, Glaciology
Start/End 01.07.2020 - 30.06.2021
Approved amount 109'555.00
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All Disciplines (2)

Discipline
Hydrology, Limnology, Glaciology
Geochemistry

Keywords (11)

dimethylsulphoniopropionate; methane production; lakes; carbon isotope fractionation; methanogenesis; phytoplankton community structures; stable isotope probing; zooplankton; biomarker; methane paradox; methane oxidation

Lay Summary (German)

Lead
Methanogenesis, the microbial production of methane during fermentation-like processes has been considered a truly anoxic process. Nevertheless, large amounts of methane appear to accumulate in oxygen-rich waters. This project aims at explaining this "methane paradox".
Lay summary
For the longest time, biogenic methanogenesis has been considered a strictly anaerobic process. Yet, the accumulation of methane in oxic surface waters of lakes and the ocean have been reported around the globe, and seems responsible for a large portion of aquatic methane emissions. Despite the importance for the global greenhouse gas budget, and for predictions on changes in lacustrine greenhouse gas emissions under future climate scenarios in particular, the potential mechanisms behind this “methane paradox” remain elusive. Combining field-sampling and incubation-experimental efforts, and employing various analytical methods to samples from Lake Lugano (Southern Switzerland), this project investigates methane production related to the exploitation and decomposition of methylated organic compounds. A main goal is to establish functional links between methane accumulation, concentrations of methanogenetic substrates and nutrients, the molecular composition of dissolved organic matter, and the planktonic community structure in surface waters of this eutrophic lake. Specifically, we will verify the uptake of methylated nutrients in the context of lacustrine aerobic methane production, and investigate the identity of the microbes involved. This project will help us gaining insight into the biogeochemical controls on global methane emissions from aquatic environments.
Direct link to Lay Summary Last update: 31.03.2020

Responsible applicant and co-applicants

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Associated projects

Number Title Start Funding scheme
169552 The “methane paradox” in Lake Lugano - understanding methane production in oxygenated waters of lacustrine environments 01.01.2017 Project funding (Div. I-III)

Abstract

With this proposal, we seek funding for a 15-month extension of the postdoctoral project "The methane paradox in Lake Lugano - understanding methane production in oxygenated waters of lacustrine environments (SNF 169552). The main objectives of SNSF project 169552 was to comprehensively test multiple hypotheses proposed to explain the aquatic “methane paradox”, the persistent CH4 supersaturation in oxic lake waters, and to understand the environmental conditions that stimulate/modulate CH4 production in under aerobic conditions. Combining field-sampling and incubation-experimental efforts, and employing stable isotopic, radio-labelling, and molecular analyses applied to samples from Lake Lugano (Southern Switzerland), we specifically proposed to investigate CH4 production related to the exploitation and decomposition of methylated organic compounds. A main goal was to establish functional links between epilimnetic CH4 accumulation, concentrations of methanogenetic substrates and nutrients, the molecular composition of dissolved organic matter (DOM), and the planktonic community structure in this eutrophic lake. During the tenure of the original project, we have broadened the scope of the original research plan by establishing a regular sampling of plankton and CH4 biogeochemistry in another Swiss Lake, Lake Cadagno. During the first 2.5 years of the project, we generated a comprehensive data set that allowed us to answer important questions that we proposed in the parent proposal. Based on results from both lakes, we could demonstrate clear seasonal variability in the magnitude of the lacustrine methane paradox, and establish links between the accumulation of CH4, feeding zooplankton, and the degradation of planktonic detritus. We could experimentally show that CH4 can be produced in oxygenated lake waters from methylated compounds both under nutrient-limited and nutrient-replete conditions. At the same time, we were able to exclude the possibility that the lacustrine “methane paradox” in the two studied lakes results from an upwelling or lateral transport of sedimentary gas. Moreover, we were able to prove that aerobic CH4 accumulation varied with different plankton communities, confirming close links to the lacustrine productivity cycle and general limnological conditions. For example, we observed putative links between the development of picocyanobacterial blooms (Synechoccocus), the concentration of dimethylsulfide (DMS, a potential substrate in CH4 formation) in subsurface waters, and the accumulation of CH4, yet, the overall amount of DMS seemed insufficient to fully account for the observed methane paradox. While canonical methanogens are virtually absent from the epilimnetic microbial communities of both lakes (less than 0.05% of OTUs), methylotrophic bacteria as well as photoautotrophs are abundant, and are potentially capable of exploiting methylated nutrients (methylphosphonate - MPn, methylamines-MA). In fact, the most recent set of incubation experiments within SNSF project 169552 supported that both methylphosponate (MPn) and methylamine (MA) are suitable substrates for CH4 generation in oxic waters of the studied lakes. Most importantly, and in contrast to literature reports, the production of CH4 was not inhibited by inorganic phosphorous (as PO43-P), indicating that P-limitation is not a pre-requisite for the exploitation of methylated P to occur in nature. Although the data so far are highly promising and provide a solid basis for at least three publications, not all of the questions originally posed could be addressed unambiguously (due to the highly complex nature of analytical techniques required to identify and quantify natural concentrations of methylated P and N compounds, as well as molecular characterization of DOM). Specifically, uptake of MPn and/or MA in the context of lacustrine aerobic CH4 production, the identity of the microbes involved, and the specific role of picocyanobacteria still needs further investigation. Given the comprehensiveness and great variety of data already generated, the project extension will provide the time required for a more in-depth statistical analysis and exploitation of the already existing datasets. In addition, we propose complementary bio-geochemical analyses of samples that have already been collected during past samplings to confirm the robustness of existing results (characterization of DOM by FTIR-ICMS, characterization and quantification of polysaccharide esters of phosphonic acids by NMR and methylated amines by solid-phase microextraction techniques), as well as incubation experiments with 13C labelled MPn and 15N labelled MA to assess uptake and metabolism of these compounds by isolated Synechoccocus strains using isotope-ratio mass-sepctrometry and NanoSims imagining. Prime objective of the project extension will be to establish whether, and to what extend, the exploitation of dissolved organic phosphorus (DOP) and methylated-N compounds occurs naturally under different trophic conditions, and what the potential rates of CH4 generation are. We will also investigate further the C isotopic fingerprint (including clumped CH4 isotope signatures) of MPn or MA-based aerobic methane production. This project extension thus promises to provide additional milestones in our efforts to understand the “methane paradox” in lakes, helping us to gain insight into the biogeochemical controls on global CH4 emissions from aquatic environments.
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