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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-18
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-18
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 Feb 2020

Submitted as: research article | 06 Feb 2020

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This preprint is currently under review for the journal BG.

Biogeochemical evidence of anaerobic methane oxidation and anaerobic ammonium oxidation in a stratified lake using stable isotopes

Florian Einsiedl1, Anja Wunderlich1, Mathieu Sebilo2, Ömer K. Coskun3, William D. Orsi3,4, and Bernhard Mayer5 Florian Einsiedl et al.
  • 1Chair of Hydrogeology, Technical University of Munich, TUM Department of Civil, Geo and Environmental Engineering, Arcisstrasse 21, 80333 München, Germany
  • 2Sorbonne Université, CNRS, IEES, F-75005, Paris, France
  • 3Department of Earth and Environmental Sciences, Paleontology & Geobiology, Ludwig-Maximilians-Universität München, 80333 Munich, Germany
  • 4GeoBio-Center, Ludwig-Maximilians-Universität München, 80333 Munich, Germany
  • 5Department of Geoscience, University of Calgary, Calgary, Alberta, Canada T2N 1N4

Abstract. Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors in deteriorating the quality of drinking water and in contributing to global climate change. The nitrate dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) have the potential to reduce nitrogen loading of aquatic ecosystems and to reduce methane emissions to the atmosphere.

Here, we report vertical concentration profiles and corresponding stable isotope compositions of CH4, NO3, NO2 and NH4+ in the water column of a stratified lake, which suggest linkages between anaerobic oxidation of methane (AOM), denitrification, and anammox. In a water depth from 12 to 20 m, a methane-nitrate transition zone (NMTZ) was observed, where δ13C values of methane and δ15N and δ18O of dissolved nitrate markedly increased in concert with decreasing concentrations of methane and nitrate. These data patterns, together with a simple 1D diffusion model show that the non-linear methane concentration profile cannot be explained by diffusion or micro-aerobic methane oxidation, and that microbial oxidation of methane coupled with denitrification under anaerobic conditions is the most likely explanation for these data trends.

In the methane zone at the bottom of the NMTZ (20 m to 22 m) δ15N of ammonium increased by 4 ‰, while ammonium concentrations decreased. In addition, a strong 15N enrichment of dissolved nitrate was observed at a water depth of 20 m, suggesting that anammox is occuring together with denitrification coupled to AOM. The conversion of nitrite to N2 and nitrate during anammox is namely associated with an inverse N isotope fractionation and may explain the observed increasing offset (Δδ15N) of 26 ‰ between δ15N values of dissolved nitrate and nitrite at a water depth of 20 m compared to the Δδ15Nnitrate-nitrite of 11 ‰ obtained in the NMTZ between a water depth of 16 m and 18 m.

The geochemcical zones were found to contain significantly different microbial communities that consist of bacteria known to be involved in denitrification with AOM (Crenothrix and NC10), and anammox (Candidatus Anammoximicrobium), confirming the presence of microbial groups potentially responsible for the proposed linkages between AOM, denitrification, and anammox. This study gives insights into the yet overlooked AOM-denitrification-anammox process in stratified lakes that can regulate methane emisssions from and nitrogen concentrations in lakes.

Florian Einsiedl et al.

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Florian Einsiedl et al.

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Short summary
Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors in deteriorating the quality of drinking water and in contributing to global climate change. The nitrate dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) have the potential to contribute to the removal of nitrate, nitrite and ammonium from aquatic ecosystems by converting them to harmless N2 while oxidizing the potent greenhouse gas methane to CO2.
Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors...
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