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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 1 | Copyright
Biogeosciences, 15, 137-157, 2018
https://doi.org/10.5194/bg-15-137-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 10 Jan 2018

Research article | 10 Jan 2018

Microbial methanogenesis in the sulfate-reducing zone of sediments in the Eckernförde Bay, SW Baltic Sea

Johanna Maltby1,a, Lea Steinle2,1, Carolin R. Löscher3,1, Hermann W. Bange1, Martin A. Fischer4, Mark Schmidt1, and Tina Treude5,6 Johanna Maltby et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Department of Marine Biogeochemistry, 24148 Kiel, Germany
  • 2Department of Environmental Sciences, University of Basel, 4056 Basel, Switzerland
  • 3Nordic Center for Earth Evolution, University of Southern Denmark, 5230 Odense, Denmark
  • 4Institute of Microbiology, Christian-Albrecht-University Kiel, 24118 Kiel, Germany
  • 5Department of Earth, Planetary, and Space Sciences, University of California Los Angeles (UCLA), Los Angeles, California 90095-1567, USA
  • 6Department of Atmospheric and Oceanic Sciences, University of California Los Angeles (UCLA), Los Angeles, California 90095-1567, USA
  • apresent address: Natural Sciences Department, Saint Joseph's College, Standish, Maine 04084, USA

Abstract. Benthic microbial methanogenesis is a known source of methane in marine systems. In most sediments, the majority of methanogenesis is located below the sulfate-reducing zone, as sulfate reducers outcompete methanogens for the major substrates hydrogen and acetate. The coexistence of methanogenesis and sulfate reduction has been shown before and is possible through the usage of noncompetitive substrates by methanogens such as methanol or methylated amines. However, knowledge about the magnitude, seasonality, and environmental controls of this noncompetitive methane production is sparse. In the present study, the presence of methanogenesis within the sulfate reduction zone (SRZ methanogenesis) was investigated in sediments (0–30cm below seafloor, cmb.s.f.) of the seasonally hypoxic Eckernförde Bay in the southwestern Baltic Sea. Water column parameters such as oxygen, temperature, and salinity together with porewater geochemistry and benthic methanogenesis rates were determined in the sampling area Boknis Eck quarterly from March 2013 to September 2014 to investigate the effect of seasonal environmental changes on the rate and distribution of SRZ methanogenesis, to estimate its potential contribution to benthic methane emissions, and to identify the potential methanogenic groups responsible for SRZ methanogenesis. The metabolic pathway of methanogenesis in the presence or absence of sulfate reducers, which after the addition of a noncompetitive substrate was studied in four experimental setups: (1) unaltered sediment batch incubations (net methanogenesis), (2) 14C-bicarbonate labeling experiments (hydrogenotrophic methanogenesis), (3) manipulated experiments with the addition of either molybdate (sulfate reducer inhibitor), 2-bromoethanesulfonate (methanogen inhibitor), or methanol (noncompetitive substrate, potential methanogenesis), and (4) the addition of 13C-labeled methanol (potential methylotrophic methanogenesis). After incubation with methanol, molecular analyses were conducted to identify key functional methanogenic groups during methylotrophic methanogenesis. To also compare the magnitudes of SRZ methanogenesis with methanogenesis below the sulfate reduction zone (>30cmb.s.f.), hydrogenotrophic methanogenesis was determined by 14C-bicarbonate radiotracer incubation in samples collected in September 2013.

SRZ methanogenesis changed seasonally in the upper 30cmb.s.f. with rates increasing from March (0.2nmolcm−3d−1) to November (1.3nmolcm−3d−1) 2013 and March (0.2nmolcm−3d−1) to September (0.4nmolcm−3d−1) 2014. Its magnitude and distribution appeared to be controlled by organic matter availability, C/N, temperature, and oxygen in the water column, revealing higher rates in the warm, stratified, hypoxic seasons (September–November) compared to the colder, oxygenated seasons (March–June) of each year. The majority of SRZ methanogenesis was likely driven by the usage of noncompetitive substrates (e.g., methanol and methylated compounds) to avoid competition with sulfate reducers, as was indicated by the 1000–3000-fold increase in potential methanogenesis activity observed after methanol addition. Accordingly, competitive hydrogenotrophic methanogenesis increased in the sediment only below the depth of sulfate penetration (>30cmb.s.f.). Members of the family Methanosarcinaceae, which are known for methylotrophic methanogenesis, were detected by PCR using Methanosarcinaceae-specific primers and are likely to be responsible for the observed SRZ methanogenesis.

The present study indicates that SRZ methanogenesis is an important component of the benthic methane budget and carbon cycling in Eckernförde Bay. Although its contributions to methane emissions from the sediment into the water column are probably minor, SRZ methanogenesis could directly feed into methane oxidation above the sulfate–methane transition zone.

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The activity and environmental controls of methanogenesis (MG) within the sulfate-reducing zone (0–30 cm below the seafloor) were investigated in organic-rich sediments of the seasonally hypoxic Eckernförde Bay, SW Baltic Sea. MG activity was mostly linked to non-competitive substrates. The major controls identified were organic matter availability, C / N, temperature, and O2 in the water column, revealing higher rates in warm, stratified, hypoxic seasons compared to colder, oxygenated seasons.
The activity and environmental controls of methanogenesis (MG) within the sulfate-reducing zone...
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