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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Oct 2019

Submitted as: research article | 23 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Acetate turnover and methanogenic pathways in Amazonian lake sediments

Ralf Conrad1, Melanie Klose1, and Alex Enrich-Prast2,3 Ralf Conrad et al.
  • 1Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch-Str. 10, 35043 Marburg, Germany
  • 2Department of Thematic Studies - Environmental Change, Linköping University, Linköping, Sweden
  • 3Departamento de Botânica, Instituto de Biologia, University Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

Abstract. Lake sediments in Amazonia are a significant source of CH4, a potential greenhouse gas. Previous studies of sediments using 13C analysis found that the contribution of hydrogenotrophic versus aceticlastic methanogenesis to CH4 production was relatively high. Here, we determined the methanogenic pathway in the same sediments (n = 6) by applying [14C]bicarbonate or [2-14C]acetate, and confirmed the high relative contribution (50–80 %) of hydrogenotrophic methanogenesis. The respiratory index (RI) of [2-14C]acetate, which is 14CO2 relative to 14CH4 + 14CO2, divided the sediments into two categories, i.e., those with an RI < 0.2 being consistent with the operation of aceticlastic methanogenesis, and those with an RI > 0.4 showing that a large percentage of the acetate-methyl was oxidized to CO2 rather than reduced to CH4. Hence, part of the acetate was probably converted to CO2 plus H2 via syntrophic oxidation, thus enhancing hydrogenotrophic methanogenesis. This happened despite the presence of potentially aceticlastic Methanosaetaceae in all the sediments. Alternatively, acetate may have been oxidized with a constituent of the sediment organic matter (humic acid) serving as oxidant. Indeed, apparent acetate turnover rates were larger than CH4 production rates except in those sediments with a R < 0.2. Our study demonstrates that CH4 production in Amazonian lake sediments was not simply caused by a combination of hydrogenotrophic and aceticlastic methanogenesis, but probably involved additional acetate turnover.

Ralf Conrad et al.
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Ralf Conrad et al.
Ralf Conrad et al.
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Short summary
Lake sediments release the greenhouse gas CH4. Acetate is an important precursor. Although Amazonian lake sediments all contained acetate-consuming methanogens, measurement of the turnover of labeled acetate showed that some sediments converted acetate not to CH4 plus CO2, as expected, but only to CO2. Our results indicate the operation of acetate-oxidizing microorganisms that couple the oxidation process to syntrophic methanogenic partners and/or to the reduction of organic compounds.
Lake sediments release the greenhouse gas CH4. Acetate is an important precursor. Although...