Preprints
https://doi.org/10.5194/bg-2021-203
https://doi.org/10.5194/bg-2021-203

  14 Sep 2021

14 Sep 2021

Review status: this preprint is currently under review for the journal BG.

Fractionation of stable carbon isotopes during acetate consumption by methanogenic and sulfidogenic microbial communities in rice paddy soils and lake sediments

Ralf Conrad1, Pengfei Liu1,2, and Peter Claus1 Ralf Conrad et al.
  • 1Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch-Str. 10, 35043 Marburg, Germany
  • 2Center for Pan-third Pole Environment, Lanzhou University, Tianshuinanlu 222, Lanzhou, 730000, China

Abstract. Acetate is an important intermediate during the degradation of organic matter in anoxic flooded soils and sediments. Acetate is disproportionated to CH4 and CO2 by methanogenic or is oxidized to CO2 by sulfate-reducing microorganisms. These reactions result in carbon isotope fractionation, depending on the microbial species and their particular carbon metabolism. To learn more about the magnitude of the isotopic enrichment factors (ε) involved, acetate conversion to CH4 and CO2 was measured in anoxic paddy soils from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) and in anoxic lake sediments from the north east (NE) and the south west (SW) basins of Fuchskuhle (Germany). Acetate consumption was measured using samples of paddy soil or lake sediment suspended in water or in phosphate buffer (pH 7.0), both in the absence and presence of sulfate (gypsum), and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, values of εac for acetate consumption were always in a range of -21 ‰ to -17 ‰, but higher in the lake sediment from the SW basin (-11 ‰). Under sulfidogenic conditions εac values tended to be slightly lower (-26 ‰ to -19 ‰) especially when aceticlastic methanogenesis was inhibited. Again, εac in the lake sediment of the SW basin was higher (-18 ‰ to -14 ‰). Determination of εCH4 from the accumulation of 13C in CH4 resulted in much lower values (-37 ‰ to -27 ‰) than from the depletion of 13C in acetate (-21 ‰ to -17 ‰), especially when acetate degradation was measured in buffer suspensions. The microbial communities were characterized by sequencing the bacterial 16S rRNA genes as well as the methanogenic mcrA and sulfidogenic dsrB genes. The microbial communities were quite different between lake sediments and paddy soils, but were similar in the sediments of the two lake basins and in the soils from Vercelli and IRR, and were similar after preincubation without and with addition of sulfate (gypsum). The different microbial compositions could hardly serve for the prediction of the magnitude of enrichment factors.

Ralf Conrad et al.

Status: open (until 26 Oct 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-203', Anonymous Referee #2, 22 Sep 2021 reply
    • AC1: 'Reply on RC1', Ralf Conrad, 22 Oct 2021 reply
  • RC2: 'Comment on bg-2021-203', Anonymous Referee #1, 28 Sep 2021 reply
    • AC2: 'Reply on RC2', Ralf Conrad, 22 Oct 2021 reply

Ralf Conrad et al.

Ralf Conrad et al.

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Short summary
Acetate is an important intermediate during the anaerobic degradation of organic matter. It is consumed by methanogenic and sulfidogenic microorganisms accompanied by stable carbon isotope fractionation. We determined isotope fractionation under different conditions in two paddy soils and two lake sediments, and also determined the composition of the microbial communities. Despite a relatively wide range of experimental conditions, the range of fractionation factors was quite moderate.
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