13 Apr 2022
13 Apr 2022
Status: this preprint is open for discussion.

Effects of brackish water inflow on methane cycling microbial communities in a freshwater rewetted coastal fen

Cordula Nina Gutekunst1, Susanne Liebner2,3, Anna-Kathrina Jenner4, Klaus-Holger Knorr5, Viktoria Unger6, Franziska Koebsch7, Erwin Don Racasa8, Sizhong Yang2, Michael Ernst Böttcher4,9,10, Manon Janssen8, Jens Kallmeyer2, Denise Otto4, Iris Schmiedinger4, Lucas Winski4,a, and Gerald Jurasinski1,10 Cordula Nina Gutekunst et al.
  • 1Landscape Ecology, University of Rostock, Rostock, 18059, Germany
  • 2Section Geomicrobiology, German Research Centre for Geosciences (GFZ), Potsdam, 14473, Germany
  • 3Institute of Biochemistry and Biology, University of Potsdam, Potsdam, 14476, Germany
  • 4Geochemistry and Stable Isotope Biogeochemistry, Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, 18119, Germany
  • 5Institute of Landscape Ecology, Ecohydrology & Biogeochemistry Group, University of Münster, Münster, 48149, Germany
  • 6Institute of Plant Science and Microbiology, Applied Plant Ecology, University of Hamburg, Hamburg, 22609, Germany
  • 7Bioclimatology, University of Göttingen, Göttingen, 37073, Germany
  • 8Soil Physics, University of Rostock, Rostock, 18059, Germany
  • 9Marine Geochemistry, University of Greifswald, Greifswald, 17489, Germany
  • 10Department of Maritime Systems, University of Rostock, Rostock, 18059, Germany
  • apresent address: FRG, University of Jena, Jena, 07743, Germany

Abstract. Rewetted peatlands can be a significant source of methane (CH4), but in coastal ecosystems, input of sulfate-rich seawater could potentially mitigate these emissions. The presence of sulfate as electron acceptor during organic matter decomposition is known to suppress methanogenesis, by favoring the growth of sulfate-reducers, which outcompete methanogens for substrate. We investigated the effects of a brackish water inflow on the microbial communities relative to CH4 production-consumption dynamics in a freshwater rewetted fen at the southern Baltic Sea coast after a storm surge in January 2019 and analyzed our data in context with the previous freshwater rewetted state (2014 serves as our baseline) and the conditions after a severe drought in 2018.

We took peat cores at four previously sampled locations along a brackishness gradient to compare soil and pore water geochemistry as well as the microbial methane and sulfate cycling communities with the previous conditions. We used high-throughput sequencing and quantitative polymerase chain reaction (qPCR) to characterize pools of DNA and cDNA targeting total and putatively active bacteria and archaea. Furthermore, we measured CH4 fluxes along the gradient and determined the concentrations and isotopic signatures of trace gases in the peat.

We found that both, the inflow effect of brackish water and in parts also the preceding drought increased the sulfate availability in the surface and pore water. Still, peat soil CH4 concentrations and the 13C compositions of CH4 and total dissolved inorganic carbon (DIC) indicated ongoing methanogenesis and little methane oxidation. Accordingly, we did not observe a decrease of absolute methanogenic archaea abundance or a substantial change in methanogenic community composition following the inflow, but found that the methanogenic community had mainly changed during the precedent drought. In contrast, absolute abundances of aerobic methanotrophic bacteria decreased back to their pre-drought level after the inflow while they had increased during the drought year. In line with the higher sulfate concentrations, the absolute abundances of sulfate reducing bacteria (SRB) increased – as expected – by almost three orders of magnitude compared to the freshwater state and also exceeded abundances recorded during the drought by over two orders of magnitude. Against our expectations, methanotrophic archaea (ANME), capable of sulfate-mediated anaerobic methane oxidation, did not increase in abundance after the brackish water inflow. Altogether, we could find no microbial evidence for hampered methane production or increased methane consumption in the peat soil after the brackish water inflow. Because Koebsch et al. (2020) reported a new minimum in CH4 fluxes at this site since rewetting of the site in 2009, methane oxidation may, however, take place in the water column above the peat soil or in the lose organic litter on the ground. This highlights the importance to consider all compartments across the peat-water-atmosphere continuum to develop an in-depth understanding of inflow events in rewetted peatlands. We propose that the changes in microbial communities and GHG fluxes relative to the previous freshwater rewetting state cannot be explained with the brackish water inflow alone, but was potentially reinforced by a biogeochemical legacy effect of the precedent drought.

Cordula Nina Gutekunst et al.

Status: open (until 04 Jun 2022)

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  • RC1: 'Review of egusphere-2022-65', Anonymous Referee #1, 05 May 2022 reply

Cordula Nina Gutekunst et al.

Cordula Nina Gutekunst et al.


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Short summary
Methane emissions decreased after a seawater inflow and a preceding drought in freshwater rewetted coastal peatland. However, our microbial and greenhouse gas measurements do not indicate that methane consumers increased. Rather, methane producers co-existed in high numbers with their usual competitors, the sulfate-cycling bacteria. We studied the peat soil interdisciplinary and aimed to cover the soil-atmosphere continuum to better understand the sources of methane production and consumption.