Effects of brackish water inflow on methane-cycling microbial communities in a freshwater rewetted coastal fen
Authors: Gutekunst Cordula Nina , Liebner Susanne , Jenner Anna-Kathrina , Knorr Klaus-Holger , Unger Viktoria , Koebsch Franziska , Racasa Erwin Don , Yang Sizhong , Böttcher Michael Ernst , Janssen Manon , Kallmeyer Jens , Otto Denise , Schmiedinger Iris , Winski Lucas , Jurasinski Gerald 
Published: August 2022

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 an 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 (Fig. 1). 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 RNA 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 the preceding
drought increased the sulfate availability in the surface and pore water.
Nevertheless, 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 in 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 preceding
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 3 orders of magnitude compared to
the freshwater state and also exceeded abundances recorded during the
drought by over 2 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 loose organic litter on the ground. This highlights the importance
of considering 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 greenhouse gas (GHG)
fluxes relative to the previous freshwater rewetting state cannot be
explained with the brackish water inflow alone but were potentially
reinforced by a biogeochemical legacy effect of the preceding drought.