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

[摘要] Rewetted peatlands can be a significant source of methane(CH 4 ), but in coastal ecosystems, input of sulfate-rich seawater couldpotentially mitigate these emissions. The presence of sulfate as an electronacceptor during organic matter decomposition is known to suppressmethanogenesis by favoring the growth of sulfate reducers, which outcompetemethanogens for substrate. We investigated the effects of a brackish waterinflow on the microbial communities relative to CH 4 production–consumption dynamics in a freshwater rewetted fen at the southernBaltic Sea coast after a storm surge in January 2019 and analyzed our datain context with the previous freshwater rewetted state (2014 serves as ourbaseline) and the conditions after a severe drought in 2018 (Fig. 1). We took peat cores at four previously sampled locations along a brackishnessgradient to compare soil and pore water geochemistry as well as themicrobial methane- and sulfate-cycling communities with the previousconditions. We used high-throughput sequencing and quantitative polymerasechain reaction (qPCR) to characterize pools of DNA and RNA targeting totaland putatively active bacteria and archaea. Furthermore, we measuredCH 4 fluxes along the gradient and determined the concentrations andisotopic signatures of trace gases in the peat. We found that both the inflow effect of brackish water and the precedingdrought increased the sulfate availability in the surface and pore water.Nevertheless, peat soil CH 4 concentrations and the 13 C compositions ofCH 4 and total dissolved inorganic carbon (DIC) indicated ongoingmethanogenesis and little methane oxidation. Accordingly, we did not observea decrease in absolute methanogenic archaea abundance or a substantialchange in methanogenic community composition following the inflow but foundthat the methanogenic community had mainly changed during the precedingdrought. In contrast, absolute abundances of aerobic methanotrophic bacteriadecreased back to their pre-drought level after the inflow, while they hadincreased during the drought year. In line with the higher sulfateconcentrations, the absolute abundances of sulfate-reducing bacteria (SRB)increased – as expected – by almost 3 orders of magnitude compared tothe freshwater state and also exceeded abundances recorded during thedrought by over 2 orders of magnitude. Against our expectations,methanotrophic archaea (ANME), capable of sulfate-mediated anaerobic methaneoxidation, did not increase in abundance after the brackish water inflow.Altogether, we could find no microbial evidence for hampered methaneproduction or increased methane consumption in the peat soil after thebrackish water inflow. Because Koebsch et al. (2020) reported a new minimumin CH 4 fluxes at this site since rewetting of the site in 2009, methaneoxidation may, however, take place in the water column above the peat soilor in the loose organic litter on the ground. This highlights the importanceof considering all compartments across the peat–water–atmosphere continuum todevelop an in-depth understanding of inflow events in rewetted peatlands. Wepropose that the changes in microbial communities and greenhouse gas (GHG)fluxes relative to the previous freshwater rewetting state cannot beexplained with the brackish water inflow alone but were potentiallyreinforced by a biogeochemical legacy effect of the preceding drought.

[学科分类] 大气科学