Microbial community adaptation to brackish water rewetting in a coastal peatland

Abstract Coastal wetlands can serve as natural laboratories for assessing the future impacts of sea-level rise and the intricacies of the effect of sulfate (SO 4 2− ) on emissions of greenhouse gases, such as methane (CH 4 ) and carbon dioxide (CO 2 ). In the case of previously drained and freshened coastal wetlands, we can observe how freshwater terrestrial microbial communities react and adapt to intrusion of SO 4 2− rich saline waters. We conducted a 3-month anoxic incubation experiment with soil extracted from a coastal peatland on the German Baltic coast which was rewetted with brackish water in late 2019 to examine how microbial communities at the site had adapted to the new conditions after two years. Soil slurries were incubated at a temperature of 15 °C at two different salinities (reflecting surface water and average peat soil water salinity) and sampled at 8 timepoints. At each timepoint 5 replicates of each treatment were destructively harvested and sampled for concentrations of CH 4 , dissolved inorganic carbon (DIC), total aqueous organic carbon, sulfate (SO 4 2− ), ammonium (NH 4 + ), and other major ions, pH values, δ 13 DIC and δ 13 CH 4 values, microbial community composition via 16S rRNA amplicon sequencing and functional gene analysis via shotgun metagenomic sequencing. Carbon, nitrogen, and sulfur elemental analysis (CNS) and X-ray fluorescence (XRF) analysis of soil cores from nearby monitoring locations were included to give background on the biogeochemical conditions of the soil. Contrary to expectations, the legacy of SO 4 2− exposure from a previous connection with the Baltic Sea, as evidenced by high sulfate concentrations, was the strongest influence on the biogeochemistry of each treatment, rather than the new salinity and SO 4 2− introduced during rewetting. The different salinities tested had little impact on the methane emissions as the microbial community was already well adapted to saline and SO 4 2− -rich conditions and displayed a considerable amount of functional gene equivalency. We conclude from our results that we need to pay more attention to the legacy effects in coastal peatlands and how they affect methane cycling and microbial community composition for years to decades.