47 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments and environmental degradation (Reusch et al., 2018). High methane emissions to the atmosphere from several locations in the Baltic Sea have been documented, in the range of 0.1-3.3 mmol m-2 day-1, particularly from coastal sites with a shallow sulfate-methane transition zone (SMTZ) in the sediment and relatively shallow water depth (Bange et al., 1994; Gülzow et al., 2013; Humborg et al., 2019). Similarly, significant methane concentrations in the water column (up to 47 nmol L-1) (Humborg et al., 2019), large benthic fluxes of methane (up to 2.6 mmol m-2 day-1) (Sawicka & Brüchert, 2017), and high porewater concentrations of methane (6 mM) (Sawicka & Brüchert, 2017) have been reported in the Baltic Sea. Previous studies indicate that ANME are key players in methane cycling in Baltic Sea sediments, with ANME-1 and ANME-2 accounting for S-AOM activity, ANME-2 potentially involved in Fe-AOM, and ANME-1 also implicated in methanogenesis (Beulig et al., 2019; Iasakov et al., 2022; Meulepas et al., 2009; Rasigraf et al., 2020). However, a mechanistic understanding of environmental and biological factors that impact AOM in the Baltic Sea and other coastal ecosystems remains elusive. Here we investigated sediments of the eutrophic Stockholm Archipelago (Almroth-Rosell et al., 2016; Conley et al., 2011; van Helmond et al., 2020), pursuing a better mechanistic understanding of the impacts of divergent bottom water redox conditions on microbial methane cycling and associated sediment biogeochemistry. We selected three sites with contrasting bottom water oxygen concentrations (oxic: [O2]aq > 63 µmol L-1; seasonally hypoxic: [O 2]aq < 63 µmol L-1; euxinic: [O2]aq = 0 µmol L-1 with free sulfide). Sediment cores taken at these sites were subjected to high-resolution geochemical characterization, 16S rRNA gene profiling, potential methane production rate measurements, metagenomic analyses, and AOM rate measurements in selected sediments incubated with different sulfide concentrations. Our study specifically aimed to identify the main controls on the abundance, distribution and activity of ANME archaea and to elucidate the impacts of differing bottom water redox conditions on methane cycling in these eutrophic coastal sediments. 2
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