60 Chapter 2 In coastal systems, electron acceptors other than sulfate may drive AOM, such as nitrate and nitrite (Haroon et al., 2013; He et al., 2016) as well as poorly ordered Fe(III)- and Mn(III/IV)-oxides (Aromokeye et al., 2020; Canfield & Thamdrup, 2009; Raghoebarsing et al., 2006). At our sites, nitrate and nitrite are exclusively present in low concentrations in the surface sediment (Figure 2) and are therefore unlike to substantially contribute to AOM activity. At Sites 5 and 7, poorly ordered Fe(III)- and Mn(III/IV)-oxides are nearly absent (Figure 3). Additionally, at both these sites, most of the reactive Fe and Mn is sulfidized (Figure 3), in line with the ambient bottom water redox conditions and relatively high porewater sulfide concentrations (Table 2; Figure 2), previously also observed for other sites in the Stockholm Archipelago (van Helmond et al., 2020). Hence, there is only limited potential for Fe- and Mn-AOM, which seem to play a larger role in oligotrophic rather than eutrophic coastal ecosystems (Beal et al., 2009; Egger et al., 2015; Lenstra et al., 2018). However, a role for Fe-AOM cannot be fully excluded, as crystalline Fe(III)- oxides and even recalcitrant Fe(II/III)-oxides (Figure 3) may also play a role in Fe-AOM (Bar-Or et al., 2017) and S-AOM (Sivan et al., 2014). Magnetite, for example, was shown to stimulate Fe-AOM activity and ANME-2a enrichment in incubations with North Sea sediments (Aromokeye et al., 2020), and goethite-dependent AOM has been suggested as a significant methane sink in paddy soils, in which hematite and magnetite-AOM were also detected (He et al., 2021). High methane production potential in the hypoxic and euxinic sites Sites 5 and 7 showed particularly high potential methane production rates (up to 2.3±0.3 and 3.3±0.4 µmol methane g-1 d-1 respectively at 2 cm depth). By contrast, at Site 3, potential methane production rates did not exceed 0.22±0.006 µmol methane g-1 d-1 at 18 cm (Figure 4, Supplementary Table 1). To examine the microbial diversity and metabolic potential, sediments were subjected to DNA extractions and high-resolution 16S rRNA gene sequencing, with selected samples also used for metagenomic sequencing. Archaeal 16S rRNA gene sequences were used to generate amplicon sequence variants (ASVs), which were clustered at family level for relative abundance visualization (Figure 4). Methanoregulaceae and Methanosaetaceae represented the two most abundant putative methanogenic families. While Methanoregulaceae had highest relative abundances of 16% in Site 3 at 34 cm, 38% in Site 5 at 50 cm, and 34% in Site 7 at 30 cm, Methanosaetaceae reached 11% in
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