33 Introduction and thesis outline In brackish and freshwater environments, metabolically highly flexible ANME species such as “Ca. Methanoperedens” use nitrate, metal oxides (iron, manganese), humic substances as electron acceptors, thereby playing a critical role in methane mitigation in these ecosystems (Cai et al., 2022; Haroon et al., 2013; Leu et al., 2020a; Pelsma et al., 2023a; Valenzuela et al., 2020) (Figure 2 and 3). Sulfate-AOM for “Ca. Methanoperedens” is a particular topic of interest, because of its enigmatic and unique occurrence in sulfate-rich freshwater systems like meromictic lakes or groundwater systems (Bell et al., 2022; Su et al., 2020), included also in Chapter 7. O2 CH4 SO4 2NO3 - NO2 - MnO2 Fe(OH)3 Methane-oxidizing bacteria or MOB ANME-2a / “Ca. Methanoperedens” ANME-1, ANME-2abc, ANME-3 (“Ca. Methanoperedens”) “Ca. Methanoperedens” “Ca. Methylomirabilis” (MOB) Methanogens Methane biofilter Lorem ipsum Figure 2. An example schematic illustrating chemical gradients relevant to methane oxidation in the water column and sediment, along with the microbial groups catalyzing these reactions. The dashed black gradient for methane indicates that methanogens are present only within the anoxic sediment interval, while the remaining methane diffuses upward to the surface. The parentheses highlight less common environments for the methanotrophic groups mentioned. Edited using Adobe Illustrator 2024. 1
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