205 Methanotrophic flexibility of “Ca. Methanoperedens” and SRB in meromictic Lake Cadagno INTRODUCTION Anaerobic oxidation of methane (AOM) is an important biological sink for this potent greenhouse gas (Knittel & Boetius, 2009; Saunois et al., 2020; Wallenius et al., 2021) in a wide range of anoxic ecosystems, including inland waters, coastal and ocean ecosystem sediments (Gao et al., 2022; Rosentreter et al., 2021; Su et al., 2022). AOM is catalyzed via reverse methanogenesis by different groups of anaerobic methane-oxidizing (ANME) archaea (Chadwick et al., 2022; Timmers et al., 2017) using a variety of electron acceptors (Glodowska et al., 2022). In marine sediments, sulfate dependent-AOM (S-AOM) is the predominant process, carried out by the ANME groups 1, 2a, 2b ,2c and 3 in syntropy with sulfate-reducing bacteria (SRB) (Metcalfe et al., 2021; Murali et al., 2023a; Yu et al., 2021). The ANME’s and SRB’s engage in direct interspecies (DIET) or extracellular electron transfer (EET) via multi-heme c-type cytochromes (MHC) or conductive pili and/or other intermediates (Krukenberg et al., 2018; Scheller et al., 2016; Wegener et al., 2015). In freshwater systems, sulfate is much less abundant and the ANME-2d group, “Candidatus (Ca.) Methanoperedens” spp., appears to drive AOM with nitrate, humic acids, or metal oxides as electron acceptors without the need for a syntrophic partner (Cai et al., 2018; Cai et al., 2022; Ettwig et al., 2016; Haroon et al., 2013; Leu et al., 2020a; Pelsma et al., 2023a; Vaksmaa et al., 2017; Valenzuela et al., 2020). However, interactions with certain guilds such as nitrite-scavenging anammox or Methylomirabilis bacteria might be beneficial for Methanoperedens (Arshad et al., 2017; Dalcin Martins et al., 2022, Chapter 4). Reports of “Ca. Methanoperedens” spp. in estuarine and marine habitats are rather scarce although enrichment cultures have been shown to withstand marine salinities (Echeveste Medrano et al., 2024b, Chapter 6; Frank et al., 2023). Enrichment cultures of “Ca. Methanoperedens” have usually been established from freshwater source material, with different electron acceptors including nitrate, manganese-, and/or iron oxides. These studies revealed that the reduction of nitrate is performed by a membrane bound nitrate reductase within the respiratory chain, while large MHC containing proteins are predicted to engage in EET for insoluble metal-oxides or electrodes (Cai et al., 2022; Leu et al., 2020a; Ouboter et al., 2024; Zhang et al., 2023). As MHCs are also responsible for the electron transfer between 7
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