225 Methanotrophic flexibility of “Ca. Methanoperedens” and SRB in meromictic Lake Cadagno Methanoperedens” representative. Group III Dsr-LP have been only described in some methanogens and “Ca. Methanoperedens” spp., but not in marine ANME (Yu et al., 2018). Consistent with Echeveste Medrano et al. (2024c) we hypothesize that, given the absence of both nitrate and nitrite reductases (nrfAH) in the sediment “Ca. Methanoperedens” MAGs and the high sulfate-to-nitrate availability, the putative role of Group III Dsr-LP sulfite reductases in the recovered MAGs is most likely linked to sulfite detoxification. Our data suggest that “Sed MAG Methanoperedens 1” and “Sed MAG Methanoperedens 2” are the most plausible candidates to engage in a syntrophic interaction with Desulfobacterota class QYQD01. They clustered closer to environmental groundwater “Ca. Methanoperedens” spp., which have more sulfate available, and some had also been found to co-occur with the Desulfobacterota class QYQD01 (Figure 3). Conversely, “MnO2 MAG Methanoperedens”, “Sed MAG Methanoperedens 3” and “Sed MAG Methanoperedens 5”, appeared to be more closely related to metal-reducing “Ca. Methanoperedens” species (Figure 3). The putative MHC-enabled EET mechanisms of “MnO2 MAG Methanoperedens” and “MAG Methanoperedens 5” resemble most those of “Ca. Methanoperedens” spp. enriched in cultures amended with metal (manganese and iron) oxides or other electron acceptors (electrode and nitrate) enrichments (Figure 3) (Cai et al., 2018; Leu et al., 2020a; Ouboter et al., 2024; Zhang et al., 2023). On the contrary, the predicted syntrophic “Sed MAG Methanoperedens 1” and “Sed MAG Methanoperedens 2” harbor less homologous MHC proteins to that of known species, potentially indicating novel functionality (Figure 4 and Supplementary Figure 3). These two candidate MAGs were also more closely related to the ones observed in the Olkiluoto Island deep subsurface (Figure 3). Our co-occurrence Desulfobacterota class QYQD01 and Methanoperedenaceae biogeography study revealed a high correlation of Methanoperedenaceae to Desulfobacterota class QYQD01 in groundwater systems (Figure 5B and Supplementary Figure 6). This putative syntropy could represent a survival strategy for Methanoperedenaceae to dispose of electrons from methane oxidation when the environmental metal oxide pool gets depleted. One of the iron-rich subsurface metagenomic studies included genomes of both Desulfobacterota class 7
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