219 Methanotrophic flexibility of “Ca. Methanoperedens” and SRB in meromictic Lake Cadagno phylum, with the order Desulfatiglandales representing the most closely related cultivated SRB (Figure 5A). We identified three additional Desulfobacterota class QYQD01 genomes in the GTBD, two of which, DeMMO_14 (Deep Mine Microbial Observatory) and PowLak16 (Powell Lake), classified as the same species, and all three co-occurred with Methanoperedenaceae (Figure 5A and Supplementary Figure 6). Both the Desulfobacterota class QYQD01 and DTXEO1 genomes were recovered from diverse environments such as hydrothermal vents, sediments, meromictic lakes, or iron-rich subsurface waters. Notably, in the iron-rich subsurface water metagenome, Desulfobacterota MAG (DeMMO3_14) was found alongside a “Ca. Methanoperedens” MAGs (DeMMO4_29 and DeMMO5_32; Figure 3). Using the tool Sandpiper, we screened the sequence read archive (SRA) for metagenomic reads of Desulfobacterota class QYQD01, to assess whether this class has been observed alongside Methanoperedenaceae before (Supplementary Table 10 and 11). Our search resulted in 62 metagenomes where their co-existence was detected (Figure 5B). A quarter of the sequence SRA descriptions belonged to marine and groundwater systems, followed by sediment, soil, freshwater, as well as lake water, and aquaculture waste (Figure 5B). We determined the metagenomic coverage of reads from Methanoperedenaceae and Desulfobacterota class QYQD01 in the 62 metagenomes in the ecosystems where they co-occurred (Figure 5B and Supplementary Figure 6A). The metagenomic coverage of Methanoperedenaceae to Desulfobacterota class QYQD01 was much higher in groundwater systems (0 to 360) than in marine ecosystems (0 to 25) (Supplementary Figure 6A). We also found that Methanoperedenaceae co-existed with Desulfobacterota (class QYQD01) in about 70% of the groundwater samples from Finland’s Olkiluoto Island, and 21% of those from an arsenic-contaminated site in China (Supplementary Figure 6B). The Olkiluoto Island deep subsurface sample source was also shared by the Methanoperedenaceae MAGs presented in the above presented genome tree, labelled as KR11_0.1_MetaG. mb.84 & KR_11_N3.mb.44 (Mehrshad et al., 2021) and N1.45 and s2.57 (Bell et al., 2022) (Figure 3). The marine metagenome samples were dominated by mangrove, seagrass, estuary or oilfield sediments (50%), followed by the Deep Horizon Spill Sediment (21%) (Supplementary Figure 6B). 7
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