252 Chapter 8 Methanoperedens” and Desulfobacterota class QYQD01, a relationship that has primarily been observed in groundwater and marine systems. To investigate potential EET mechanisms, we used reference “Ca. Methanoperedens” spp., which have experimental evidence (metatranscriptomics) for the involvement of specific MHCs for EET with varying electron acceptor availability. In Chapter 7, we hypothesized that “Sed MAG Methanoperedens 1 and 2” are the most likely candidates for interaction with “MAG Desulfobacterota QYQD01” based on previously described depth-profile 16S rRNA gene sequence similarity, their close relationship to groundwater species, and the fewer homologous MHCs compared to metalreducing species. In support of this, reports from the Olkiluoto groundwater site, where putative “Ca. Methanoperedens” spp. N1.45 and S2.57 interact with Desulfobacterales family ETH-SRB1, also suggest a “Ca. Methanoperedens”-SRB-driven S-AOM process (Bell et al., 2022). The Desulfobacterota class QYQD01 we recovered exhibited potential mechanisms for interaction with “Ca. Methanoperedens”, including type IV pili, type VI secretion systems, adhesins, and large MHCs, which could facilitate EET. Remarkably, in groundwater systems, the ratio of “Ca. Methanoperedens” to Desulfobacterota class QYQD01 tends to increase, suggesting that this partnership may be a survival strategy when the metal oxide pool becomes depleted (Chapter 7). In marine systems, “Ca. Methanoperedens” and “Desulfobacterota class QYQD01” coexist at similar ratios but in lower abundance, indicating that while “Ca. Methanoperedens” can tolerate marine salinities (as discussed in Chapter 6), they may be outcompeted by the canonical ANME-SRB consortia typically found in these environments. This competitive dynamic is intriguing, particularly as “Ca. Methanoperedens”, including those from Lake Cadagno, might possess sulfite detoxification mechanisms (Group III Dsr-LP sulfite reductases), which were explored in Chapter 5 under sulfide exposure. This observation suggests that, in addition to sulfate preference, there is a niche differentiation between marine ANME-SRB S-AOM and “Ca. Methanoperedens”-driven S-AOM processes, potentially influenced by these adaptive syntrophic or detoxification strategies.
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