224 Chapter 7 DISCUSSION This study deepens our knowledge of the poorly explored potential syntrophic interaction between “Ca. Methanoperedens” spp. and the Desulfobacterota class QYQD01 in Lake Cadagno sediments (Figure 1-2 and Supplementary Figure 1). After screening of the SRA, we also described the potentially widespread co-existence of Desulfobacterota class QYQD01 and “Ca. Methanoperedens” spp. (Figure 3 and 5) across many groundwater and marine ecosystems (Figure 5B and Supplementary Figure 6). Long-term microcosm incubations under different electron acceptor conditions resulted in the enrichment of “MnO2 MAG Methanoperedens”, which constituted up to about 19.5% (Figure 3) of the microbial community, which aligns well with the almost ~ 20% abundance of one of the “Ca. Methanoperedens” ASVs observed (ZOTU1150) by Su et al. (2020). Molybdate addition to the manganese oxide incubation resulted in strong AOM inhibition and disappearance of the highly enriched “Ca. Methanoperedens” spp. (Supplementary Table 1 and Supplementary Figure 1). This observation, coupled to the increase in non-canonical sqr harboring SRB from the class Anaerolineales (including sqr) (Figure 1 and Supplementary Table 6) for the same enrichment, is suggestive of cryptic sulfur cycling. Sulfate can be produced from reoxidation of sulfide or sulfide minerals present in the sediment with MnO2 (Supplementary Table 1) (Yao & Millero, 1996) allowing indirectly for sulfate reduction to occur. All incubations showed a loss of putatively syntrophic “MAG Desulfobacterota class QYQD01” (Figure 2). This is especially surprising for the sulfate enrichment, but the build-up of sulfide (488.6 µM) could explain the general sulfide-toxicity derived inhibition observed for other ANMEs (Supplementary Table 1) (Dalcin Martins et al., 2022, Chapter 4). Recently, Group III Dsr-LP sulfite reductases have been linked to sulfide toxicity in a “Ca. Methanoperedens BLZ2” enriched culture (Echeveste Medrano et al., 2024a, Chapter 5) and to sulfite detoxification in the model methanogen Methanococcus maripaludis (Day Leslie et al., 2024). In the current study, we recovered one Group III Dsr-LP protein in “Sed MAG Methanoperedens 1” and “Sed MAG Methanoperedens 5” and two for “Sed MAG Methanoperedens 2” and the enriched “MnO2 MAG
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