226 Chapter 7 QYQD01 and two “Ca. Methanoperedens” spp. labelled as DeMMO (Figure 4A and 5A) (Casar et al., 2021). They observed several genes encoding for proteins involved in iron cycling in six different fracture fluids with varying chemistry. Here, one clear difference between sites was the high sulfate concentration, ranging from 0.88 mM to 42.79 mM. The Methanoperedenaceae appeared to contribute to iron reduction the most in site D6, where sulfate was the most available from all sites. In Casar et al. (2021), SRB genomes from the “Desulfobacterales” taxa were also suggested to contribute with 2-4% to the relative metagenomic iron reduction. The second Desulfobacterota class QYQD01 genome (PowLake16_MAG 11) (Figure 5A) was sampled from the meromictic Powell Lake resembling Lake Cadagno as well as co-occurred with “Ca. Methanoperedens” species (Supplementary Figure 6A, indicated with an arrow) (Haas et al., 2019). We further explored “MAG Desulfobacterota class QYQD01” for genomic features that could be indicative of a syntrophic lifestyle with “Ca. Methanoperedens” spp. Our observations resulted in high congruence with genomic traits described for syntrophic SRB partners of marine ANME (Murali et al., 2023b; Skennerton et al., 2017), including: the putative loss of Ech hydrogenase, type IV pili formation and type VI secretion system, presence of adhesins and the conservation of large MHCs for DIET. In this regard, the largest MHC in our “MAG Desulfobacterota class QYQD01” was contained in a four-sequence operon structure that resembled the one described for SEEP-SRB 1 marine SRB as well as homologous organisms presented in the same study (Sed_Bac_MAG_1_p_Desulfobacterota_c_QYQD01_ contig_10778 ORF, 1 to 4) (Supplementary Table 7 and 8). The analyzed SEEP-SRB MHC operon structure included one 26 and another 16 heme c-type cytochromes followed upstream by a PPIase domain and downstream by a beta propeller fold protein (Skennerton et al., 2017). Our “MAG Desulfobacterota class QYQD01” also included a 26 and 12 (not 16) heme c-type cytochrome with a PPIase domain but since the contig in our study/MAG was broken downstream, we were not able to conclude whether this operon also included a gene encoding a beta-propeller fold protein. Compared to the genomic signature traits of marine SRB associated with ANME, our SRB lacked Ech hydrogenases. This observation contrasts with the NiFe group 1b hydrogenase reported for the Olkiluoto Island Deep subsurface in Finland, where sulfate-AOM was proposed via Desulfobacterales family ETH-SRB1
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