67 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments Nine candidate genes encoding electron-carrying ferredoxins were identified, as well as several FrhB/FdhB/FpoF paralogs (Supplementary Table 4). A single subunit fpoF could be part of the F420H2 dehydrogenase complex fpoDCBAONMLKJ1J2IH, and an frhB-fqoF (K00441, K22162) gene fusion could encode a protein to couple ferredoxin oxidation to F420H2 production, as potential Fpo/Fqo-dependent ferredoxin oxidation pathways (Chadwick et al., 2022). Moreover, three other genes with homology to FrhB of Candidatus Methanoperedens nitroreducens strain BLZ1 (Arshad et al., 2015) were found: (i) the first as a single subunit, (ii) the second immediately upstream of mvhD, hdrA2, another mvhD, then hdrABCC, and (iii) the third as 2xfrhB-fsrC fusion (K00441-K00441-K21816). The genes fsrNC encode an F420-dependent sulfite reductase in Methanocaldococcus jannaschii (Johnson & Mukhopadhyay, 2005) (EC: 1.8.98.3), which detoxifies sulfite while reducing it to sulfide. Furthermore, the succinate dehydrogenase membrane subunits sdhCD were absent, while sdhAB were present and sdhB was fused with tfrB (K00239, K00240-K18210), which encodes the CoM/CoB-fumarate reductase subunit B (EC: 1.3.4.1) characterized in Methanobacterium thermoautotrophicum strain Marburg (Heim et al., 1998). In M. thermoautotrophicum, TfrA harbors FAD-binding motifs and the catalytic site for fumarate reduction, while TfrB harbors one [2Fe-2S] cluster, two [4Fe-4S] clusters, and the catalytic site for CoM-S-H and CoB-S-H oxidation. Therefore, we hypothesize that in ANME-2 represented by MAG 011, electrons from succinate oxidation could be used to generate heterodisulfide for the first step in methane oxidation instead of flowing to the electron transport chain. Finally, a complete Rnf complex, involved in ferredoxin recycling and proton gradient generation in ANME (Yan & Ferry, 2018), was identified, as well as a downstream c7 family-octaheme cytochrome as previously reported in ANME-2 (Wang et al., 2014). Other cytochromes were also identified in this genome: three c3-family cytochromes containing 11 or 12 heme-binding motifs, an S-layer protein containing 21 heme-binding motifs, and two FeGenie-identified outer membrane cytochrome omcZ sequences with 7 and 8 heme-binding motifs (Supplementary Table 4), which could mediate extracellular electron transfer to a syntrophic partner or metallic terminal electron acceptor. 2
RkJQdWJsaXNoZXIy MTk4NDMw