63 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments sediment g-1 d-1 as measured in triplicate methanogenic incubations, in which error bars are generally smaller than black circles. Sulfate and methane porewater concentrations are expressed in mmol L-1. The arrow to the right indicates decreasing bottom water (BW) oxygen concentrations from Site 3 to 7. cmbsf; centimeters below the seafloor. In total, 144 metagenome-assembled genomes (MAGs) of high (>90% complete, <5% contaminated) and medium quality (>50% complete, <10% contaminated) were reconstructed from 12 co-assembled samples and screened for methane metabolism marker genes. No particulate or soluble methane monooxygenaseencoding genes, which are diagnostic of aerobic methane oxidation potential, were found in binned and unbinned contigs. Five genes encoding a methyl-coenzyme M reductase alpha subunit (mcrA) related to methane production were identified in the following four genomes. MAG 009 Methanoregulaceae had potential for hydrogenotrophic methanogenesis, and MAG 010 Methanosarcinaceae had potential for methanogenesis from H2 and CO2, formate, acetate (acss), H2 and methanol (mtaA), H2 and mono-(mtmBC) and trimethylamine (mttC), but contained two mcrA genes. While MAG 015 Methanomassiliicoccales had potential for methanogenesis from H2 and methanol (mtaA), MAG 016 Methanomassiliicoccales had potential for methanogenesis from H2 and methanol (mtaA), di-(mtbC) and trimethylamine (mttC). We also identified an unbinned mcrA sequence with a best blastp hit to “Candidatus Methanofastidiosum methylthiophilus” (KYC53403.1 NCBI accession number), which has been proposed to perform methanogenesis from methanethiol, dimethylsulfide, 3-methylmercaptopropionate, and 3-mercaptopropionate (Nobu et al., 2016). We used normalized mapped read (NMR) values of genes and normalized genome coverage (ngCOV) of MAGs as a qualitative proxy for microbial abundances with the sole purpose of comparisons between samples in this study, and interpret these data strictly in the context of sediment biogeochemistry and microbial activity rates. NMR values of mcrA genes indicated that MAG 010 Methanosarcinaceae, MAG 015 Methanomassiliicoccales, and MAG 009 Methanoregulaceae accounted for the most significant methanogen mcrA NMR changes across sites (Supplementary Figure 2), with highest NMR values in Site 5, then Site 7, and lowest in Site 3. The summed normalized genome coverage (ngCOV) of four methanogen MAGs were 2
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