61 Sulfide toxicity as key control on anaerobic oxidation of methane in eutrophic coastal sediments Site 3 at 19 cm, 20% in Site 5 at 42 cm, and 24% in Site 7 at 38 cm. Other identified putative methanogenic families within Euryarchaeota included Methanosarcinaceae, Methanofastidiosaceae, Methanomicrobiaceae, Methanospirillaceae, Methanobacteriaceae, Methanocorpusculaceae, poorly resolved Methanocellales and Methanomicrobiales families, Methanocellaceae, and Methanothermobacteriaceae, and the putative methanotroph family Methanoperedenaceae. Within the phylum Verstraetearchaeota, the putative methanogenic families Methanomethyliaceae and Methanomethylophilaceae were identified, and, within the phylum Thermoplasmatota, Methanomassiliicoccaceae. An ANME-2a-2b family had highest relative abundances, among archaeal sequences, of 32% at 2.5 cm depth at Site 3, 12% at 26 cm in Site 5, and 1% at 50 cm depth in Site 7. The putative ammonium-oxidizing Crenarchaeota family Nitrosopumilaceae reached 70% relative abundance in Site 3 at 1.25 cm and was minor in the other two sites. Of 7192 archaeal ASVs, 71% could not be classified at the family level, and their summed relative abundances varied between 22 and 86%. However, only 6% of archaeal ASVs could not be classified at phylum level. The large proportion of unclassified archaeal ASVs at family level could indicate taxonomic novelty or, alternatively, overestimation of biodiversity, a common limitation of 16S-based studies (Sun et al., 2013). 2
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