Page 44 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 2. The diversity of methane cycling microorganisms
methylotrophic methanogenesis pathways of Methanosarcinales strains (Kurth, Op den Camp and Welte 2020). Tertiary amines like choline (N,N,N-trimethylethanolamine) and betaine (N,N,N-trimethylglycine) can function as specific methanogenic substrates in specialized Methanococcoides and Methanolobus strains (Kurth, Op den Camp and Welte 2020). The list of methanogenic substrates was recently further expanded to include the direct use of methoxylated aromatic compounds by methoxydotrophic Methermicoccus shengliensis (Methanosarcinales) found in coal beds and oil reservoirs (Cheng et al. 2007; Mayumi et al. 2016). Two novel classes, Methanonatronarchaeia and “Candidatus Methanofastidiosa”, were also recently discovered (Nobu et al. 2016a; Sorokin et al. 2017, 2018). Methanonatronarchaeum thermophilum and “Candidatus Methanohalarchaeum thermophilum”, which are closely related to Halobacterium, were detected in and enriched from hypersaline soda lakes (Sorokin et al. 2017, 2018). “Candidatus Methanofastidiosum methylthiophilus” has the metabolic potential for methanogenesis through methylated thiol reduction using a methylated-thiol:coenzyme M methyltransferase (Nobu et al. 2016b). These findings indicate that methanogenic archaea might include more extremophilic and metabolically versatile members than those currently known.
The observation of the acetoclastic “Candidatus Methanothrix paradoxum” in oxygenated soils (Angle et al. 2017) and indications of methanogenesis under oxic conditions (Wagner 2017) are striking since methanogens are considered obligate anaerobes. The occurrence of CH4 production in oxic environments could alter our view of CH4 dynamics in methanogenic ecosystems.
It remains a debate whether methanogenesis occurs outside the phylum Euryarchaeota. The discovery of Bathyarchaeota and V erstraetearchaeota metagenome-assembled genomes (MAGs) comprising methyl-coenzyme M reductase (MCR) genes indicates that methanogenesis might be more widespread in the archaeal domain than previously thought (Evans et al. 2015; Vanwonterghem et al. 2016). A recent study on MCR diversity in Archaea has shown that MCR-encoding MAGs are widespread an anoxic ecosystems (Wang et al. 2019). These MCRs are, however, linked to the metabolism of non-CH4 alkanes (Wang et al. 2019). In addition, several studies consider Bathyarchaeota anaerobic heterotrophs that assimilate sedimentary organic carbon compounds (Lazar et al. 2016; Xiang et al. 2017).
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