Page 259 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

experiments. This suggests that, although the potential for nitrate-dependent AOM was found, the environmental conditions were apparently not suitable for the process to be detected.
A study on a Tibetan alpine wetland found both denitrifying anaerobic methane oxidizing (DAMO) bacteria and archaea in different soil layers (Xie et al. 2020). A study by Winkel et al. on deep submarine permafrost found “Candidatus Methanoperedens”-related sequences, indicating the potential for nitrate- and iron-dependent AOM (Winkel et al. 2018). In this case, their observations were linked to high, millimolar-range nitrate concentrations, which provide an electron acceptor for nitrate-dependent AOM. With currently low nitrogen loads in pristine peatlands and permafrost ecosystems, nitrogen-AOM is unlikely to provide an important CH4 sink in these environments (Salmon et al. 2016; Mao et al. 2020; Ramm et al. 2020).
Novel discoveries within the phylum Euryarchaeota
There is a vast microbial diversity within methane cycling microorganisms. Although biological processes that generate methane can be found outside the phylum Euryarchaeota, the metabolic process of energy conservation through methanogenesis is so far considered unique to Euryarchaeota (see Chapter 2). In the last decades, dedicated enrichment procedures, metagenomics surveys, and single-cell technologies have enabled the identification of several new groups of methane cycling microorganisms.
Methanogens are currently classified into one class and seven orders. In the last decade two novel taxa, the Methanofastidiosa and Methanomassiliicoccales were added (Dridi et al. 2012; Nobu et al. 2016a). In 2018, the discovery of the extremely halophilic methyl-reducing Methanonatronarchaeum thermophilum and “Candidatus Methanohalarchaeum thermophilum” led to the proposition of a new class of Euryarchaeota, the Methanonatronarchaeia (Sorokin et al. 2017, 2018; Sorokin and Merkel 2019). They are commonly found in sediments and soil systems (Zhang et al. 2020). Sequencing-based studies also commonly detect Methanomassiliicoccales-related sequences in permafrost environments (Barbier et al. 2012; Gittel et al. 2014; Yang et al. 2017; Grodnitskaya et al. 2018; Winkel et al. 2018). Methanomassiliicoccales are subdivided in an environmental and a gastrointestinal tract (GIT) clade, although there is no clear link with the locations, and the GIT clade is also observed in soils (Dolan et al. 2016; Nkamga and Drancourt 2016). We also observed Methanomassiliicoccales-related sequences in the study on permafrost soils exposed to long-
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