Page 45 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Verstraetearchaeota also appear to utilize sugars as carbon compounds (Vanwonterghem et al. 2016). Further research on the role of MCR-containing Archaea will increase our understanding of their ecosystem role.
2.2 Anaerobic oxidation of methane
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Before CH4 produced by methanogens reaches the atmosphere, anaerobic methanotrophs oxidize CH4 using a suite of electron acceptors (Conrad 2020a). The CH4 that passes this anoxic filter can ultimately be converted by aerobic CH4-oxidizing bacteria (Fig. 1).
Sulfate-dependent anaerobic methane oxidation
The anaerobic oxidation of methane (AOM) was long considered impossible due to the high activation energy needed to break the C-H bonds (439 kJ mol−1) (Thauer and Shima 2008). The discovery of counter-gradients of sulfate (SO42-) and CH4 changed this view and indicated habitats with active AOM (Reeburgh and Heggie 1977). Sulfate-dependent anaerobic oxidation of CH4 (S-AOM) is particularly intriguing since the reaction has a relatively low Gibbs free energy change of approximately -21 kJ mol−1. In marine ecosystems, S-AOM is carried out by a consortium of ANME archaea in cooperation with SO42--reducing bacteria, or possibly by ANME employing interspecies electron transfer (Knittel and Boetius 2009; Milucka et al. 2012; Scheller et al. 2016). An inverted and modified methanogenesis pathway has been proposed for the catalysis of AOM by ANME (Scheller et al. 2010; McGlynn et al. 2015; Timmers et al. 2017).
ANMEs are divided into three distinct groups: ANME-1 (Methanosarcinales- and Methanomicrobiales-related), ANME-2 (Methanosarcinales) and ANME-3 (Methanococcoides-related) (Knittel et al. 2005; Nauhaus et al. 2005; Stadnitskaia et al. 2005). The 16S rRNA gene phylogeny indicates that ANME groups are not monophyletic with each other, and the phylogenetic distance between subgroups is large, with 16S rRNA gene nucleotide sequence similarities of 75%–92% (Knittel and Boetius 2009). A recent study on ANME-1 in estuarine sediments suggests a potential switch between AOM and methanogenesis depending on which process is energy-yielding at a given time point (Kevorkian et al. 2020).
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