Page 43 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Methanogens use a range of electron donors that are mainly produced during the anaerobic degradation of organic matter (Fig. 1). Before CH4 reaches the atmosphere, it can be oxidized by a range of anaerobic and aerobic methanotrophs that can use a suite of electron acceptors that are present in the environment (Fig. 1). This chapter provides an overview of the diversity of CH4 cycling microorganisms.
2.1 Methane production
Microbial methanogenesis was first described by Omelianski in 1890 and later experimentally confirmed by Söhngen (1906), who was the first to describe the ‘fat rod’ Methanothrix soehngenii, which produces CH4 from acetate (Huser, Wuhrmann and Zehnder 1982). Methanogens are dependent on fermentative and syntrophic processes that convert organic compounds to methanogenic substrates (Kotsyurbenko, Nozhevnikova and Zavarzin 1993; Schink 1997). Methanogens that use H2/CO2 and methylated compounds as substrates were subsequently isolated and characterized (Plugge and Stams 2010). Acetate usage appears to be limited to the genera Methanosarcina and Methanothrix (Jetten, Stams and Zehnder 1992).
Currently, there are eight well-established methanogenic orders: Methanosarcinales, Methanomicrobiales, Methanobacteriales, Methanococcales, Methanopyrales, Methanocellales, and the more recently discovered Methanomassiliicoccales, Methanonatronarchaeales and “Candidatus Methanofastidiosa” (Garrity, Bell and Lilburn 2004; Thauer et al. 2008; Dridi et al. 2012; Iino et al. 2013; Lyu and Lu 2015; Nobu et al. 2016b; Sorokin et al. 2018). Methanomassiliicoccales species (Methanomassiliicoccus luminyensis) were first discovered in human feces and use hydrogen as an electron donor to reduce methanol or methylamines to CH4 (Dridi et al. 2012). In anoxic sediments, the concerted action of acetogens and methanogens can result in the breakdown of methoxylated aromatic compounds like trimethoxybenzoate (Finster, King and Bak 1990). The acetogens cleave off the methoxy-groups and produce dimethylsulfide and methanethiol, which can subsequently be used by methylotrophic methanogens (Methanosarcina semesiae, Methanomethylovorans hollandica) employing several unique methyltransferases (Finster, Tanimoto and Bak 1992; Lomans et al. 1999; Lyimo et al. 2000). In addition, other methylated sulfur compounds, such as dimethyl sulfide and methylmercaptopropionate, can be converted by adapted
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