Page 48 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 2. The diversity of methane cycling microorganisms
“Candidatus Methanoperedens nitroreducens” that coupled the reduction of particulate forms of iron and manganese to methane oxidation. Cai et al. (2018) described a 1,100 day enrichment of “Candidatus Methanoperedens ferrireducens” that used methane to reduce Fe(III), possibly using several highly expressed multiheme cytochrome c proteins. Most recently, Leu et al. (2020) reported on the enrichment of novel members of “Candidatus Methanoperedens manganicus” and “Candidatus Methanoperedens manganireducens” in a bioreactor inoculated with an enrichment of “Candidatus Methanoperedens ferrireducens” and fed with methane and Mn(IV) oxides. These studies provide valuable insights into Fe-AOM organisms that can provide an important natural CH4 sink in anoxic environments.
2.3 Aerobic oxidation of methane
Methane that is not oxidized by anaerobic methanotrophs can reach the oxic layer of sediment or soil and undergo conversion by aerobic methanotrophs. Aerobic microbial oxidation of CH4 was first described in 1906 (Söhngen 1906). Based on the isolation and description of numerous aerobic CH4-oxidizing bacteria (MOB), it was long assumed that microbial CH4 oxidation was only possible under oxic conditions (Whittenbury, Phillips and Wilkinson 1970). MOB belong to Alphaproteobacteria (type II), Gammaproteobacteria (type I) and the phylum Verrucomicrobia (Trotsenko and Murrell 2008; Op den Camp et al. 2009; Semrau, DiSpirito and Yoon 2010). Aerobic methanotrophs are found in virtually all ecosystems, from acidic permafrost-affected peatlands (Dedysh et al. 2000, 2004) to volcanic mud pots with temperatures up to 70°C and pH values as low as 1 (Dunfield et al. 2007; Pol et al. 2007).
Taxonomically, based on their difference in metabolism and morphology, proteobacterial methanotrophs have been divided into two groups: type I and type II. Type I methanotrophs belong to γ-Proteobacteria and are grouped in the family Methylococcaceae, which to date includes 18 genera (Bowman 2011; Knief 2015). They prevail in both terrestrial and marine CH4-rich environments, utilizing CH4 for both assimilation and energy generation (Hanson and Hanson 1996). Type II methanotrophs are represented by α-Proteobacteria, including five known genera (Knief 2015), which mostly thrive in terrestrial environments low in ambient CH4 by utilizing even atmospheric concentrations (Holmes et al. 1999; Bull et al. 2000). They assimilate only about half of their C from CH4, with the rest originating from CO2 (Hanson and
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