Page 49 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Hanson 1996). Studies on the competition between members of type I and type II MOB have shown that CH4, nitrogen, and copper availability can be crucial factors for success (Graham et al. 1993; Ho et al. 2013a).
The volcanic aerobic Methylacidiphilum methanotrophs belong to the phylum Verrucomicrobia (Op den Camp et al. 2009; van Teeseling et al. 2014). The verrucomicrobial methanotrophs use the Calvin cycle for CO2 fixation (Khadem et al. 2012) and are able to grow
as Knallgas bacteria on hydrogen and oxygen (Carere et al. 2017; Mohammadi et al. 2017a). 2 These methanotrophs express hydroxylamine oxidoreductase, nitrite reductase, and nitric oxide
reductase to counteract the nitrosative stress induced by high ammonium concentrations in mud volcanoes (Mohammadi et al. 2017b). The growth of verrucomicrobial methanotrophs is dependent on rare earth elements (lanthanides), which are incorporated into the active center of an XoxF-type methanol dehydrogenase (Pol et al. 2014; Picone and Op den Camp 2019). The unique properties of verrucomicrobial MOB are a striking example of the microbial diversity and physiology that remains to be explored and discovered.
Atmospheric CH4 levels were long considered too low to sustain microbial methanotrophy, but CH4 oxidation at atmospheric levels has been described in upland soils (Dunfield et al. 1999). Culture-independent studies of these soils, which have high-affinity CH4 oxidation capacity, detected novel methanotrophic bacteria within Alpha- and Gammaproteobacteria, named upland soil cluster (USC) α and γ (Knief, Lipski and Dunfield 2003; Kolb et al. 2005; Ricke et al. 2005). Recently, Pratscher et al. (2018) obtained an 85% complete draft genome of the USC- α genus within Beijerinckiaceae using combined metagenomics and targeted cell enrichments with fluorescence-activated cell sorting (FACS). In addition, recent studies have indicated that classic MOB can thrive under extremely low oxygen conditions by apparently coupling fermentative metabolism to nitrate reduction (Kits et al. 2015; Kits, Klotz and Stein 2015; Oswald et al. 2016; Gilman et al. 2017). In addition, genomic analysis of a Methylobacter species from anoxic lake water indicated the potential for mixed-acid fermentation and H2 production as potential adaptations to oxygen limitation (van Grinsven et al. 2020). Together with the observations of methanogenesis under oxic conditions, these findings may alter our understanding of the microbial controls on CH4 fluxes.
47