Page 240 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 10. Co-culturing methanogens and methanotrophs in an MBR
alphaproteobacterial family Methylocystaceae and possess pmoA genes encoding the particulate methane monooxygenase that oxidizes methane at atmospheric levels (Dunfield et al. 1999; Kravchenko et al. 2010). M. sporium is a suitable candidate for the co-cultivation set- up due to its high substrate affinity (Murrell, Mcdonald and Gilbert 2000). In addition, for M. trichosporium, which is closely related to M. sporium, Km values for methane were as low as 0.8-2.0 μM depending on the strain (Joergensen and Degn 1983). Furthermore, M. sporium has often been used in previous methanogenic-methanotrophic cultures (Shen, Miguez and Bourque 1996; Miguez et al. 1999). Acetate measurements (Fig. 4) showed acetate consumption by the methanotrophic culture. Genome data confirmed the presence of genes encoding acetate kinase (AckA), acetate-CoA ligase (ACSS), and phosphate acetyltransferase (pta) in both genome bins. It is known that methanotrophs can metabolize acetate, but to our knowledge, growth on acetate and other carbon-carbon bond substrates had been thought to be limited to Methylocella and Methylocapsa species (Dedysh, Knief and Dunfield 2005; Dunfield et al. 2010). The carbon fixation pathways showed the alphaproteobacterial type II pathway of formaldehyde conversion to L-Serine via 5,10-methylenetetrahydromethanopterin (5,10- methylene THMPT).
Co-culture studies could significantly contribute to our current knowledge on methanogen- methanotroph interactions in the environment. The metabolic processes that drive ecosystem- scale GHG fluxes are dependent on the activity of both the aerobic and anaerobic microbial community members (McCalley et al. 2014). Especially the interplay between methanogens and aerobic methanotrophs is relevant, since this determines the types and quantities of GHG fluxes into the atmosphere. The use of pre-defined methanogen-methanotroph co-cultures enables the study of environmental effectors including temperature, pH, substrate, and oxygen availability on methane fluxes and methanogen-methanotroph interactions under controlled conditions. These studies could provide experimental evidence to better estimate wetland GHG fluxes. Methanogen-methanotroph interactions in the environment have recently gained more attention with the contradictory observations of oxic water column methanogenesis and methanogenesis in oxic soils (Bogard et al. 2014; Angle et al. 2017), aerobic methanotrophic activity in anoxic lake waters and sediments (Oswald et al. 2016; Martinez-Cruz et al. 2017), and fermentative activity of gammaproteobacterial methanotrophs under oxygen limitation
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