Second, several publically available methanotroph genomes are not yet completely assembled, and absence of evidence does not provide see more evidence of absence. Third, the required pathway reactions could be performed by proteins whose sequence bears little or no resemblance to experimentally characterized enzymes. Clearly, more research is needed to elucidate how facultative

methanotrophs assimilate carbon from multicarbon compounds into biomass, and the increasing availability of genome sequences represents as much a great asset as a sobering reminder of our ignorance. It has been confirmed that facultative methanotrophy does indeed exist, but corresponding isolates can only utilize a small number of organic acids and ethanol to support growth, i.e., sugars cannot be used, possibly due to lack of sugar transporters and/or lack of key steps of the glycolytic pathway. Also, to date, no methanotrophs of the gammaproteobacterial phylum have conclusively been shown to be facultative. These methanotrophs present

several key differences to Alphaproteobacteria methanotrophs including, as noted above, the lack of a complete TCA cycle, as well as their utilization of the RuMP pathway for growth. One Gammaproteobacteria methanotroph, M. capsulatus Bath, has been found to have genes for the E1 and E2 subunits Selleckchem Obeticholic Acid of the 2-ketoglutarate dehydrogenase (Ward et al., 2004). At this Olopatadine time, it is

unclear under what conditions, if any, these genes are transcribed, and active enzyme synthesized. The absence of 2-ketoglutarate dehydrogenase activity may limit growth of Gammaproteobacteria methanotrophs with alternative multicarbon compounds, as well as the fact that isocitrate lyase and malate synthase are apparently missing in these microorganisms (Trotsenko & Murrell, 2008). Further, the acetate assimilation pathways described above do not lead to the production of intermediates of the RuMP pathway. Accordingly, and unlike Alphaproteobacteria methanotrophs that utilize the serine cycle, Gammaproteobacteria methanotrophs appear to be unable to use these pathways for carbon assimilation from multicarbon compounds. This may help explain why all known facultative methanotrophs utilize the serine cycle and not the RuMP pathway for carbon assimilation. We suggest that more effort be invested to isolate Gammaproteobacteria methanotrophs from environments with high acetate concentrations, for example, peat bogs and acidic forest soils, to determine if such conditions promote facultative growth in a broader phylogenetic range of methanotrophs. Molecular evidence indicates that such methanotrophs exist in these environments, particular peat bogs, but that they do not represent a significant fraction of the overall methanotrophic population (Dedysh, 2009).