g. malate and glyoxylate), because a variety of acetate assimilation pathways convert acetate into 20s Proteasome activity these compounds (e.g. the glyoxylate shunt of the tricarboxylic acid cycle, the ethylmalonyl-CoA pathway, the citramalate cycle, and the methylaspartate cycle). In this review, we summarize the history of facultative methanotrophy, describe scenarios for the basis

of facultative methanotrophy, and pose several topics for future research in this area. Aerobic methanotrophs are widely distributed in the environment, found wherever methane : air interfaces develop, including in wetlands, bogs, agricultural, forest and urban soils, rice paddies, groundwater, landfill cover soils, among many other locations (Semrau et al., 2010). These cells play a critical role in the global carbon cycle by utilizing methane as a source of carbon and energy – it is estimated that in soils, aerobic methanotrophs consume ∼30 Tg methane year−1 (Kolb, 2009). It was initially widely believed

that aerobic methanotrophs were obligate, i.e., that these microorganisms could only grow utilizing methane or methanol, and in some cases, other C1 compounds such as formaldehyde, formate, and methylamine, but not compounds with carbon–carbon bonds (Bowman, 2006). The cause for obligate methanotrophy is still unresolved (Wood et al., 2004), and, interestingly, many reports have recently been published of methanotrophs that also able

to utilize Enzalutamide multicarbon PFKL compounds as sole growth substrates (Semrau et al., 2010). Hence, it appears that facultative methanotrophy may be more common than originally thought. In this review, the history and basis of facultative methanotrophy is summarized, as well as the implications and applications of such metabolism. The defining characteristic of a methanotroph is its ability to utilize methane as its sole carbon and energy source, and there are at least two forms of the key enzyme involved in the initial oxidation of methane to methanol, the methane monooxygenase (MMO). Most but not all methanotrophs express a membrane-bound or particulate methane monooxygenase (pMMO), while some can either express in addition, or as the unique form, a cytoplasmic, or soluble methane monooxygenase (sMMO). Phylogenetically, aerobic methanotrophs belong primarily to the Alpha- and Gammaproteobacteria, although recently aerobic methanotrophs have also been found that belong to the Verrucomicrobia phylum (Op den Camp et al., 2009; Semrau et al., 2010). The alphaproteobacterial methanotrophs can be further divided in the Beijerinckiaceae and Methylocystaceae families, while the gammaproteobacterial methanotrophs belong to the Methylococcaceae family.