## 2 Methane Metabolism in Methanotrophs  
Methanotrophs play important roles in global methane cycling and in the degradation of harmful substances. They use methane as a sole carbon source and were isolated in several conditions where methane and oxygen are both present. Freshwater and sediments are sinks of atmospheric methane. A variety of different methanotrophs inhabit these environments, and _Methylomonas_, _Methylobacter_, _Methyllosarcina_, _Methyllococcus_, and _Methyllosoma_ are predominant. Landfills and rice field soils are major sources of atmospheric methane. Studies have reported that landfills and rice field soils contain a variety of methanotrophs, such as _Methylomonas_, _Methylobacter_, _Methyllosarcina_, _Methylomicrobium_, _Methylococcus_, _Methyloladlum_, _Methylocystis_, and _Methyllosinus_. The distribution and abundance of methanotrophs in these methane-generating environments are affected by oxygen availability. Methanotrophs are also found in various extreme environments such as hot springs, the Antarctic, peatbogs, and deep-sea environments [15]. Methanotrophs are bacteria that have the ability to assimilate methane. This has always been considered a very unique microbial function. Over the past decade, knowledge of methane metabolism has been broadened by the help of systems biology approaches [16]. However, it also highlighted a large amount of fundamental knowledge gaps of methanotrophs that must be addressed to exploit the full potential of methanotrophs [17]. Oxidation of methane is carried out by methane monooxygenase (MMO), a membrane-associated particulate methane monooxygenase (pMMO) and soluble methane monooxygenase (sMMO) [18; 19; 20]. After that, methanol dehydrogenase (MDH) oxidized methanol into formaldehyde, which is then assimilated through C1 metabolism [18; 19; 20]. While calcium-dependent MDH encoded by _mxaF1_ has been well-investigated, a novel MDH (i.e., lanthanide-dependent MDH encoded by _xoxF_) has also been discovered [21]. The role of _xoxF_ in methane and methanol oxidation and the divergence and wide distribution of these enzymes among bacterial taxa demonstrated the importance of _xoxF_ enzymes environmentally, and furthermore, _xoxF_ may be ancestral to _mxaF_. There are two types of methanotrophs--\(\gamma\)-proteobacteria (Type I) and \(\alpha\)-proteobacteria (Type II). The distinction between these groups is based on the process of assimilating formaldehyde, the organization and arrangement of cell membranes, and related characteristics such as cell morphology [22; 23]. Typically, type I methanotrophs assimilate formaldehyde through the RuMP pathway (including Type X _Methylocolatum_ and _Methyllococcus_). They also form a flat, disc-shaped inner cytoplasm and have a TCA cycle lacking \(\alpha\)-ketoglutarate dehydrogenase activity [24]. However, there have been many novel findings related to the methane metabolism of this group that have been reported recently. For example, \({}^{13}\)C carbon tracings were used to discover a complete oxidative TCA cycle operating during methanotrophic growth in _Methylomicrobium buryatense_ 5 GB1 [25]. Others demonstrated that the Embden Meyerhof pathway (EMP) is the main route for C1 assimilation in _Methylomicrobium alcaliphium_ 20Z and _Methylomonas_ sp. LW13 [26; 27]. The assimilation of formaldehyde in type II methanotroph is through the serine pathway. First, formaldehyde is oxidized to formate, subsequently incorporating into biomass via tetrahydrorofolate-linked pathways and the serine cycle. The serine cycle first converts formaldehyde and CO\({}_{2}\) into C3 and C4 compounds, converts them to acetyl-CoA, and sends them to the TCA cycle (carbon dioxide accounts for about 50% of the total carbon) [18]. It has been hypothesized that an efficient pathway for C1 assimilation requires the coupling of the serine cycle with a glyoxylate regeneration pathway such as a glyoxylate shunt or an ethylmalonyl-CoA (EMC) pathway.