The properties of methanotrophic bacteria have been reviewed extensively by Hanson and Hanson (1996). This section summarizes the aspects most relevant for understanding functional methanotrophic ecology in landfill cover soils (see also Chapter 10).
Methanotrophs are Gram-negative bacteria that are able to use CH4 as the sole carbon and energy source. They are a subset of the methylotrophs, which are able to use one-carbon compounds more reduced than HCOOH as sources of carbon and energy. What distinguishes methanotrophs from the other methylotrophs is the use of the methane monooxygenase (MMO) enzyme, which catalyzes the oxidation of CH4 to methanol.
The current taxonomy of methano-trophs was first established by Whittenbury et al. (1970), based on morphological and biochemical characteristics. A more comprehensive study including numerical taxonomy and DNA-DNA hybridization has necessitated some revisions (Bowman et al., 1993, 1994, 1995). Two types of methano-trophs, labelled type I and type II, were distinguished. Bowman and his co-workers consistently refer to these types as 'group I' and 'group II'. As of 1995, type I included the genera Methylococcus, Methylomicrobium, Methylobacter and Methylomonas, and formed the family Methylococcaceae. Type II included the genera Methylosinus and Methylocystis.
Type I methanotrophs form a distinct branch within the gamma subdivision of the Proteobacteria. They use a particulate MMO (pMMO) to oxidize CH4. Formaldehyde, a CH4 oxidation product, is assimilated using the ribulose monophosphate pathway (RuMP). Most type I methanotrophs cannot fix N2 (absence of nitrogenase activity). Exceptions are some Methylomonas and Methylococcus species.
Type II methanotrophs form a distinct branch within the alpha subdivision of the Proteobacteria. They use pMMO, but in the absence of copper a soluble enzyme (sMMO) is produced in most type II metha-notrophs and in some type I methanotrophs. This enzyme has broad substrate specificity and enables these microorganisms to oxidize chlorinated hydrocarbons and aromatic hydrocarbons. Type II methanotrophs assimilate formaldehyde via the serine pathway. They are also able to fix N2.
The RuMP pathway is more efficient than the serine pathway. Consequently, type
I methanotrophs tend to outgrow their type
II counterparts, unless inorganic nitrogen or copper limitation provides an advantage for type II species, expressing sMMO or nitro-genase activity.
In this taxonomy, Methylococcus occupies a special position within the type I genera. Species from this genus are mildly thermophilic, express sMMO and nitroge-nase activity, and sometimes use the serine pathway. For these reasons this genus was formerly referred to as type X. This nomenclature became irrelevant with the discovery of other genera with unusual properties and is no longer in use.
Novel methanotrophic species and genera are continuously being discovered. New type I genera include the psychro-philic Methylosphaera (Bowman et al., 1997), the thermophilic Methylocaldum (Bodrossy et al., 1997) and Methylothermus (Tsubota et al., 2005), the strongly clustering Methylosarcina (Wise et al., 2001) and the halophilic Methylohalobius (Heyer et al., 2005). A new type II genus is the mildly acidophilic Methylocella (Dedysh et al., 2000, 2004; Dunfield et al., 2003). Dedysh et al. (2002) even suggested a 'type III' classification for the new genus Methylocapsa, which consists of mildly acidophilic bacteria not expressing sMMO, but using the serine pathway. This genus belongs to the a-Proteobacteria and its closest relation is Methylocella.
Wise et al. (1999) discovered that the result of methanotrophic isolation by enrichment series depends on the strength of the nitrate mineral medium used, indicating that results of enrichments are not necessarily representative of the in situ ecology.
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