Another possibility is that atmospheric methane oxidizers do not survive on methane alone, but also consume other substrates. Until recently, all methanotrophs were thought to be obligately methylotrophic. They grow on methane, methanol and, in some cases, formate, formaldehyde and methylamines, but not on any compounds containing carboncarbon bonds (Bowman, 2000). Therefore, potential alternative substrates for methano-trophs are limited. One of these substrates, methanol, has been shown to increase the atmospheric methane uptake rate when added to soil and methanotrophic cultures (Benstead et al., 1998; Jensen et al., 1998). The reducing power obtained from methanol oxidation presumably fuels MMO.
Dedysh et al. (2005) recently proved that several strains of the genus Methylocella are able to use the multicarbon compounds acetate, pyruvate, succinate, malate and etha-nol in addition to one-carbon compounds. Methylocella is the first clear example of a facultative methanotroph. With this discovery it now seems more plausible that metha-notrophs in soil can survive on alternative compounds in addition to methane and methanol. Acetate stimulated atmospheric methane oxidation in a tundra soil (West and Schmidt, 1999). This could be a secondary effect via enhanced methanogenesis, but the existence of facultative methano-trophs suggests that direct stimulation of the methanotroph population by acetate is also possible. Unfortunately, Methylocella contain only sMMO, which has a lower affinity for methane than pMMO has, and is therefore less likely to be used for atmospheric methane oxidation. The methane oxidation threshold for Methylocella is much higher than 1.7 ppmv (Dedysh et al., 2005). It remains to be seen whether there are facultative methane oxidizers with a pMMO.
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