The hypothesis generally used to explain the low -K^pp) of soil methane uptake is that certain methanotrophs have developed a specialized enzyme to allow survival on the constant low level of atmospheric methane. However, there are several problems (described earlier) in interpreting the apparent kinetic coefficient Km(app). A better measure of oligotrophy is specific affinity (aO), defined as Vmax/Km, or the slope of the first-order section of a Michaelis-Menten curve. Specific affinity directly indicates how fast limiting substrate is metabolized. Measured specific affinities of cultivated methanotrophs are too low to supply maintenance energy requirements at 1.7 ppmv methane (Conrad, 1999; Knief and Dunfield, 2005). To grow on atmospheric methane alone, they would need either higher cellular Vmax(app) values (i.e. more enzyme) or lower Ks values. The existence of the putative 'high-affinity' methanotroph of Bender and Conrad (1992) is therefore supported by energetic calculations, although it is probably better defined as a 'high-specific-affinity' methanotroph.

Other evidence indicates that methano-trophs obtain all or some of their maintenance energy from atmospheric methane. In forest soils, 14CH4 is incorporated into cell material even at atmospheric concentrations (Roslev et al., 1997). Potential methanotro-phic activity declines when some soils are completely deprived of CH4 (Schnell and King, 1995; Gulledge et al., 1998), although this is not always the case (Benstead and King, 1997; Gulledge et al., 1998).

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