Identification of the functionally active methanotroph population in a peat soil microcosm by stable-isotope probing

The active population of low-affinity methanotrophs in a peat soil microcosm was characterized by stable-isotope probing. "Heavy" C-13-labeled DNA, produced after microbial growth on (CH4)-C-13, was separated from naturally abundant C-12-DNA by cesium chloride density gradient centrifugation and used as a template for the PCR. Amplification products of 16S rRNA genes and pmoA, mxaF, and mmoX, which encode key enzymes in the CH4 oxidation pathway, were analyzed. Sequences related to extant type I and type 11 methanotrophs were identified, indicating that these methanotrophs were active in peat exposed to 8% (vol/vol) CH4. The C-13-DNA libraries also contained clones that were related to beta-subclass Protcobacteria, suggesting that novel groups of bacteria may also be involved in CH4 cycling in this soil.