<p><strong>Abstract.</strong> <i>Sphagnum</i> peatlands are important ecosystems in the methane cycle. Methanotrophs living inside the dead hyaline cells or on the <i>Sphagnum</i> mosses are able to act as a methane filter and thereby reduce methane emissions. We investigated in situ methane concentrations and the corresponding activity and diversity of methanotrophs in different <i>Sphagnum</i> dominated bog microhabitats. In contrast to the Northern Hemisphere peat ecosystems the temperate

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... th American peat bogs are dominated by one moss species; <i>Sphagnum magellanicum</i>. This permitted a species-independent comparison of the different bog microhabitats. Potential methane oxidizing activity was found in all <i>Sphagnum</i> mosses sampled and a positive correlation was found between activity and in situ methane concentrations. Substantial methane oxidation activity (23 μmol CH₄ gDW⁻¹ day^&amp;minus;1) was found in pool mosses and could be correlated with higher in situ methane concentrations (>35 μmol CH₄ l⁻¹ pore water). Little methanotrophic activity (&amp;lt;0.5 μmol CH₄ gDW⁻¹ day^&amp;minus;1) was observed in living <i>Sphagnum</i> mosses from lawns and hummocks. Methane oxidation activity was relatively high (>4 μmol CH₄ gDW⁻¹ day^&amp;minus;1) in <i>Sphagnum</i> litter at depths around the water levels and rich in methane. The total bacterial community was studied using 16S rRNA gene sequencing and the methanotrophic communities were studied using a <i>pmo</i>A microarray and a complementary <i>pmo</i>A clone library. The methanotrophic diversity was similar in the different habitats of this study and comparable to the methanotrophic diversity found in peat mosses from the Northern Hemisphere. The <i>pmo</i>A microarray data indicated that both alpha- and gammaproteobacterial methanotrophs were present in all <i>Sphagnum</i> mosses, even in those mosses with a low initial methane oxidation activity. Prolonged incubation of <i>Sphagnum</i> mosses from lawn and hummock with methane revealed that the methanotrophic community present was viable and showed an increased activity within 15 days. The high abundance of methanotrophic <i>Methylocystis</i> species in the most active mosses suggests that these might be responsible for the bulk of methane oxidation.</p>