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Results of an in vitro assay revealed that root-associated methane consumption was a common attribute of diverse emergent wetland macrophytes from a variety of habitats. Maximum potential uptake rates (V(maxp)) varied between about 1 and 10 μmol g (dry weight)-1 h-1, with no obvious correlation between rate and gross morphological characteristics of the plants. The V(maxp) corresponded to about 2 x 108 to 2 x 109 methanotrophs g (dry weight)-1, assuming that root-associated methanotrophs have cell- specific activities comparable to those of known isolates. V(maxp) varied seasonally for an aquatic grass, Calamogrostis canadensis, and for the cattail, Typha latifolia, with highest rates in late summer. V(maxp) was well correlated with ambient temperature for C. canadensis but weakly correlated for T. latifolia. The seasonal changes in V(maxp), as well as inferences from apparent half-saturation constants for methane uptake (K(app); generally 3 to 6 μM), indicated that oxygen availability might be more important than methane as a rate determinant. In addition, roots incubated under anoxic conditions showed little or no postanoxia aerobic methane consumption, indicating that root-associated methanotrophic populations might not tolerate variable oxygen availability. Hybridization of oligodeoxynucleotide probes specific for group I or group II methylotrophs also varied seasonally. The group II-specific probe consistently hybridized to a greater extent than the group I probe, and the relative amount of group II probe hybridization to C. canadensis root extracts was positively correlated with V(maxp).

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Applied and Environmental Microbiology

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