ASSOCIATIONS OF METHANOTROPHS WITH THE ROOTS AND RHIZOMES OF AQUATIC VEGETATION

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 mu mol 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 10(8) to 2 x 10(9) 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 mu M), 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.