In situ localization of two methanotrophic strains in the rhizosphere of rice plants.

Methanotrophic bacteria play an important role in regulating the methane flux from rice fields to the atmosphere. The abundance of methane-oxidizing bacteria is very heterogeneous along the rice root as revealed by most probable number (MPN) counts and by scanning confocal laser microscopy (SCLM). Two methanotrophic strains. Rp1 and Rp2, were isolated from high dilutions of MPN counts from the rhizoplane of rice roots grown in natural rice field soil. We used monoxenically grown rice plants to relocalize these strains on and in rice roots by means of polyclonal antisera and 16S rRNA probes. Strain Rp1 was even detected in xylem vessels. The two isolates also were able to recolonize roots grown in natural soil under competitive conditions. We found large colonies in deep grooves on the root surface between iron oxide precipitates. Some cells also were observed in the root cortex. The distribution of methane-oxidizing bacteria probably reflects the changing availability of methane and oxygen in the rhizosphere