Characterization of methanotrophic bacteria isolated from a subtropical paddy field

Isolation and characterization of methane-oxidizing bacteria from a subtrop ical paddy field of Ishigaki, Japan, were performed. Two strains, namely R1 6 and R18, of Methylosinus-like organisms were isolated from methane enrich ment cultures of rice rhizosphere. The isolates were motile, and stained Gr am-negative. Cells of strains R16 and RLS were short rods, while slightly c urved cells and coccobacillus-like cells were also observed for strains R16 and R18, respectively. Strain R16 was slightly smaller in size than R18. T he organisms had an optimal pH range of 6.3-6.8 and a temperature optimum o f 29-30 degrees C. Methane and methanol were used as sole carbon and energy sources. Nitrate and ammonium sulfate could serve as nitrogen sources, The acetylene reduction method showed that the strains were able to fix nitrog en. On the basis of morphological and physiological properties, and 16S rRN A gene sequence analysis, the organisms were identified as genus Methylosin us. Strain R16 shows the highest 16S rRNA gene similarity of 95.5% with Met hylosinus trichosporium, whereas R18 shows the highest 16S rRNA gene simila rity with Methylosinus sporium (95.6%). (C) 1999 Federation of European Mic robiological Societies. Published by Elsevier Science B.V. All rights reser ved.