f Characterization of Methanobrevibacter arboriphilicus SA Isolated from a Paddy Field Soil and DNA-DNA Hybridization among M. arboriphilicus strains

We isolated a methanogenic strain, designated strain SA (= DSM 7056), from an enrichment culture inoculated with a Japanese paddy field soil. Cells of this strain were strictly anaerobic, nonmotile, short rods and stained gram positive. The strain was able to use H2-CO2or formate as a methanogenic substrate. It required vitamins, but not acetate, for growth. Growth was fastest at 35 to 40°C. Methane was produced most rapidly at pH 6.0 to 7.5. The cellular lipid composition of strain SA was similar to that of M. arboriphilicus A2 (= DSM 2462). The G+C content of the DNA was 26.4 mol%. Strain SA had DNA-DNA hybridization values of more than 70% with M. arboriphilicus DH1T(= DSM 1125T). On the basis of phenotypic and genotypic characteristics, we identified strain SA as M. arboriphilicus. In the course of our identification work, the genetic heterogeneity of M. arboriphilicus was revealed by the results of DNA-DNA hybridization experiments. Although strain AZ (= DSM 744) should be classified as a member of a species distinct from the species containing the other four strains studied (DH1T, A2, DC [= DSM 1536], and SA), further phenotypic characterization will be required before a new species can be proposed.