Soil Microorganisms

Recent studies on the colony formation of soil bacteria opened the way to categorize soil bacteria into colony forming curve (CFC) groups of different growth rates. A bacterial culture collection comprising organisms from every CFC group is called an ecocollection (EC). One eco-collection of a paddy soil and two eco-collections of a grassland soil were comparatively studied. Sequencing 16S rDNA base sequences of strains from the three eco-collections revealed that strains of each CFC group were diverse and showed a trend to make clusters. In the case of the paddy soil eco-collection all strains belonged to Proteobacter alpha- and beta- subdivisions and Cytophaga/Flexibacter/Bacteroides division were oligotrophic. But, in the case of the grassland soil eco-collections, only the strains of Proteobacter alpha-subdivision were collectively oligotrophic. Based on these results the organization of the soil bacterial community and its evolutionary background were discussed.

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The gusA-marked Rhizobium meliloti YAO 3 M 2 and YAO 3 M 6 mutants were constructed by inserting transposon mTn 5 SSgusA 20 from Escherichia coli S 17-1 lambda-pir into the genome of Rhizobium meliloti YAO 3, a promising acid-tolerant strain with a high nitrogen-fixing ability isolated from Korean soil. Alfalfa (Medicago sativa L.) plants inoculated with the gusA-marked mutant showed a comparable nodule number and nitrogenase activity to those of the plants inoculated with the wild strain, while nodule occupancy by each mutant did not differ from that of the wild strain. These findings suggest that transposon mutagenesis did not alter the characteristics of the parent strain and both gusA-marked mutant strains could be used in the characterization of the nodulation and competitiveness of R. meliloti YAO 3. The colonization of the root surface and inside of the nodule by the gusA-marked mutants was easily determined due to the production of a blue precipitate after incubation of the nodulated roots in the GUS substrate X-gluc, while the unmarked strain was recognized by the presence of unstained bacteroids in the nodules.

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Malate dehydrogenases [EC 1.1.1.37, MDH] were purified as electrophoretically homogeneous proteins from Nitrosomonas sp. TK 794 (ammonia-oxidizing bacterium, Ns) and Nitrobacter agilis ATCC 14123 (nitrite-oxidizing bacterium, Nb). The molecular weights of the MDHs were 61.0 kDa (Ns-MDH) and 107.5 kDa (Nb-MDH), respectively, as indicated by gel filtration. Analysis by SDS-PAGE showed that these MDHs apparently consisted of two (Ns-MDH) and three (Nb-MDH) similar subunits with molecular weights of 35.5 and 35.9 kDa, respectively. The MDHs showed oxaloacetate-reducing activity with NAD^+. The K_m of the enzymes for oxaloacetate and NADH were estimated as 67 μM, 25 μM (Ns-MDH) and 74 μM, 53.6 μM (Nb-MDH), respectively. Optimal pH and temperature for the reaction were approximately 6.5-7.0, 60℃ (Ns-MDH) and 8.0-8.5, 45℃ (Nb-MDH), respectively. The enzymes were stable up to 60℃ (Ns-MDH) and 30℃ (Nb-MDH) and in the pH range between 5.5 and 9.5 (Ns-MDH) and 9.0 and 10.0 (Nb-MDH) after standing at 50℃ for 30 min. The Nb-MDH activity was strongly inhibited by N-ethylmaleimide and p-chloromercuribenzoate unlike the Ns-MDH activity. The N-terminal amino acid sequence of Nb-MDH was found to be ARNKIGLIGSGQIGGTLAHLIGLKELGNVVMFNIANGVPQ.

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The difference in the composition of the arbuscular mycorrhizal fungal flora under six different cropping systems in a Light-colored Andosol of Japan was investigated. Six cropping systems included 'fallow (weeding)-barley', 'fallow (no weeding)-barley' 'soybean-barley', 'maize-barley', 'cotton-barley' and 'sugar beet-barley'. Total number of spores after the 1 st cropping in the summer season was high in the fallow (no weeding), soybean, maize and cotton plots, and low in the fallow (weeding) and sugar beet plots. Common purslane, Henry crabgrass and common nightshade were dominant in the fallow (no weeding) plots. Pot experiment showed that in the case of Henry crabgrass the number of arbuscular mycorrhizal fungal spores increased significantly. After the 2 nd cropping (barley) in the winter season, a higher number of spores of Glomus spp. was found than after the 1 st cropping. In this study, it was demonstrated that the arbuscular mycorrhizal fungal flora was influenced by cropping systems in a Light-colored Andosol of Japan.

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