Occurrence of hydrogen-oxidizing Ralstonia species as primary microorganisms in the Mt. Pinatubo volcanic mudflow deposits

Ralstonia eutropha is an aerobic, hydrogen-oxidizing, facultative chemolithotrophic bacterium. Recently, R. eutropha-related bacteria have been found to predominate in the cultural bacterial communities of fresh mudflow deposits from Mt. Pinatubo, the Philippines. In the present study, the R. eutropha-related strains were examined for their chemolithotrophic growth in the presence of H-2, O-2, and CO2 (85:5:10) and for their phylogenetic relationship with R. eutropha. Eleven of the 12 strains tested grew in the presence of H-2, O-2, and CO2, although the growth was not as rapid as that of the well-characterized H-2-oxidizing strain, R. eutropha H16. To examine the hydrogenase activity, membrane and soluble fractions were prepared from R. eutropha-related strain 1245 and its activity was assayed with methylene blue and NAD as electron acceptors. A significant hydrogenase activity was detected in both membrane and soluble fractions of the cell-free extract. The 16S rDNA analysis of the 12 strains of R. eutropha-related bacteria revealed that they could be classified into two clusters, both of which were clearly separated from the cluster of R. eutropha sensu stricto. The probable ecological niche of H2-oxidizing Ralstonia species in the volcanic mudflow deposits was examined.