PHYLOGENETIC POSITION OF THE GENUS HYDROGENOBACTER

The genus Hydrogenobacter consists of extremely thermophilic, obligate ly chemolithotrophic organisms that exhibit anaerobic anabolism but ae robic catabolism. Preliminary studies of the phylogenetic position of these organisms based on limited 16S ribosomal DNA sequence data sugge sted that they belong to one of the earliest branching orders of the B acteria. In this study, the complete 16S ribosomal DNA sequences of tw o type strains, Hydrogenobacter thermophilus TK-6 and Calderabacterium hydrogenophilum Z-829, and another isolate, Hgdrogenobacter sp. strai n T3, were determined, and the phylogenetic positions of these organis ms were examined. Our results revealed that the two type strains are m embers of a single genus, the genus Hydrogenobacter. Our results also verified the previous conclusion that the Aquifex-Hydrogenobacter comp lex belongs to a very early branching order, the ''Aquificales.'' With in this order, the relationships among the various organisms are such that only a single family, the ''Aquificaceae,'' can be recognized at this time. Given the early branching point of the ''Aquificales,'' the characteristics of these organisms support the view that the last com mon ancestor of existing life was thermophilic and suggest that this a ncestor may have fixed carbon chemoautotrophically.