The genus consists of extremely thermophilic, obligately chemolithotrophic organisms that exhibit anaerobic anabolism but aerobic catabolism. Preliminary studies of the phylogenetic position of these organisms based on limited 16S ribosomal DNA sequence data suggested that they belong to one of the earliest branching orders of the . In this study, the complete 16S ribosomal DNA sequences of two type strains, TK-6 and Z-829, and another isolate, sp. strain T3, were determined, and the phylogenetic positions of these organisms were examined. Our results revealed that the two type strains are members of a single genus, the genus . Our results also verified the previous conclusion that the complex belongs to a very early branching order, the “.” Within this order, the relationships among the various organisms are such that only a single family, the “,” can be recognized at this time. Given the early branching point of the “,” the characteristics of these organisms support the view that the last common ancestor of existing life was thermophilic and suggest that this ancestor may have fixed carbon chemoautotrophically.


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