Phylogenetic Analysis of Methanogenic Bacteria
The most recent extensive phylogenetic tree of the methanogenic bacteria presented the relationship of twenty two species and was derived from the comparison of 16S ribosomal RNA oligonucleotide catalogs (Whitman, W. B., 1985, Methanogenic bacteria, in The Bacteria, vol. 8, Archaebacteria, C. R. Woese and R.S. Wolfe ed., Academic Press, Inc., New York). This work has been expanded here. The complete sequence of the 16S ribosomal RNA was obtained for twenty nine species and a new tree was derived. The overall structure of the tree is conserved. However since the comparison of complete sequences distinguishes between fast and slow evolving organisms, additional information can be obtained. The hierarchy of branching shows that the Methanococcaceae family has the deepest branching, followed by the Methanobacteriaceae, the Methanomicrobiaceae and the Methanosarcinaceae. It appears that the groups branching more recently (Methanomicrobiaceae and Methanosarcinaceae) have evolved relatively faster than the Methanococcaceae and Methanobacteriaceae This observation matches the increase in structural and metabolic diversity observed from the Methanococcaceae to the Methanosarcinaceae. Also, extreme thermophilicity, a characteristic assumed to be ancestral and shared with all the sulfur-dependent Archaebacteria, is only found among the Methanococcaceae and the Methanobacteriaceae.