FIG. 1.

Phylogenetic relationships within the methanogenic archaea, derived from 16S evolutionary distances produced by the Kimura two-parameter correction model (12) and constructed using the neighbor-joining method (18). The tree was bootstrap resampled (4) 1,000 times. Only bootstrap values greater than 50% are shown on the internal nodes. The scale bar equals an average of five nucleotide substitutions per 100 positions. The GenBank accession numbers for these nucleotide sequences are given in parentheses. Higher taxonomic groupings are indicated as follows: 1, Methanobacteriales; 2, novel group of uncultivated archaea; 3, Thermoplasmatales; 4, Methanosarcinales; 5, Methanomicrobiales; 6, Halobacteriales; 7, Methanococcales.