A 16S rDNA-based phylogenetic analysis of 12 thermoacidophilic members of Archaea demonstrates the
Sulfolobales to be a monophyletic group within the Crenarchaeota. Several phylogenetic dusters are
evident, sometimes not in agreement with the present taxonomic ranking. The four Sulfolobus species,
represent three well separated branches. Further taxonomic features suitable to distinguish these groups
are so far lacking, however. Therefore, a description of higher ranks appears not to be useful at present.
The same is true for Acidianus brierleyi, which is dearly separated from the type species Acidianus
infernus. On the other hand, Desulfurolobus ambivalens appears as a dose relative of Acidianus infernus
and, therefore does not represent a different genus. In line with its other physiological and biochemical
similarities we assign Desulfurolobus ambivalens to the genus Acidianus as Acidianus ambivalens comb.
nov. (type strain DSM 3772).