Unrooted 16S phylogenentic tree (constructed using ClustalW and Phylip 3.5c available on http://mobyle.pasteur.fr) of extremely thermoacidophilic archaea (Topt ≥60°C, pHopt <4), compiled from [8,9,11,14,72,73]. Genomes are denoted as Ssequenced or Sipsequencing in progress, according to www.genomesonline.org. Organisms denoted in “ ” have not yet been described in full detail. Accession numbers for 16S rRNA are listed in parentheses. Sso = S. solfataricus (SSOr03), “Sis”=”S. islandicus” strain M14A (AY247895), Ssh=S. shibatae (M32504), Sto=S. tokodaii (ABO22438), “Syang”= ”Sulfolobus yangmingensis” (AB010957), Sowh= Sulfurisphaera ohwakuensis (D85507), Saci= S. acidocaldarius (D14876), “Sthur” = “Sulfolobus thuringensis” (X90485), Sazo=Stygiolobus azoricus (D85520), Smet= S. metallicus (U40813), Msed=M. sedula (Msed_R0026), Mpru= Metallosphaera prunae (X90482), Mhak=Metallosphaera hakonensis (D86414), Abr=Acidianus brierleyi (X90477), “Aten”= ”Acidianus tengchongensis” (AF226987), Ainf=A. infernus (X89852), Aam=Acidianus ambivalens (D85506), Asuf=Acidianus sulfidivorans (AY907891), Aac=Acidolobus aceticus (AF191225), Cmaq=Caldivirga maquilingensis (ABO13926), Posh=Picrophilus oshimae (X84901), Pto=Picrophilus torridus (PTOr02), Taci=Thermogymnomonas acidicola (AB269873), Tvn=Thermoplasma volcanium (Tvnr04), DHVE2 represented by 16S of A. boonei (DQ451875). No 16S sequences are available for extreme thermoacidophiles Sulfurococcus yellowsonensis and Sulfurococcus mirabilis.