prokaryotes - bacteria see Domain Bacteria Woese et al. 1990
   Domain "Archaea" Woese et al. 1990
     Phylum "Euryarchaeota" Garrity and Holt 2001
       Class Thermoplasmata Reysenbach 2002 [monotypic]
         Order Thermoplasmatales Reysenbach 2002
           Family Picrophilaceae Schleper et al. 1996 [monotypic]
             Genus Picrophilus Schleper et al. 1996