Estimation of genome sizes of hyperthermophiles

Genomes of various hyperthermophilic and extremely thermophilic prokaryotes were analyzed with respect to size, physical organization, and 16S rDNA copy number. Our results show that all the genomes are circular, and they are in the size range of 1.6-1.8 Mb for Pyrodictium abyssi, Methanococcus igneus, Pyrobaculum aerophilum, Archaeoglobus fulgidus, Archaeoglobus lithotrophicus, and Archaeoglobus profundus (the two bacteria Fervidobacterium islandicum and Thermosipho africanus possess genomes of 1.5-Mb size). A systematic study of all validly described species of the order Sulfolobales revealed the existence of two classes of genome size for these archaea, correlating with phylogenetic analyses. The Metallosphaera-Acidianus group, plus Sulfolobus metallicus, have genomes of ca. 1.9 Mb; the other members of the order Sulfolobales group possess genome > 2.7 Mb. The special case of stygiolobus azoricus is discussed.