[Viral diversity in deep-sea hydrothermal vents and in hot terrestrial springs].

Our knowledge of the diversity of the viruses infecting prokaryotic micro-organisms from extreme environments still remains very rudimentary. With about 5 150 viruses of prokaryotes described to date, only forty, were isolated from Archaea (Halophiles, methanogens, thermoacidophiles or hyperthermophiles). Nevertheless, the studies undertaken recently on hyperthermophilic Archaea from terrestrial or oceanic hydrothermal environments suggest the existence of an impressive morphological and genomic viral diversity. Among the different morphotypes observed, the lemon-shaped type prevailed but rigid rods, filaments and unique pleomorphic morphologies never yet observed were also detected. The majority of these new viruses was isolated from the phylum Crenarchaeota, mostly among representatives of the order Sulfolobales, whereas only one virus was described in the hyperthermophilic members of the phylum Euryarchaeota. Analysis of the genomes of these new viruses show that 90 to 100 % of the predicted proteins were not related to anything previously reported. The viruses of the hyperthermophiles thus represent an important reservoir of new proteinic structures and new biological functions.