Abstract The tolerance limits of extremophiles in term of temperature, pH, salinity, desiccation, hydrostatic pressure, radiation, anaerobiosis far exceed what can support non-extremophilic organisms. Like all other organisms, extremo- philes serve as hosts for viral replication. Many lines of evidence suggest that viruses could no more be regarded as simple infectious ‘‘fragments of life’’ but on the contrary as one of the major components of the biosphere. The exploration of niches with seemingly harsh life conditions as hypersaline and soda lakes, Sahara desert, polar environments or hot acid springs and deep sea hydrothermal vents, permitted to track success- fully the presence of viruses. Substantial popula- tions of double-stranded DNA virus that can reach 10 9 particles per milliliter were recorded. All these viral communities, with genome size ranging from 14 kb to 80 kb, seem to be geneti- cally distinct, suggesting specific niche adaptation. Nevertheless, at this stage of the knowledge, very little is known of their origin, activity, or impor- tance to the in situ microbial dynamics. The continuous attempts to isolate and to study viru- ses that thrive in extreme environments will be needed to address such questions. However, this topic appears to open a new window on an unexplored part of the viral world. Keywords Bacteriophages Viral diversity Viral abundance Extreme environments Deep sea subsurface environment Deserts Hot springs Hydrothermal vents Hypersaline habitats Polar ecosystems Introduction Extremophiles include organisms from the three domains of life, Archaea, Bacteria and Eukarya, which thrive in extreme environments that are characterized by physico-chemical conditions close to the limit values in which an organism can live. As bacteria and archaea are almost omni- present on the planet and have evolved for over 3.5 billion years, ‘‘extremophile’’ conjures up images of prokaryotes, especially from the domain Archaea. Although archaea are present in many moderate environments, they are still primarily considered extremists, flourishing in habitats that brave the physical limits for life, such as sulfur-rich hot acid springs and geysers, Marc Le Romancer and Me ´ lusine Gaillard contributed equally to this work. M. Le Romancer (&) M. Gaillard C. Geslin D. Prieur Laboratoire de Microbiologie des Environnements Extre ˆ mes, IUEM, Technopo ˆ le Brest Iroise, 29280 Plouzane ´ , France e-mail: marc.leromancer@univ-brest.fr Rev Environ Sci Biotechnol DOI 10.1007/s11157-006-0011-2 123 REVIEW PAPER Viruses in extreme environments Marc Le Romancer Me ´ lusine Gaillard Claire Geslin Daniel Prieur Received: 3 March 2006 / Accepted: 30 May 2006 Ó Springer Science+Business Media B.V. 2006