Institut des
NanoSciences de Paris
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Séminaire général

Life in extreme hot habitats

par David Prangishvili

Studies on microbial diversity in geothermally heated ecosystems were stimulated in 70s by a finding that hyperthermophiles represent one of the primary phenotypes of the newly discovered domain Archaea. To date hyperthermophiles have been isolated from all possible water-containing biotopes of elevated temperatures : in different regions of active volcanism and active tectonics, from terrestrial hot springs and mud holes, as well as submarine hydrothermal systems. All known species are well adapted to their biotopes, being able to grow at high temperatures and extremes of pH, redox potential and salinity. Their optimal growth temperatures are above 80°C, and most of them do not grow below 60°C. The highest growth optimum, at 106°C, has Pyrolobus fumarii. Hyperthermophiles can use inorganic reactions as energy source and CO2 as carbon source, can be primary producers of organic matter and are good candidates for life without sunlight in the depth, like the deep sea or below the surface. The isolated and cultured hyperthermophiles have been assigned to 10 orders of Archaea encompassing 20 genera, and 2 orders of Bacteria. In addition to the cultured species, in different regions of high volcanism were detected by 16S rRNA sequences a plethora of putative hyperthermophiles which can not be cultivated at present. Apparently, hyperthermophiles are much more diverse on the Earth than expected in a recent past. The hyperthermophiles are close to the root of the tree of life, and it has been repeatedly suggested that the common ancestor could have been a hyperthermophile. Anyway, phylogenetic analysis clearly indicates that at least archaeal ancestor was a hyperthermophile. An unexpected and prominent feature of life in hot habitats is the surprising diversity of double-stranded (ds) DNA viruses, contradicting the widespread notion of a limited biodiversity in extreme environments. Known dsDNA viruses infecting hyperthermophilic hosts represent a diverse collection of unique morphotypes, not encountered among dsDNA viruses of mesophilic or moderately thermophilic hosts. Genome sequences confirm a specific nature of hyperthermophilic viruses, suggested by their structural uniqueness. With the current coverage of viral and cellular genomes a considerable majority of viral genes have no detectable homologs in other viruses or cellular life forms. The origin and nature of the biodiversity of hyperthermophilic viruses raise intriguing evolutionary questions.