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University of Helsinki (2014)

Evolution in salt : Genomic and structural approaches to halophilic archaea and their viruses

Jaakkola, Salla

Titre : Evolution in salt : Genomic and structural approaches to halophilic archaea and their viruses

Auteur : Jaakkola, Salla

Université de soutenance : University of Helsinki

Grade : Doctoral 2014

Résumé
Extremely saline environments include salt lakes, evaporation ponds, and terrestrial environments, such as salt deserts and underground halite deposits. They are inhabited by halophilic microbes that require salt for living. Cell densities in hypersaline waters can be as high as 107-108 cfu/ml, and most of the cells are archaeal. The number of viruses can be ten times higher than that of the cells. In buried halite, the cell counts are generally low, but viable bacteria and archaea have been isolated from samples up to Permian in age (250-280 million years).
Icosahedral tailless virus types seem to be common in hypersaline waters, based on microscopic studies. However, only few such viruses have been isolated and studied. In this thesis Haloarcula hispanica icosahedral virus 2 (HHIV-2) was studied using virological, biochemical, sequencing, lipidomic, and cryo-electron microscopy methods. HHIV-2 infects a halophilic euryarchaeal host and is virulent. It is icosahedral, tailless, and contains an inner membrane. The properties of HHIV-2 were compared to two structurally related viruses : haloarchaeal virus SH1 and thermophilic phage P23-77. The comparison revealed the evolutionary stability of the virion capsid structure, in contrast to the host-interacting structures of viruses. It was also established that different virus capsid assembly pathways can lead to identical capsid architecture.
Drill core samples from deeply buried halite deposits were used for isolating halophilic microbes. Nine novel unique archaeal strains belonging to Halobacterium and Halolamina were obtained. No bacteria or viruses could be isolated. Three archaeal isolates from 40 million years old halite were found to be polyploid. Polyploidy is connected to higher mutation resistance, which might positively affect the survival of cells inside halite deposits. One unique isolate was obtained from 123 million years old halite. The complete genomic sequence of this isolate was resolved. Based on sequence data and DNA-DNA hybridization, the isolate represented a novel species, and was named Halobacterium hubeiense. The isolate was found to be closely related to halophilic archaea residing in surface habitats.

Présentation (HELDA)

Page publiée le 16 janvier 2015, mise à jour le 4 avril 2018