| Summary: | Two approaches can be used for exploring the microbial diversity of soil samples: analysis of the total DNA using metagenomics approches and reactivation of giant viruses capable of infecting protozoa of the genus Acanthamoeba castellanii. We produced 12 metagenomes from frozen and unfrozen soils as well as reactivated 4 new viruses of wich Mollivirus kamchatka was the most unexpected. Since the reactivation in 2015 of two new giant viruses: pithovirus and mollivirus from a 30,000 years old permafrost sample no other mollivirus isolate have been described. The reactivation of M. kamchatka from a modern cryosol sample was therefore the opportunity to carry out a comparative genomic analysis between these two isolates. These analyzes revealed that the majority of genes are under strong negative selection pressure. In addition, the presence of a "super core-genome", common between pandoraviruses and molliviruses, has led to the Molliviridae being proposed as a new family of giant viruses sharing a distant evolutionary history with pandoraviruses. Particular attention was paid to the study by electronic microscopy of the structure of the mollivirus capsid, making it possible to precisely describe the stages of the assembly of the capsids of this new viral family. The discovery of a new pandoravirus, P. Y4, in a cryosol sample exhibiting peculiarities in terms of genome structure was an opportunity to examine the fine structure of pandoravirus’ genomes. Finally, the analysis of the metagenomes made it possible to confirm the presence of reactivated viruses in the samples and raised many questions about the diversity of truly infectious microorganisms contained in its soils Deux approches ont permis l'exploration de la biodiversité microbienne d'échantillons de cryosol: l’analyse de l'ADN total des échantillons par métagénomique et la réactivation de virus géants capables d'infecter des protozoaires du genre Acanthamoeba castellanii. Ici, 12 métagénomes de sols gelés et non gelés ont été produits. De plus, 4 ...
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