Viruses are the most abundant biological entities observed in environmental samples. They display an extraordinary morphological and genetic richness. In addition to their pathogenicity, viruses are now considered as of major influence on biogeochemical cycles, microorganisms' population regulation and more generally on the evolution of cellular genomes throughout the history of life. Viral metagenomics, i.e. the random sequencing of encapsidated nucleic acids in samples, has provided important new insights into viral diversity. These data reveals an overwhelming genetic diversity in the biosphere's viral communities sampled so far, especially in marine environments. Generally, the type of biome or environmental niche from which samples were obtained seems to be the main determinant of its composition. In addition, virome sequences obtained are generally quite distant from the reference sequences available in databases. This fact leads to the inescapable conclusion that nearly all of the virosphere's vast population is currently unknown to science, and emphasizes the need for pushing efforts toward surveying and characterizing the viral diversity.
Keywords: diversity; metagenomics; virus.