Viruses of the A rchaea

Abstract

Viruses infecting members of Archaea, the third domain of life, constitute an integral, yet unique part of the virosphere. Many of these viruses, specifically the species that infect hyperthermophilic hosts, display morphotypes – for example, bottle shaped, spindle shaped, droplet shaped, coil shaped, bacilliform – not known to be associated with the other two cellular domains, Bacteria and Eukarya. The distinctiveness of the hyperthermophilic archaeal viruses extends to their genome sequences: a large majority of the predicted genes yield no sequence matches in public databases and encode proteins with exceptional structures and unknown functions. Moreover, the ways in which these viruses interact with their hosts are also unique, as indicated by a unique virion egress mechanism, which involves formation of pyramidal portals on the cell surface. Some viruses that infect extremely halophilic Archaea are morphologically highly similar to head‐tail bacterial viruses of the order Caudovirales and apparently share an ancestry with them. Identified archaeal viruses almost exclusively carry double‐stranded deoxyribonucleic acid (DNA) genomes and only a few species have single‐stranded DNA genomes. Key Concepts Archaea, the third domain of life, same as the two other domains, Bacteria and Eukarya, is parasitised by viruses. Known species of archaeal viruses have been isolated from environments with extreme conditions – extremely high temperatures, exceeding 80 °C, or extremely high salinity. Known archaeal viruses have DNA genomes, most of which are double stranded. Morphological diversity of archaeal viruses exceeds that of bacterial viruses (bacteriophages). Archaeal viruses from extreme thermal environments are enveloped and have diverse unusual morphologies, indicative of multiple independent origins. Most known viruses from highly saline waters resemble bacteriophages and share an ancestry with them. Most of the genes of hyperthermophilic archaeal viruses have no homologues in other known viruses. The knowledge on viruses of Archaea contributes to the understanding of the origin and evolution of viruses and virus–host interactions.

Marine archaea and archaeal viruses under global change

“…Research on archaea has increased exponentially over the last few years, but marine archaea and the viruses able to infect them have received little attention despite their global relevance 1 – 6 ( Figure 1 ). Moreover, most of the current knowledge on archaea and archaeal viruses is based on culturable extremophiles inhabiting peculiar high-temperature, high-salinity, or low-pH environments 7 – 9 , while the archaeal taxa most represented in the ocean remain almost completely uncultured 10 – 16 , as well as their viruses 17 . Several new archaeal phyla have been described in the last 15 years thanks to gene surveys, metagenomics studies, and single-cell next-generation sequencing projects 16 , 18 – 22 .…”