Novel viruses of <i>Haloquadratum walsbyi</i> expand the known archaeal virosphere of hypersaline environments

Solar salterns represent unique systems with low diversity microbial communities that serve as an excellent model for studying the evolution and ecology of archaeal viruses and the interactions with their hosts. This is particularly relevant for the extremely abundant "square" archaeon Haloquadratum walsbyi, for which isolated viruses have remained elusive despite the fact that this microbe governs the salt-saturated ponds of most solar salterns worldwide. In this work, we have used cutting-edge imaging techniques, based on virus fluorescence in situ hybridization (virusFISH), and a combination of -omic techniques, at both population and single-cell levels, to provide an in-depth characterization of the Hqr. walsbyi virosphere. Our analyses have led to the identification of a new subfamily of tailed low-GC dsDNA viruses, which we propose to name "Haloquadravirinae", with host assignment confirmed by virusFISH in natural samples. Haloquadraviruses can represent more than 50% of the viral community in solar saltern viromes and infect nearly 40% of square cells in natural environments. The genetic imprint of these viruses, which are globally distributed in hypersaline environments, has provided insights into the structure of their virions and their potential life strategy. Along with the identification of other virus-like elements associated with Hqr. walsbyi through single-cell genomics, this work expands our current understanding of the archaeal virosphere.