Tunturi virus isolates and metagenome-assembled viral genomes provide insights into the virome of Acidobacteriota in Arctic tundra soils

Acidobacteriota are phylogenetically and physiologically diverse soil bacteria that play important roles in key ecological processes. Still, surprisingly little is known about their viruses. Here, we report five virus isolates, Tunturi 1-5, that were obtained from Arctic tundra soils, Kilpisjärvi, Finland (69°N), using Tunturibacter spp. strains as hosts. The new virus isolates have tailed particles with podo- (Tunturi 1, 2, 3), sipho- (Tunturi 4), or myovirus-like (Tunturi 5) morphologies. The dsDNA genomes of the viral isolates are 63–98 kbp long, except Tunturi 5, which is a jumbo phage with a 309-kbp genome. Tunturi 1 and Tunturi 2 share 88% overall nucleotide identity, while the other three are not related to one another. Over half of the open reading frames in Tunturi genomes have no homologs in the NCBI nr database. Viral diversity in Kilpisjärvi soils was further assessed using a metagenomic approach. A total of 1938 viral operational taxonomic units (vOTUs) were extracted from bulk soil metagenomes, of which 46 represented >90% complete virus genomes. Almost all vOTUs (98%) were assigned to the class Caudoviricetes. For 338 vOTUs, putative hosts were predicted, including Acidobacteriota and other common soil phyla involved in carbon and nitrogen cycling. We also observed 16 putative Terriglobia-associated proviruses in Kilpisjärvi soils, being related to proviruses originating from soils and other distant biomes. Approximately genus- or higher-level similarities could be observed between Tunturi viruses, Kilpisjärvi vOTUs, and other soil vOTUs, suggesting some shared viral diversity across soils. On a local scale, viral communities, including acidobacterial vOTUs, were habitat-specific, being driven by the same environmental factors as their host communities: soil moisture, SOM, C, N content and C:N ratio. This study represents a comprehensive analysis of Acidobacteriota-associated viruses residing in Arctic tundra soils, providing isolates as laboratory models for future studies and adding insights into the viral diversity and virus-host interactions in these climate-critical soils.