New Finnlakevirus isolate FLiP-2 provides insight into the ecology of ssDNA phages in Flavobacterium hosts

Life cycle details or ecological impact are well characterized only for a few ssDNA phages. The Finnlakeviridae family includes one species, Finnlakevirus FLiP. Here, using the same Flavobacterium host and sampling location, we isolated a new strain designated FLiP-2, with 96,7 % genetic identity to the original isolate FLiP. To understand the ecology of this Flavobacterium-infecting phage species, we explored the host interactions of the two FLiP strains and a dsDNA Flavobacterium phage MaF61 under various conditions representing those encountered in their natural habitats in boreal lakes including different temperatures, anoxic conditions, and in the presence of different nutrients and antibiotics. While FLiP and FLiP-2 had similar virion stability outside the host, they exhibited significant differences in plaque morphology and infectivity. FLiP-2 could not replicate in the presence of ampicillin, whereas FLiP thrived even at high concentrations. Both strains of Finnlakevirus FLiP propagated better under or after stress exposure compared to MaF61. Additionally, Finnlakevirus FLiP plaques appeared far from the original infection site, particularly in response to stress, suggesting latent presence within a motile or filamentous bacterium. In conclusion, Finnlakevirus FLiP showed remarkable flexibility in host-interactions being well adapted to fluctuating conditions in boreal freshwaters.