Characterisation of three novel giant viruses reveals huge diversity among viruses infecting Prymnesiales (Haptophyta)

•We describe a new diversity of algal viruses within the Mimiviridae family.•We report the so far largest viruses infecting Prymnesiales.•Phylogeny of algal viruses does not reflect the phylogeny of their hosts.•Cross-species infectivity is widespread among algae-infecting viruses.•Our results enhance our understanding on the evolutionary history of giant viruses.

We have isolated three novel lytic dsDNA-viruses from Raunefjorden (Norway) that are putative members of the Mimiviridae family, namely Haptolina ericina virus RF02 (HeV RF02), Prymnesium kappa virus RF01 (PkV RF01), and Prymnesium kappa virus RF02 (PkV RF02). Each of the novel haptophyte viruses challenges the common conceptions of algal viruses with respect to host range, phylogenetic affiliation and size. PkV RF01 has a capsid of ~310 nm and is the largest algal virus particle ever reported while PkV RF01 and HeV RF02 were able to infect different species, even belonging to different genera. Moreover, PkV RF01 and HeV RF02 infected the same hosts, but phylogenetic analysis placed them in different groups. Our results reveal large variation among viruses infecting closely related microalgae, and challenge the common conception that algal viruses have narrow host range, and phylogeny reflecting their host affiliation.