Cooccurrence of Broad- and Narrow-Host-Range Viruses Infecting the Bloom-Forming Toxic Cyanobacterium Microcystis aeruginosa

The bloom-forming toxic cyanobacteriumMicrocystis aeruginosa is thought to have diversified its population through the interactions between host and viruses in antiviral defense systems. However, current knowledge of viral genomes and infection programs is limited to those ofMicrocystis virus Ma-LMM01, which was a narrow host range in which it can escape from the highly abundant host defense systems. Our metagenomic approaches unveiled the cooccurrence of narrow- and broad-host-rangeMicrocystis viruses, which included fifteen viral genomic fragments fromMicrocystis blooms that were classified into three groups. Interestingly,Microcystis antiviral defense genes were expressed against viral infection in the environment, unlike what was seen in a culture experiment with Ma-LMM01. Given that viruses with a broad host range often induce antiviral responses within alternative hosts, our findings suggest that antiviral responses inhibit viral reproduction, especially that of broad-range viruses like those in group II. This paper augments our understanding of the interactions betweenM. aeruginosa and its viruses and fills an important knowledge gap.