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

Viruses play important roles in regulating the abundance and composition of bacterial populations in aquatic ecosystems. The bloom-forming toxic cyanobacterium is predicted to interact with diverse cyanoviruses, resulting in population diversification. However, current knowledge of the genomes from these viruses and their infection programs is limited to those of virus Ma-LMM01. Here, we performed a time series sampling at a small pond in Japan during a bloom and then investigated the genomic information and transcriptional dynamics of -interacting viruses using metagenomic and metatranscriptomic approaches. We identified 15 viral genomic fragments classified into three groups, groups I (including Ma-LMM01), II (high abundance and transcriptional activity), and III (new lineages). According to the phylogenetic distribution of strains possessing spacers against each viral group, the group II-original viruses interacted with all three phylogenetically distinct population types (phylotypes), whereas the groups I and III-original viruses interacted with only one or two phylotypes, indicating the cooccurrence of broad- (group II) and narrow (groups I and III)-host-range viruses in the bloom. These viral fragments showed the highest transcriptional levels during daytime regardless of their genomic differences. Interestingly, expressed antiviral defense genes in the environment, unlike what was seen with an Ma-LMM01 infection in a previous culture experiment. Given that broad-host-range viruses often induce antiviral responses within alternative hosts, our findings suggest that such antiviral responses might inhibit viral multiplication, mainly that of broad-host-range viruses like those in group II. The bloom-forming toxic cyanobacterium 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 of 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-range viruses, which included fifteen viral genomic fragments from blooms that were classified into three groups. Interestingly, 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 between and its viruses and fills an important knowledge gap.