Abstract
While resequencing Microcystis aeruginosa (PCC7806) and its nontoxigenic mutant (PCC7806 ΔmcyB), we discovered identical unreported plasmids in both strains. These strains were separated in culture over 25 years ago, resulting in sequence divergence among their chromosomes. RNA-seq data demonstrated these plasmids were transcriptionally active during chemostat growth. Moreover, in situ metatranscriptomes from Lake Erie revealed genes like those on the PCC7806 plasmid were expressed in the environment. As we investigated plasmids in Microcystis, we found that M. aeruginosa NIES-298 also had a putatively conserved plasmid, but with phage-like features. To gain an understanding of the ecological relevance of these plasmids, we examined Lake Erie metatranscriptomes and found that transcript abundance for predicted plasmid-like contigs was significantly higher than predicted virus-like contigs across the microbial community: this trend was also present when metatranscriptomic reads were mapped to Microcystis-infecting phage and Microcystis-specific plasmid genomes. Our observations demonstrate a potential ecological importance and stability of these extrachromosomal elements in Microcystis. Additionally, this work draws attention to the potential overlap between Microcystis plasmid and phage genomes, and how this may complicate molecular investigations.