Comparative Ecophysiology and the effects of the mobilome on Microcystis physiology and genomic architecture

Harmful cyanobacterial blooms (HCABs) are an increasing threat to the preservation of freshwater systems. One of the top offenders of HCAB formation in freshwater systems is Microcystis aeruginosa, a toxic cyanobacterium with a global distribution. The toxin produced by Microcystis, called microcystin, is a hepatotoxic secondary metabolite. While microcystin poses an ecological threat, why the compound is produced by Microcystis is still unknown. In Chapter 2, we performed an ecophysiological comparison of a toxic wildtype Microcystis strain, and a non-toxic Microcystis mutant to elucidate microcystin’s role in the cell. This was done during chemostat growth under optimal and suboptimal growth temperatures, and involved ROS assays, photo-physiological measurements, microcystin quota, and a transcriptomic time series. In Chapter 3, we investigated transposon movement between the toxic Microcystis wildtype and non-toxic mutant, which were thought to be genetically identical. In some instances, natural genomic rearrangements that occurred during long-term batch culture growth altered phenotype, which may give insight into microcystin’s cellular role. These investigations were done using comparative genomics approaches after genome resequencing, growth assays, PCR validation, and transcriptomics. In Chapter 4, we found a yet undescribed identical plasmid in our toxic Microcystis wildtype and non-toxic Microcystis mutant, despite being a well-used system to study microcystin production for >20 years, which is how long ago the mutant was created. In this chapter we used protein models to help elucidate the plasmid gene functions, investigated gene flow between the plasmid and genome, and looked at plasmid activity in vitro and in situ using transcriptomics and meta-transcriptomics. As little is known about Microcystis plasmids in general, we extended this work looking at plasmids in other Microcystis strains, and the role they play in host physiology, cyanophage infection, and their ecological relevance.