Doctoral Dissertations

Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Microbiology

Major Professor

Steven W. Wilhelm

Committee Members

Shawn R. Campagna, Elizabeth M. Fozo, Erik R. Zinser

Abstract

Toxic species of cyanobacteria form noxious blooms in freshwater ecosystems that provide critical drinking water, fisheries, and recreational needs worldwide. Blooms are caused principally by nutrient over-enrichment of water bodies and are exacerbated by increasing temperatures caused by global climate change. Blooms have increased in frequency and severity due to changes in land use and climate patterns that are projected to continue or accelerate in the future. Against this backdrop, there is a need for improved understanding of the ecological constraints governing cyanobacterial blooms. This dissertation presents studies addressing several aspects of this question. We demonstrated that seasonally-relevant cool temperatures dramatically increase microcystin quota in lab cultures of Microcystis aeruginosa NIES-843, providing an additional explanation for the observed pattern in which blooms tend to have higher toxin content in spring relative to mid-summer. Data from monitoring stations in Lake Erie illustrate the potential for incorporating a temperature-driven toxin phenotype into bloom prediction models. Following speculation in the literature and using E. coli as a model organism, we tested the hypothesis that microcystin-LR serves as an allelopathic agent against heterotrophic bacteria by means of inducing oxidative and envelope stress within these bacteria. We found no evidence to support this hypothesis, casting doubt on this putative role, at least for bacteria whose response is similar to E. coli. To facilitate additional research into the viral ecology of Cylindrospermopsis blooms, we sequenced and analyzed the genomes of Cylindrospermopsis raciborskii Virus (CrV) and its host C. raciborskii Cr2010. We show that CrV likely represents a clade of viruses infecting and perhaps lysogenizing filamentous cyanobacteria. Metagenomic data indicates CrV-like viruses are distributed across distant locals. The CrV genome is unique in containing an intact CRISPR array and an unusual 12-kb inverted repeat. The host genome contains a CRISPR-Cas system targeting the genome of CrV, but is apparently non-functional, perhaps due to lack of a key gene. Collectively, these features make CrV/Cr2010 an exciting system in which to investigate virus/host interactions in a bloomforming cyanobacterium.

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