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Date of Graduation
5-15-2025
Semester of Graduation
Spring
Degree Name
Master of Science (MS)
Department
Department of Biology
First Advisor
Louie Wurch
Second Advisor
Ray Enke
Third Advisor
Harry Hu
Abstract
Cyanobacterial harmful algal blooms (cHABs) occur globally, excluding Antarctica, and often involve toxin-producing species such as Microcystis aeruginosa. These blooms threaten both ecosystems and public health, making it essential to understand their underlying drivers. Cyanobacteria generate reactive oxygen species (ROS) during photosynthesis, which can cause cellular damage unless neutralized. Some, like M. aeruginosa, lack the genetic capacity to detoxify ROS and rely on associated bacterioplankton for protection. To investigate the role of phycosphere bacteria in mitigating ROS stress, we conducted an incubation experiment using samples collected throughout the 2023 Lake Erie bloom season. Samples were treated with 10 µM hydrogen peroxide (H2O2) and collected at five timepoints (0, 30 min, 2 hr, 8 hr, 24 hr) for analyses including cell counts, fluorescence, microcystin concentration, and RNA-Seq. We focused on bacterial genes involved in oxidative stress defense—catalase (katG), superoxide dismutase (sodB), and alkyl hydroperoxide reductase (ahpC)—normalized to the housekeeping gene rpoB across 23 genera previously linked to Microcystis or oxidative stress response. Results showed cyanobacterial decline following H2O2 exposure. Four bacterial genera—Chitinilyticum, Exiguobacterium, Limnobacter, and Polynucleobacter—exhibited significant changes in gene expression for all three enzymes. These genera represented three functional response types: immediate “first responders,” delayed “long-term defenders,” and those with reduced expression post-exposure, suggesting possible energy conservation strategies. Each genus displayed distinct roles depending on the enzyme assessed, underscoring functional differentiation in microbial responses to ROS. To further explore these interactions, scanning electron microscopy (SEM) confirmed direct attachment of heterotrophic bacteria to Microcystis cells, supporting the hypothesis that cyanobacteria outsource oxidative stress defense to surrounding microbial communities. This integrated molecular and imaging approach emphasizes the importance of microbial partnerships in the persistence and resilience of high-biomass cHABs, offering new insights into bloom dynamics and potential mitigation strategies.
