Laura Reitz Dissertation Defense

Cyanobacterial harmful algal blooms (cHABs) are the proliferation of a diverse community of photosynthetic bacteria in marine and freshwater ecosystems. cHABs are globally distributed and pose risk to humans and wildlife health due to their ability to produce secondary metabolites that have toxic effects, known as cyanotoxins. Changes in the climate such as increased temperature and precipitation, which causes excess loading of nutrients such as nitrogen and phosphorus from runoff into waterways, promote proliferation of cHABs. Microcystis spp. are often the dominant genera in freshwater cHABs globally and produce the cyanotoxin microcystin, which has hepatotoxic effects. In the western basin of Lake Erie, annual cHABs dominated by Microcystis spp. have occurred May through September for over four decades, often yielding microcystin concentrations exceeding the World Health Organization and United States federal Environmental Protection Agency’s drinking water guideline of 1-1.6µg/L.

In this dissertation, three studies used genomic analyses to identify specific cyanobacterial genotypes involved in cyanotoxin production and investigated how they interact with other microbes in cHABs, leveraging Lake Erie as a model system for similar blooms in other regions. In Chapter 2, single-copy core genes were identified as markers of Microcystis strain diversity and were found via comparative genomics to resolve Microcystis phylogenies better than previously identified marker genes. The markers identified were also found to be useful in inferring Microcystis gene content and phenotypes such as potential production of secondary metabolites such as cyanotoxins. Chapter 3 characterized and linked mcy genotypes with microcystin congeners via spatiotemporal profiling. Results show that the diversity of microcystin congener type is due to a combination of environmental and genetic diversity. Even with genetic diversity not being the sole driver of congener diversity, specific single nucleotide polymorphisms were identified to be associated with biochemically important domains within the mcy operon and to be strongly correlated to the increased abundance of specific congeners. Lastly, in chapter 4, the cyanobacterium Pseudanabaena, which is commonly associated with
Microcystis and inhabits the Microcystis phycosphere, was genetically and metabolomically characterized to provide insight potential interactions between these two organisms. Pseudanabaena was found to have the capability to produce many different compounds which could have mutualistic and potentially allelopathic effects on Microcystis and its associated microbiome. Together, these studies highlight the genetic variability in cyanobacteria and how it can be an indicator of production for specific natural compounds such as cyanotoxins, which could influence how the organism interacts with other community members and its impact on the environment. Additionally, it serves as a basis for future studies to elucidate mechanisms of genetic regulation of production of microcystins.