| For billions of years, cyanobacteria have strongly influenced Earth's oceans and atmosphere. More recently, anthropogenic transformation of freshwater ecosystems via nutrient loading and climatic warming have contributed toward the increasing dominance of cyanobacteria among phytoplankton communities. One of the most common bloom-forming cyanobacteria in temperate freshwater ecosystems is Microcystis, which produces the hepatotoxin microcystin. However, the manner in which specific environmental conditions allow Microcystis to bloom and synthesize this toxin is not fully understood. For my dissertation, I utilized whole transcriptome sequencing and reverse transcriptase quantitative PCR (RT-qPCR) assays to understand how differing nutrient concentrations and sources influenced the growth, dominance and global gene expression patterns in cultures and wild populations of Microcystis. Using RT-qPCR, I found that P scavenging and alkaline phosphatase genes were highly upregulated under low inorganic phosphorus (P) conditions but were unresponsive to organic P sources. I used whole transcriptome shotgun sequencing to identify a suite of nitrogen (N) and P transporter genes that Microcystis upregulated in response to N and P limitation and repeatedly found that Microcystis contains less microcystin under low N conditions. Exploring this link further, I found that under N starvation, microcystin synthetase (mcy) genes involved in peptide synthesis were downregulated while genes involved in tailoring and transport of microcystin were upregulated. Furthermore, I was able to identify previously uncharacterized genes that may be important to Microcystis for growth on organic matter and surviving N limited conditions. I also documented the temporal choreography of the expression of various N transporter genes and the uptake of specific nitrogenous compounds as well as the dynamics of bicarbonate uptake and bicarbonate transporter gene expression. Lastly, gene expression patterns observed in culture were investigated within natural populations of Microcystis in western Lake Erie. Under low N conditions, Microcystis decreased production of microcystin and displayed lowered expression of protease inhibitors that may deter grazing. Increased N led to upregulation of these genes and increased microcystin concentrations. I observed that Microcystis dominated the lowest P regions of western Lake Erie, highly expressing P scavenging genes, whereas competing cyanobacteria became scarce under such conditions. Populations of Microcystis in western Lake Erie also displayed a vast genetic capability to adapt to ever changing conditions, altering genomic structure through expression of transposases and expressing numerous phage defense related genes. This dissertation has provided new insight into the nutritional ecology of the harmful cyanobacterium Microcystis and suggests that given its capacity to thrive under low inorganic P conditions and its non-diazotrophic status, reductions of P, but not N, have allowed it to take over from N-fixers. Managed reductions in both N and P may be required to control blooms of this alga. |
