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Cylindrospermopsis raciborskii is an invasive, toxin producing cyanobacterium of great ecological significance. This thesis explores the genomic plasticity and adaptive strategies of C. raciborskii strains isolated from different locations worldwide. It further explores the conservation of essential primary metabolic machinery of these strains, and discusses the potential causes of gene loss in these organisms. While strains isolated from Australia exhibited over 97% genomic similarity and 100% similarity within the cylindrospermopsin (CYN) biosynthesis (cyr) gene cluster, comparison of strains from China, Australia and Brazil highlighted diversity within the species, suggesting that geographic location could possibly play important roles in shaping the C. raciborskii genome. Nucleotide sequence based analyses exhibited strain clustering as per geographic location. Toxigenicity and taxonomic identity were not found to impact strain divergence. We report for the first time a varying spatial arrangement of the cyr biosynthesis gene cluster isolated from Chinese strains, compared to Australian strains. We, therefore hypothesize, that the arrangement of this cluster is influenced by the geographic location of isolates and not by overall strain relatedness as considered earlier. Additionally, we examined the effect of nitrogen (N) and phosphorus (P) on growth and ecotype-related changes in production of cylindrospermopsins by C. raciborskii. Significantly higher growth rates were observed, only when N and P were co-supplied. In contrast, toxin cell quotas increased significantly in treatments irrespective of whether N was supplied. These increased toxin quotas correlated with an increase in the proportion of cyrA to 16S genes in the C. raciborskii population, therefore, attributing changes in ecotype dominance as the most likely factor driving changes in toxin production between treatments. Finally, studies to gauge the changes in the transcriptional regulation of the cyr gene cluster under different intensities of light, low (10 µE) and high (100 µE) and pCO2, 500 ppm (L CO2) and 1300 ppm (H CO2), revealed that cylindrospermopsin production is constitutive and is not affected by the physiological parameters of light intensity and pCO2. Overall, this thesis highlights the influence of environmental drivers and location, on the divergence of C. raciborskii, thus, cautions water managers against the rampant threat of this cyanobacterium to water bodies worldwide.

The release of the neurotoxic alkaloid saxitoxin by cyanobacterial cells was previously thought to occur primarily after cell lysis, yet recent evidence also suggests active toxin export by membrane transporters. Transporter proteins associated with STX biosynthesis in Cylindrospermopsis raciborskii T3 (sxtF and sxtM) and Anabaena circinalis 131C (naDt) were predicted to be involved in the export of STX from cyanobacterial cells. The main aim of this project was to characterise the transporters associated with STX biosynthesis, by investigation of their genetic prevalence, functional substrates and specific regulation. An sxtM homologue was discovered in A. circinalis 131C, as part of an sxt cluster, and found to be uniquely associated with STX-producing strains. Bioinformatic and phylogenetic analysis showed that the translated sxt transporters clustered with the NorM prokaryotic MATE sub-family and membrane topology analysis predicted 12 membrane-spanning regions. To characterise the functional substrates of the putative STX-transporters, they were heterologously expressed in the antibiotic-sensitive E. coli strain KAM32. Expression of the sxt MATES complemented host sensitivity to the cationic fluroquinolone antibiotics, ciprofloxacin and ofloxacin. Disruption of gene homologues of naDt and the sxt MATE genes in Synechocystis sp. PCC6803 yielded mutant strains with increased sensitivity to the toxic organic cations, methyl viologen and acriflavine. Transcription of the putative STX transporters, and the putative STX biosynthesis gene sxtA, was studied in C. raciborskii T3 and A. circinalis 131C under alkali and Na+ stress. Alkali stress (pH 9) decreased total STX levels in A. circinalis 131C and was correlated with a down-regulation of the putative transport and biosynthetic genes. In C. raciborskii T3, alkali stress promoted higher extracellular but lower intracellular STX levels, which also correlated with large increases in transcription of the putative STX transport genes.