Comparative genomics and toxin regulation in the cyanobacterium Cylindrospermopsis raciborskii

Abstract

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.