Physiogenomics of Cylindrospermopsis raciborskii and Raphidiopsis brookii (Cyanobacteria) with Emphasis on Evolution, Nitrogen Control and Toxin Biosynthesis

Abstract

The freshwater toxic cyanobacteria Cylindrospermopsis raciborskii CS-505 and Raphidiopsis brookii D9 belong to the order Nostocales of filamentous, heterocystous (nitrogen fixation) morphotypes. However, Raphidiopsis spp. do not develop heterocyst nor fix N2, contradicting the morphological classification. This thesis presents an integrated study of evolutionary relationships linked to nitrogen (N) metabolism and toxin production in filamentous cyanobacteria. The genome sequencing and comparison of CS-505 and D9 revealed the smallest genomes of filamentous cyanobacteria and an extent of similarity corresponding to conspecific isolates, therefore recommending a taxonomical reclassification of the genera Raphidiopsis. The presence of heterocyst-specific genes and their expression under alternative N-sources in D9, together with differences in genomic plasticity, allowed us to propose divergent evolutionary pathways for both species. Late transcriptional responses were observed in C. raciborskii CS-505 under N-deprivation, contrasting with those of other heterocystous cyanobacteria. The pattern of biosynthetic gene regulation suggested that additional mechanisms of N-control and post-transcriptional mechanisms are involved in toxin production. The role of repetitive elements and transposases in gene regulation may be critical but this aspect requires further investigation.