AME 82:105-110 (2018)  -  DOI: https://doi.org/10.3354/ame01877

ABSTRACT: Cylindrospermopsis raciborskii is a bloom-forming diazotrophic cyanobacterium, the worldwide success of which has been attributed to its phenotypic plasticity and to the presence of ecotypes with different environmental preferences. C. raciborskii strains from Oceania and Asia are able to produce cylindrospermopsin, a potent nitrogen-rich cytotoxin. There is no agreement about the biological role of cylindrospermopsin or which environmental factors trigger its production in this species. In the present study, we analyzed the combined effect of nitrate and phosphate availability on the relative expression of a gene involved in cylindrospermopsin synthesis (cyrA) and on toxin production in C. raciborskii CYP011K. The strain was grown under different combinations of nitrate and phosphate availability. We found that, besides cyrA expression being detected in all treatments, the combination of nitrate deprivation and phosphate repletion was the condition significantly inducing cyrA expression and toxin production, concomitant with a higher growth rate. Thus, we demonstrated that cylindrospermopsin production in C. raciborskii is positively related to nitrogen fixation. We hypothesize that under nitrogen-fixing conditions, the production and export of cylindrospermopsin would be a functional strategy for competition with other phytoplankters via a mechanism involving the inhibition of reduced glutathione synthesis and induction of oxidative damage.

KEY WORDS: Cylindrospermopsis raciborskii · cyrA · Cylindrospermopsin · Cyanotoxins · Nitrogen availability