Functional genomics of the Baltic Sea cyanobacterium Nodularia spumigena in relation to nitrogen fixation, heterocyst differentiation and toxin production

Abstract: Nodularia spumigena is a filamentous, heterocystous cyanobacterium that dominates the annual, toxic cyanobacterial blooms in the Baltic Sea. The blooms are believed to have increased in recent years, due to eutrophication by phosphate and/or nitrogen. Toxicity of these blooms is well-documented and attributed to the hepatotoxin nodularin. The focus of this study was to characterize nitrogen fixation and toxin production of Nodularia spumigena, in response to various levels of combined nitrogen and phosphate. In the presence of combined nitrogen, several Nodularia spumigena strains lost aerobic nitrogen fixation activity while the heterocyst frequency along the filaments remained unchanged. The expression of nifH dropped in response to the treatment, while the expression patterns of ntcA and hetR (key genes controlling heterocyst differentiation) were not affected. It is therefore concluded that in Nodularia, nitrogen fixation and heterocyst differentiation are uncoupled. Protein profiling during ammonium supplementation revealed differential expression of 28 proteins. Proteins involved in energy metabolism, carbon and nitrogen fixation were down-regulated during ammonium supplementation while stress-related proteins were up-regulated, further indicating that N. spumigena does not benefit from ammonium supplementation. Analysis of the nodularin synthetase (nda) gene expression demonstrated an increase of all the nine nda genes upon phosphate depletion and decrease of expression in the presence of combined nitrogen. In contrast to shifts in expression, the intra- and extracellular nodularin concentrations did not vary significantly, suggesting a transcriptionally-independent regulation of nodularin biosynthesis. Together, these data suggest that neither nitrogen nor phosphate alone, especially the low levels encountered in the Baltic Sea, affects heterocyst formation and nodularin biosynthesis.