Nitrogen and phosphorus significantly alter growth, nitrogen fixation, anatoxin-a content, and the transcriptome of the bloom-forming cyanobacterium, Dolichospermum.

While freshwater cyanobacteria are traditionally thought to be limited by the availability of phosphorus (P), fixed nitrogen (N) supply can promote the growth and/or toxin production of some genera. This study characterizes how growth on N(2) (control), nitrate (NO(3) (-)), ammonium (NH(4) (+)), and urea as well as P limitation altered the growth, toxin production, N(2) fixation, and gene expression of an anatoxin-a (ATX-A) - producing strain of Dolichospermum sp. 54. The transcriptomes of fixed N and P-limited cultures differed significantly from those of fixed N-deplete, P-replete (control) cultures, while the transcriptomes of P-replete cultures amended with either NH(4) (+) or NO(3) (-) were not significantly different relative to those of the control. Growth rates of Dolichospermum (sp. 54) were significantly higher when grown on fixed N relative to without fixed N; growth on NH(4) (+) was also significantly greater than growth on NO(3) (-). NH(4) (+) and urea significantly lowered N(2) fixation and nifD gene transcript abundance relative to the control while cultures amended with NO(3) (-) exhibited N(2) fixation and nifD gene transcript abundance that was not different from the control. Cultures grown on NH(4) (+) exhibited the lowest ATX-A content per cell and lower transcript abundance of genes associated ATX-A synthesis (ana), while the abundance of transcripts of several ana genes were highest under fixed N and P - limited conditions. The significant negative correlation between growth rate and cellular anatoxin quota as well as the significantly higher number of transcripts of ana genes in cultures deprived of fixed N and P relative to P-replete cultures amended with NH(4) (+) suggests ATX-A was being actively synthesized under P limitation. Collectively, these findings indicate that management strategies that do not regulate fixed N loading will leave eutrophic water bodies vulnerable to more intense and toxic (due to increased biomass) blooms of Dolichospermum.