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

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 (control), nitrate (NO ), ammonium (NH ), and urea as well as P limitation altered the growth, toxin production, N fixation, and gene expression of an anatoxin-a (ATX-A) - producing strain of 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 or NO were not significantly different relative to those of the control. Growth rates of (sp. 54) were significantly higher when grown on fixed N relative to without fixed N; growth on NH was also significantly greater than growth on NO . NH and urea significantly lowered N fixation and gene transcript abundance relative to the control while cultures amended with NO exhibited N fixation and gene transcript abundance that was not different from the control. Cultures grown on NH exhibited the lowest ATX-A content per cell and lower transcript abundance of genes associated ATX-A synthesis (), while the abundance of transcripts of several 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 genes in cultures deprived of fixed N and P relative to P-replete cultures amended with NH 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 .