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 N₂ (control), nitrate (NO₃ ^-), ammonium (NH₄ ⁺), and urea as well as P limitation altered the growth, toxin production, N₂ fixation, and gene expression of an anatoxin-a (ATX-A) - producing strain of <i>Dolichospermum</i> 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 NH₄ ⁺ or NO₃ ^- were not significantly different relative to those of the control. Growth rates of <i>Dolichospermum</i> (sp. 54) were significantly higher when grown on fixed N relative to without fixed N; growth on NH₄ ⁺ was also significantly greater than growth on NO₃ ^-. NH₄ ⁺ and urea significantly lowered N₂ fixation and <i>nifD</i> gene transcript abundance relative to the control while cultures amended with NO₃ ^- exhibited N₂ fixation and <i>nifD</i> gene transcript abundance that was not different from the control. Cultures grown on NH₄ ⁺ exhibited the lowest ATX-A content per cell and lower transcript abundance of genes associated ATX-A synthesis (<i>ana</i>), while the abundance of transcripts of several <i>ana</i> 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 <i>ana</i> genes in cultures deprived of fixed N and P relative to P-replete cultures amended with NH₄ ⁺ 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 <i>Dolichospermum</i>.