The role of nitrogen and phosphorus limitation in controlling cellular saxitoxin quotas in the harmful cyanobacteria, Dolichospermum and Aphanizomenon

Toxin production by cyanobacteria that cause harmful algal blooms (CyanoHABs) can be influenced by nitrogen (N) and phosphorus (P), yet little is known regarding how the macronutrient balance affects their saxitoxin (STX) content. Here, we report on the influence of N and P limitation on cellular STX quotas of multiple strains of the freshwater cyanobacteria, Dolichospermum circinale and Aphanizomenon gracile . Strains were grown semi-continuously and exposed to different concentrations of N and P after which growth rates, photosynthetic efficiency, N 2 fixation rates, alkaline phosphatase activity (APA), and STX cell quotas were quantified. Despite diazotrophic capabilities, growth rates of the cyanobacteria could be limited by low levels of N or P. For all strains, low P conditions (0–1 μM orthophosphate; 0–31 μg L −1 ) led to significant and up to five-fold increase in STX quotas and APA, indicating that P limitation within this range of P concentrations increased cellular STX quotas. N 2 fixation rates were also significantly reduced by P limitation. There were differential responses to P limitation across strains, with D . circinale strain ACMB05 exhibiting greater reductions in growth rate and photosynthetic efficiency at low P, whereas growth rates of D. circinale strain ACBU02 and A. gracile changed less despite increased STX quotas and APA. Low P conditions increased the abundance of the saxitoxin congeners GTX2, STX, and dcGTX2, which are among the more potent congeners, thereby further enhancing total toxicity of cells under low P conditions. This study demonstrates that, although N or P availability can constrain the growth of these diazotrophic cyanobacteria, P limitation consistently increased their cellular STX quotas. While mitigation of P may restrict CyanoHABs proliferation, this study identifies a potential trade-off between bloom biomass and cellular toxicity under low P conditions.