Nutrient‐driven regulation of saxitoxin gene expression and toxin production in <i>Raphidiopsis raciborskii</i> (Cyanobacteria)

Raphidiopsis raciborskii is a diazotrophic cyanobacterium, globally distributed in aquatic environments and known for forming toxic blooms, thereby affecting ecosystem services. South American strains are producers of saxitoxins, potent neurotoxins harmful to humans and animals. This study examined the effect of nutrient availability on saxitoxin production in two toxic R. raciborskii strains. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used to investigate the transcriptional response of the saxitoxin sxtA4 gene under nitrogen and phosphorus gradients and the intracellular toxin concentration was measured by high performance liquid chromatography (HPLC). Results showed that the sxtA4 gene expression was generally upregulated at lower nutrient conditions. Positive correlations were observed among transcripts of sxtA4 and genes related to metabolic processes (ntcA, nifH, and pstS), an indication that nutrient stress may affect sxt gene regulation. Intracellular saxitoxin concentration increased slightly under moderate nitrogen reduction (10%), although not always significantly. Under phosphorus reduction, despite the observed upregulated transcription of sxtA4, total saxitoxin concentration significantly decreased, a possible consequence of hindered metabolic fitness. Interestingly, nutrient availability also affected the profiles of toxin analogs produced by R. raciborskii. Because different analogs exhibit variable toxicity, the presence of certain variants may enhance the toxic potential of an entire population under shifting environmental stressors. The responses observed in this study indicate the need for further investigations to identify the mechanisms controlling toxicity. This is particularly relevant as nutrient reduction may control cyanobacterial growth but not necessarily their toxin production.