Low Temperature and Cold Stress Significantly Increase Saxitoxins (STXs) and Expression of STX Biosynthesis Genes sxtA4 and sxtG in the Dinoflagellate Alexandrium catenella

Toxic dinoflagellate Alexandrium spp. produce saxitoxins (STXs), whose biosynthesis pathway is affected by temperature. However, the link between the regulation of the relevant genes and STXs’ accumulation and temperature is insufficiently understood. In the present study, we evaluated the effects of temperature on cellular STXs and the expression of two core STX biosynthesis genes (sxtA4 and sxtG) in the toxic dinoflagellate Alexandrium catenella Alex03 isolated from Korean waters. We analyzed the growth rate, toxin profiles, and gene responses in cells exposed to different temperatures, including long-term adaptation (12, 16, and 20 °C) and cold and heat stresses. Temperature significantly affected the growth of A. catenella, with optimal growth (0.49 division/day) at 16 °C and the largest cell size (30.5 µm) at 12 °C. High concentration of STXs eq were detected in cells cultured at 16 °C (86.3 fmol/cell) and exposed to cold stress at 20→12 °C (96.6 fmol/cell) compared to those at 20 °C and exposed to heat stress. Quantitative real-time PCR (qRT-PCR) revealed significant gene expression changes of sxtA4 in cells cultured at 16 °C (1.8-fold) and cold shock at 20→16 °C (9.9-fold). In addition, sxtG was significantly induced in cells exposed to cold shocks (20→16 °C; 19.5-fold) and heat stress (12→20 °C; 25.6-fold). Principal component analysis (PCA) revealed that low temperature (12 and 16 °C) and cold stress were positively related with STXs’ production and gene expression levels. These results suggest that temperature may affect the toxicity and regulation of STX biosynthesis genes in dinoflagellates.