Synergistic effects of temperature and nutrients on growth and saxitoxin content of the cyanobacterium Raphidiopsis raciborskii

The eutrophication of aquatic ecosystems combined with the rise in global temperature has favored the frequency and intensity of harmful cyanobacterial blooms. Although the importance of temperature and nutrients on biomass, biovolume, and toxin production by the cyanobacterium Raphidiopsis raciborskii (Woloszynska) Aguliera et al. has been studied, the synergistic effects of these factors have not received due attention. We investigated the individual and combined effects of high nitrogen, phosphorus, and temperature on the biomass, cell volume, and saxitoxin (STXs) content of R. raciborskii . The cyanobacterium was exposed to high nitrogen (N +) and phosphorus (P +) concentrations at two temperatures, 24 and 30 °C, for 8 days. Raphidiopsis raciborskii biomass was highest under conditions P + and N + at 30 °C. The highest growth rates were observed at 24 °C in cultures exposed to the N + conditions. The changes in cell volume induced by nutrients, temperatures, and incubation time were interactively significant. Cultures incubated at 30 °C had significantly reduced cell volume regardless of the nitrogen condition. Condition P + stimulated higher STXs production by the cyanobacterium at 24 °C compared to the other experimental conditions tested. Linear regression models indicated that P + had a strong relationship with biomass and saxitoxins production at the investigated temperatures—24 and 30 °C. Our results suggest a synergy of nutrient and temperature variation promotes the proliferation of R. raciborskii , supporting the existing hypothesis on the geographic expansion of this species. However, it is not yet clear whether elevated temperatures could reduce the toxicity of STX-producing strains.