Physiological responses of Raphidiopsis raciborskii (Cyanobacteria) strains to water conductivity: effect of sodium and magnesium ions

Cyanobacterial blooms dominated by Raphidiopsis raciborskii, a potential producer of saxitoxin (STX), are commonly reported in continental water bodies of semiarid regions with high conductivity (> 1000 mu S cm(-1)). STX production is proposed as a cellular protection mechanism against salt stress. Here, we test the hypothesis that high conductivity favors R. raciborskii growth and that STX is advantageous under this condition. We included four tropical strains (two STX+ and two STX-) cultured for 12 days in control (450 mu S cm(-1)) or in media with NaCl (10 mM) or MgCl2 (5 mM) (1500 mu S cm(-1)). High conductivity (either NaCl or MgCl2) had neutral or positive effects on growth for all strains. Total STX content did not differ in response to salt concentrations. All strains could tolerate higher conductivity without signals of stress, as noted for stable photosynthetic parameters and protein expression profiles. For the strain with higher growth in salt, accumulation of intracellular carbohydrates occurred in early times (3 days). In conclusion, high conductivity is a favorable condition for R. raciborskii, with no clear relation with the ability to produce STX. Pronounced intraspecific variability was observed, a fact that must be considered for a better understanding of this species.