Effects of fluoride and chloride on the growth of Chlorella pyrenoidosa

To compare the toxic action of fluoride (F⁻) and chloride (Cl⁻) on freshwater algae, effects of F⁻ and Cl⁻ on Chlorella pyrenoidosa growth were investigated by determination of the algal biomass and model analysis. Results showed that the exponential growth equation (D = D₀ + ae^bT) may be used to fit the relationship between cell density (D) of C. pyrenoidosa and exposure time (T), but F⁻ and Cl⁻ have significantly different effects on parameters a (initial quantity of algae at the exponential growth stage) and b (growth constant). In the range of experimental concentrations, F⁻ inhibited C. pyrenoidosa growth, and percent inhibition increased with increasing exposure time and F⁻ concentration. By contrast, Cl⁻ either inhibited or enhanced C. pyrenoidosa growth depending on Cl⁻ concentration and exposure time, and percent inhibition increased with increasing Cl⁻ concentration but decreased with increasing exposure time. The dose–response equation: I = min + (max − min)/(1 + 10^lgEC50−lgC) may be used to fit the relationship between percent inhibition (I) and F⁻ or Cl⁻ concentration (C); however, F⁻ was distinct from Cl⁻ in terms of effects on parameter ‘min’. The EC50 values of F⁻ and Cl⁻ ranged from 118 to 170 mg/L and 19.73 to 36.33 g/L, respectively; the latter is much higher than the former. Thus F⁻ has higher toxicity than Cl⁻.