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 + aebT) 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 + 10lgEC50−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−.
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