Effects of Salicylic Acid on Heavy Metal Resistance in Eukaryotic Algae and Its Mechanisms

Heavy metal pollution and water eutrophication are still the main issues to be solved in the environmental field. To find a biological control method for Cd(2+)-contaminated water or combined eutrophication and Cd(2+) pollution water, the effects of salicylic acid on heavy metal Cd(2+) resistance in eukaryotic algae Scenedesmus obliquus and Chlorella pyrenoidosa and its mechanisms were studied. The results showed that the inhibition rates of 3.0 mg/L Cd(2+) stress group at 96 h were 67.0% on S. obliquus and 61.4% on C. pyrenoidosa and their uptake of Cd(2+) was 0.31 mg/g and 0.35 mg/g, respectively. When adding the different concentrations of salicylic acid while stressed by 3.0 mg/L Cd(2+), the hormesis phenomenon of low dose stimulation and high dose inhibition could be seen, and the inhibition rates of 30 mg/L~90 mg/L salicylic acid addition groups were significantly lower than those of the Cd(2+) stress group alone, which were statistically significant (p < 0.05) and the absorption of Cd(2+) was dramatically improved. Except for the 120 mg/L salicylic acid addition group, the chlorophyll fluorescence parameters (Fv/Fm and YII), glutathione peroxidase (GSH-Px) and glutathione-S-transferase (GST) activities of all the other concentration groups were significantly higher than those of the Cd(2+) stress group alone, p < 0.05.; the algal cell morphology in low concentration groups (30 mg/L and 60 mg/L) was also less damaged than those in the Cd(2+) stress group alone. These indicate that the low concentrations of salicylic acid can counteract or protect the algal cells from Cd(2+) attack, the mechanisms, on the one hand, might be related to the chelation of heavy metals by salicylic acid, resulting in the decrease of the toxicity of Cd(2+); on the other hand, low concentrations of salicylic acid can stimulate the growth of these two algae, improve their photosynthetic efficiency and antioxidant capacity, as well as maintain the relative integrity of algal morphological structure.