Comparative ecotoxicity of single and binary mixtures exposures of cadmium and zinc on growth and biomarkers of Lemna gibba

Multiple heavy metals affect root response, iron plaque formation, and metal bioaccumulation of Kandelia obovata

Multiple heavy metal pollution in mangrove wetlands is serious. Kandelia obovata seedlings were cultured in pots in which lead (Pb), zinc (Zn) and copper (Cu) were added separately and in combinations. The results showed that heavy metal stress improved the rate of root oxygen leakage, enhanced root activity, and reduced root porosity. The root under single heavy metal stress was impacted by the addition of other heavy metals, demonstrating antagonistic or synergistic effects. Iron plaque (IP) formation was improved under single Zn or Cu stress, and inhibited in binary stress of Pb + Cu. The adsorptions of IP on heavy metals in combined stress (Pb, 62–116 μg g⁻¹; Zn, 194–207 μg g⁻¹; Cu, 35–52 μg g⁻¹) were higher than that in single stress (Pb, 18 μg g⁻¹; Zn, 163 μg g⁻¹; Cu, 22 μg g⁻¹). K. obovata accumulated higher levels of heavy metals in root (Pb, 7–200 μg g⁻¹; Cu, 4–78 μg g⁻¹), compared with IP (Pb, 18–116 μg g⁻¹; Cu, 22–52 μg g⁻¹), stem (Pb, 3–7 μg g⁻¹; Cu, 9–17 μg g⁻¹), and leaf (Pb, 2–4 μg g⁻¹; Cu, 4–7 μg g⁻¹). Correlation analysis showed that single and binary stresses affected K. obovata, with more significant effect of trinary stress. Regression path analysis showed that multiple heavy metal stress firstly affected root, then indirectly contributed to IP formation, as well as heavy metal in IP and root; at last, heavy metal in IP directly contributed to heavy metal bioaccumulations in root.

Comparative ecotoxicity of single and binary mixtures exposures of cadmium and nickel on growth and biomarkers of Lemna gibba

The aim of the present study was to investigate the toxicity effects of cadmium-nickel (Cd-Ni) after single and mixtures exposures over the macrophyte Lemna gibba. Effects were assessed on growth, as frond number and fresh weight and biochemical parameters, such as total protein content and activity of antioxidant enzymes. Plants were exposed to single Cd and Ni in concentrations that ranged between 0.13-33 mg/L and 0.18 and 11.82 mg/L, respectively. For binary mixtures, individual metal IC50 values were used for selection of the evaluated concentrations. The experimental design consisted in three different ratios based on the concept of toxic units (TU), each ratio was evaluated by five different concentrations. Both single and mixture treatments were performed for 7 days following the conditions according to OECD (2006). Single and mixture exposures affected plant growth and the biomarkers of the antioxidant response. Growth parameters showed a differential sensitivity after individual metal exposures. Cd was more toxic for L. gibba plants when fresh weight was considered, but on the contrary, considering frond number, Ni was the most toxic metal. IC50-7d, based on growth rate calculated on frond number were 17.8 and 2.47 mg/L, and on fresh weight were 1.08 and 3.89 mg/L, for Cd and Ni respectively. LOEC values for Cd were obtained at 2.06 and 1.03 mg/L, for frond number and fresh weight, respectively; while for Ni, these values were 0.92 and 11.82 mg/L. The three evaluated ratios for binary mixtures resulted in a high toxicity considering the same response variables in single metal exposures. Ratio 1 (2/3 TU Cd-1/3 TU Ni) was the most toxic considering both frond number and fresh weight, showing percentage inhibition of growth rates of 96 and 90%, respectively for the highest concentration. A modification of the protein content was observed in single, but especially in the mixture treatments. The activity of catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APOX; EC 1.11.1.11) and guaiacol peroxidase (GPOX; EC 1.11.1.7) was also affected in single and mixtures assays. APOX and GPOX showed a higher increase of its activities respect the controls after mixture treatments than for single metal treatments. Such optimization of the antioxidant system could be one of the causes of the antagonistic toxicity observed in mixture exposures. Concentration addition (CA) reference model, based on frond number, in Cd-Ni mixtures was not a good predictor to evaluate toxicity from dissolved metal concentration since the results showed that toxicity was less than additive, with an average of ΣTU = 2.17. The observed antagonisms resulted to be stronger in mixtures with higher metal concentrations.