Toxicity of single and ternary mixtures of heavy metals and 2,4-dichlorophenoxyacetic acid to Chlorella vulgaris alkaline phosphatase activity

The influence of mixtures of metals and pesticide on the activity of algal hydrolytic enzyme alkaline phosphatase was investigated. Toxicity of unary and ternary combinations of Copper (Cu2+), Zinc (Zn2+), Lead (Pb2+), Chromium (Cr2+), Cadmium (Cd2+) ions and 2,4-dichlorophenoxyacetic acid (2,4-D) was assessed via inhibition of phosphatase enzyme activity of Chlorella vulgaris. The effects of the amalgamated ternary mixtures were studied in their fixed percentage ratio of 20:40:40 and 40:30:30. Results obtained showed that all the unary heavy metal ions were toxic to algal phosphatase activity, with a concentration dependent inhibition of alkaline phosphatase activity. The analysis of eco-toxicity concentration (EC50) of toxicant ternary mixtures of metals/ pesticides mixtures in 20:40:40 ratios show that the ternary mixtures 2,4-D/Pb2+/Cd2+and 2,4-D/Zn2+/Cd2+ were more toxic, followed by 2,4-D/Cu2+/Cr2+, 2,4-D/Pb2+/Zn2+, 2,4-D/Cu2+/Zn2+ and 2,4-D/Cu2+/Cd2+. Whereas among the 40:30:30 mixture ratios 2,4-D/Pb2+/Cd2+ and 2,4-D/Zn2+/Cd2+ were most toxic. The decline in the trend of their toxicity strength was followed by 2,4-D/Cu2+/Cr2+, 2,4-D/Cu2+/Zn2+, 2,4-D/Cu2+/Cd2+, 2,4-D/Cu2+/Pb2+ and 2,4-D/Pb2+/Zn2+. In addition, the least toxic amongst them were the 2,4-D/Pb2+/Cr2+, 2,4-D/Zn2+/Cr2+ and 2,4-D/Cr2+/Cd2+ ternary mixtures. Toxicity index analysis of the mixture interaction effect showed that the mixtures of heavy metals with 2,4, D were mostly antagonistic with toxic index (TI) >>1, However, exceptions were the 2,4-D/Pb2+/Cd2+ and 2,4-D/Pb2+/Cd2+mixture which were additive and synergistic with TI=1 and TI<1 respectively. From the foregoing study, the toxicity of the mixtures exhibited largely sigmoidal relationship, increased composition of heavy metal in the mixture resulted in a potentiation of 2,4-D toxicity. These results indicate that mixtures of heavy metals with largely non-toxic 2,4-D may pose ecological risk to freshwater microalgae Chlorella vulgaris, presenting antagonistic, additive and synergistic toxicity interactions based on their relative occurrence in the mixture.