Mechanism Of Joint Toxicity Of Pyrethroid Insecticides And Metals In Toxicity Evaluation For Highly Toxic Sediments

With the rapid development of economy in recent decades, more and more contaminants have been discharged into aquatic environment, which caused serious deteriation of aquatic ecosystem. As sediment is an important component of aquatic environment, it is important for ecological risk assessment and management of aquatic environment that evaluating sediment toxicity effectively. Previous research on sediment pollution in urban waterways of the Pearl River Delta(PRD) reported sediments collected in this area were highly toxic to benthic organisms and whole sediment dilution was necessary before toxicity evaluation. At present, two sediment dilution methods were commonly used, including dilution based on dry weight and total organic carbon content. However, the influence of the dilution methods on the results of toxicity evaluation for highly toxic sediments is still unclear, raising the needs for further study. Besides, joint toxicity of mixtures is also an important issue in sediment toxicity assessment. In real environment, there is no such thing as a single chemical exposure. In general, many chemicals co-existed in the environment. However, the majority of sediment ecological risk assessments so far were based on the evaluation of single chemical. The interaction between chemicals may change the toxicity of individual chemicals; thus the accuracy of sediment risk assessment would be biased if we only considered the toxicity of single chemical. Studies on joint toxicity between organic chemicals and metals were limited and the mechanisms of their joint toxicity were even scarcely studied. Previous studies suggested that pyrethroids and metals were two main toxicity contributors to the highly toxic sediments in urban area of the PRD. To improve the accuracy in assessing risk of the sediments contaminated by pyrethroids and metals, it is of great importance to better understand the interaction between the contaminants as well as the reasons for the interaction. So, in order to study the joint toxicity of these two contaminants to benthic invertebrate Chironomus dilutus, the frequently detected permethrin and cadmium in this area were selected as the representative pyrethroid and metal, respectively. In addition, the mechanisms of the joint toxicity were also explained from the perspectives of bioavailability and toxicokinetics.Firstly, a sediment spiked with cypermethrin was diluted using the two commonly used whole sediment dilution methods, including dilution based on dry weight and total organic carbon. The results of 10-d spiked sediment toxicity tests with C. dilutus suggested that the toxicity data, expressed as median lethal concentration(LC50), LC25 and LC75, were significantly different for the same set of sediment samples when dilution methods varied. The LC50 of cypermethrin obtained using the dilution method based on dry weight was significantly different from the value in the literature, but the value from the dilution based on total organic carbon was similar to the literature value. The results were also validated using the highly toxic sediments collected in the PRD after dilution with the two methods, suggesting the preference of diluting sediment based on total organic carbon when evaluating the toxicity of highly toxic sediments. Interestingly, the LC50 values acquired from the freely dissolved concentrations of cypermethrin were not significantly different between the two dilution methods, which indicated that the freely dissolved concentrations of chemicals measured by the passive sampling methods were independent on sediment dilution methods and were more accurate toxicity metrics than sediment concentrations. So it was preferable to include bioavailability-based metrics in sediment toxicity evaluation.Secondly, permethrin and cadmium were spiked into sediment at various mixture ratios and doses in this study. Then 10-d sediment toxicity testing was conducted with C. dilutus using lethality as toxic endpoint. A median effect/combination index-isobologram, which was widely used in pharmacology to investigate drug interactions, was applied to evaluate the interaction between sediment-bound permethrin and cadmium. Antagonistic interaction was observed in the midges for all treatments. Comparatively, cadmium-dominated group(the ratio of toxicity contribution from permethrin and cadmium was 1:3) showed stronger antagonism than equitoxicity(1:1) and permethrin-dominated groups(3:1). In addition, the antagonism was also confirmed by the assessments using two traditional models, concentration addition and independent action. The reasons for the observed antagonism were also elucidated from two perspects, including bioavailability and activities of metabolic enzymes. The bioavailability of permethrin, expressed as the freely dissolved concentrations in sediment porewater and measured by solid phase microextraction, was not altered by the addition of cadmium, suggesting change in the bioavailability of permethrin was not the reason for the antagonistic interaction. On the other hand, the activities of metabolic enzymes, glutathione S-transferase(GST) and carboxylesterase(CarE) in the midges being exposed to mixtures of permethrin and cadmium were significantly higher than those in the midges which were exposed to permethrin solely. Cadmium considerably enhanced the detoxifying processes of permethrin in the midges, which largely explained the observed antagonistic interaction between permethrin and cadmium.Finally, in order to better understand the reasons for the joint toxicity, mechanistic explanation for the role of toxicokinetics of pyrethroids in the antagonistic interaction was studied. The goal was achieved in the current study by evaluating the impact of cadmium on toxicokinetic parameters of permethrin in C. dilutus, and by explaining the interaction through quantifying the activity and gene expression of biotransformation-related enzymes. Toxicokinetic parameters were simulated using a first-order kinetic model for the organisms in bioaccumulation tests. No obvious toxicity was observed for C. dilutus during the bioaccumulation tests. Bioconcentration factors and uptake and elimination rate constants for permethrin were not significantly changed with the addition of cadmium at sublethal level. So two more sensitive indicators, enzymatic activity and gene expression, were selected to explore the influence of cadmium on the toxicokinetics of permethrin. Compared with the midges exposed to permethrin solely, the activity of enzymes, including GST, CarE, catalase and lipid peroxidation were also not significantly changed with the addition of cadmium. Yet, the activities of metabolism-related enzymes(GST and CarE) showed an elevating tendency with adding cadmium. Furthermore, the expression of metabolism-related genes, including cytochrome P450 and GST genes were significantly up-regulated in C. dilutus exposed to a mixture of permethrin and cadmium compared with permethrin only. Although co-exposure to cadmium did not induce toxicokinetic changes of permethrin to C. dilutus, it did enhance the activity of metabolic enzymes which were encoded by the metabolism-related genes, suggesting an acceleration of biotransformation of permethrin to less toxic metabolites in the midges. The results served as a confirmation to the previous conclusion and further explained the reasons of antagonism mechanically.As the sediments collected in urban waterways of the PRD exerted high toxicity to benthic organisms and pesticides and metals were the main toxicity contributors, the influence of sediment dilution methods and the joint toxicity between pesticides and metals on sediment toxicty assessment were systematically analyzed in this study. Permethrin and cadmium were seleted as the representative pyrethroid and metal for joint toxicity analysis and the mechanisms for the interaction was also eludicated from the perspetives of bioavailability and toxicokinetics. This study provides an effective method for investigating joint toxicity of organic chemicals and metals. In the meantime, evaluating joint toxicity of mixtures and its mechanisms would also provide theoretical foundation for ecological risk assessors and regulators to develop more effective environmental quality criteria.