Toxicity Enhancement with Cu2

It is of value to note that the mixture of CuSO4 with other chemicals brought about the marked synergistic effects. In this regard, it seems to be of necessity to examine what kind of roles copper ion plays in the mixtures. Copper ions have generally been used in the form of CuO or Cu2O in the antifouling industry owing to their poor solubility in water, which leads to a constant release of copper ions from the surface of a ship hull coated with paints. Since their poor solubility makes it difficult to prepare the samples with appropriate concentrations in the present study, CuSO4 was used in place of CuO or Cu2O. Single chemical toxicities of Irgarol 1051, Ziram, Cu-pt, or Zn-pt were evaluated in the presence of high concentration of SO2since ASW used for the sample preparation contains a high concentration of SO4-(2700 m gl ), but no Cu2+. Their toxicities, however, were much lower in comparison with those detected in the presence of Cu2+. Therefore, the increased toxicities of these chemicals in combination with CuSO4 would be conceivably attributed to the presence of Cu2+ rather than SO42-.

To confirm the assumption mentioned above, CuCl2 was also used in place of CuSO4 and mixed with Zn-pt, Irgarol 1051, Ziram, and Cu-pt. As shown in Figure 8, INH (%) values in the presence of 0.05 mgl of CuCl2 were found to be less than 5% at 30min of incubation. The inhibitory activities of Zn-pt, Cu-pt, Ziram, and Irgarol 1051 were less than 10% at the concentrations of

0.04, 0.05, 0.08, and 9.0 mgl \ respectively. When each of them was combined with 0.05 mgl of CuCl2, all of the INH (%) values became higher than 60%. The degrees of toxicity enhancement were in good agreement with the values of both MTI and INH (%) calculated for their combinations with CuSO4, as shown in the table inserted in Figure 8. These results strongly suggest that the marked synergistic effects of these chemicals in combination with CuSO4 or CuCl2 are ascribed to the presence ofCu2+.

Copper ions are widely distributed in the natural environments and serve as metal element prerequisite for the growth of most of plants and animals. In marine environments, however, the level of concentrations of Cu2+ have been detected as a complex with various kinds of chemicals. Cu2+ in most of organisms is little accumulated due to its nonlipophilicity, but suppression of mitosis through glutathione reduction and breakage of the cellular defense against oxygen-free radicals might be brought about, probably due to the passive diffusion of Cu2+ into the cells. The most bioavailable and toxic form of unbounded Cu2+ is thought to be the free hydrated ion form, Cu(H2O)6+. Chemical form of ionic copper is governed by external pH, salinity, and concentration levels of dissolved organic matter. Thus, the toxicity of Cu2+ to bacteria also depends on not only the individual species but also the physiological and environmental conditions. The mechanism of toxicity enhancement might be ascribed to the formation of lipophilic organic copper complexes between Cu2+ and some antifouling

CuCl2 Zn-pt Zn-pt + CuCl2

Irgarol 1051 Irgarol 1051 + CuCl2

Ziram Ziram + CuCl2

2.32

2.44

2.36

Combinations

INH (%)

MTI

Zn-pt + CuSO4

93

2.20

Irgarol 1051 + CuSO4

67

>1.73

Ziram + CuSO4

84

2.60

Cu-pt + CuSO4

89

Figure 8 Effects of Cu on the toxicity enhancement. BLI reductions were obtained for the single chemicals (open symbols) and their combinations (shaded symbols) in the presence of 0.3 M NaCI, from which the INH (%) values were calculated at 30 min of incubation. For both single chemicals and their mixtures, the concentrations of CuCl2, Zn-pt, Irgarol 1051, Ziram, and Cu-pt were fixed to be 0.05, 0.04, 9.0, 0.08, and 0.05 mg l_1, respectively. The numbers shown at the right-hand side of bars are MTI values for each chemical mixed with CuCl2 at 30 min of incubation. The numbers in the table inserted are the values of INH (%) and MTI which were calculated based on the BLI changes for the mixtures of 0.04 mg l _1 of CuSO4 with the same concentrations of Zn-pt, Irgarol 1051, Ziram, and Cu-pt as the case of CuCl2 at 30 min of incubation. All the data were obtained from triplicate experiments, averages of which were described.

chemicals, which are able to diffuse across the plasma membrane of the cells more easily in comparison with the inorganic state of Cu2+. Organic copper complexes in the cell cytosol conceivably dissociate and exchange the transport ligands with intracellular complexes. Similar mechanism on the toxicity enhancement against micro-algae was observed for the combination of dithiocarbamate and some heavy metals. On the basis of the present results showing that synergistic effects occurred in combinations with Cu2+, the authors suggest that these antifouling chemicals promote the uptake of a variety of toxic heavy metals into the cell cytoplasm through the formation of complexes in the aquatic environments.

Until recently, much attention has been paid to the toxicity of single chemicals rather than mixed systems. Interactions of mixed chemicals have rarely been studied, probably due to the large amounts of analytical work to determine the actual status of chemicals present in natural environments. There are some data of interaction effects consisting of several antifouling chemicals combined with methyl-t«t-butyl ether (MTBE), a fuel oxygenate that is added to gasoline. Thus, MTBE frequently contaminates the aquatic systems and possibly forms some complexes with a variety of chemicals. Hernando et al. examined the interactions between MTBE and some of the antifouling

Table 2 MTI values of the mixtures consisting of methyl-tert-butyl ether (MTBE) and some antifouling chemicals for V. fischeri and D. magna

Mixture

V. fischeri

D. magna

Diuron-MTBE

6.7

6.3

DCF-MTBE

-0.7

3.4

Irgarol 1051-MTBE

6.1

6.0

SEA-NINE 211-MTBE

-5.0

-4.6

chemicals using two kinds of microorganisms, V. fischeri and Daphnia magna. As shown in Table 2, the mixtures examined resulted in toxicity enhancement, except for SEA-NINE 211 and MTBE. Therefore, it is necessary to pay attention to the fate of some general ionic and organic materials, since they possibly enhance their toxicity by forming complexes with each other.

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