Combined toxicity effects of MTBE and pesticides measured with Vibrio fischeri and Daphnia magna bioassays

Abstract

Methyl-tert-butyl ether (MTBE), a fuel oxygenate that is added to gasoline, commonly contaminates aquatic systems, many of which are already contaminated with pesticides. The toxic effects (EC₅₀ value) of several pure pesticides (Diuron, Linuron, Dichlofluanid, Sea nine, Irgarol and tributyltin (TBT)) were measured and compared with the EC₅₀ value of the pesticide mixed with MTBE, using the Vibrio fischeri and Daphnia magna acute toxicity assays. The interaction between chemicals was evaluated in terms of the effects of mixing on the EC₅₀ value (i.e. the concentration (mg/L) of a compound or mixture that is required to produce a 50% change in a toxic response parameter) and the time required to generate the toxic response. Presence of MTBE enhanced the EC₅₀ value of several pesticides (Diuron, Dichlofluanid, TBT and Linuron) and/or the toxic response manifested more rapidly than with pure pesticides. Toxicity enhancements were quite substantial in many cases. For example, the presence of MTBE increased the toxicity of Diuron by more than 50% when tested with the V. fischeri assay (5, 15 and 30 min exposure). Also, the toxic response manifested itself within 5 min whereas without the MTBE the same response arose in 30 min. Presence of MTBE increased the toxicity of Dichlofluanid by 30% when measured with the D. magna assay. Toxicities of only two pesticides (Sea nine and Irgarol) were not raised by the presence of MTBE.

Introduction

Because of its widespread use as a fuel oxygenate in gasoline, methyl-tert-butyl ether (MTBE) commonly contaminates groundwater, drinking water [1], coastal waters, and other aquatic bodies. MTBE concentrations as high as 200 mg/L have been detected in groundwater supplies located close to leaking underground gasoline storage tanks. MTBE concentrations of 0.088 mg/L have been found in lakes [1] and concentrations in coastal waters can range from 0.005 to 0.8 mg/L [2].

In response to the inputs of MTBE into aquatic ecosystems, numerous toxicity studies on different marine species and freshwater organisms have been reported. Acute and chronic toxicity of MTBE alone appears to be low towards marine species [3]. However, in the environment, species are exposed to large number of chemicals, and therefore, the toxicological studies about the adverse effects and potential risk of the mixtures of chemicals, is growing. How the combination of MTBE and pesticides affects the toxicities of individual compounds is an important question that impacts on the health of the aquatic environment. This work evaluates the effects of MTBE on toxicity of the pesticides Diuron, Dichlofluanid, Sea nine 211, Irgarol 1051, Linuron and Tributyltin (TBT). These specific compounds are found frequently in marinas, rivers, and lakes [4], [5], [6]. Diuron, Linuron and Irgarol are biocides, which act as inhibitors of photosystem-II and are used to eradicate algae. Dichlofluanid is widely used as a protective fungicide in agriculture. Sea nine is broadly toxic to marine bacteria, algae and barnacles [7]. TBT, an active ingredient of antifoulant paints used on ships, is extremely toxic to aquatic organisms [7].

Compounds in mixtures are known to interact with biological systems in ways that can greatly alter the toxicity of individual compounds [8], [9], [10], [11]. Mixture effects on the organisms can be antagonistic, additive or synergistic. For the effect assessment of mixtures, three theoretical concepts of joint action (“Simple similar action” and “Simple dissimilar action”) or interaction of combination of chemicals have been widely used in toxicological studies [12]. The models of joint action allow us to describe mathematically, the additive effects—considering that the chemicals of a mixture act by non-interactive processes. However, when the combined effect resulting in a stronger (synergism, potentiation) or weaker effect (antagonism, inhibition) than that expected on the basis of additivity, interactive processes are considered. For these cases, the mechanisms of physicochemical and/or biological nature are considered as well as the interaction which can occur in the toxicokinetic phase (processes of uptake, distribution, metabolism and excretion) or/and toxicodynamic phase (effects of the chemicals on the receptor, cellular target or organ). For example, induction and inhibition of an enzyme by one toxicant can modify an organism's response towards other toxicants. Also, one compound may compete with another for participation in a metabolic process [12].

For the assessment of the contamination in aquatic ecosystems, many types of bioassays have been used. The test organisms incorporated in these bioassays include species of different group as microorganisms, plants, invertebrates and fish [13]. Zooplankton (in particular the genus Daphnia) is frequently used in ecotoxicological tests because this is one of the groups most sensitive to toxic chemicals and this organism occupies a central position in the lentic food chain [14]. Bacteria Vibrio fischeri is another test organism widely accepted in toxicity test. Both V. fischeri and D. magna assays are rapid, relatively inexpensive, simple and reproducible [15], [16], [17], [18]).

The goals of this study were

• To evaluate the individual and combined toxic effects of different pesticides and the fuel oxygenate (MTBE) using bacteria (V. fischeri) and crustacean (Daphnia magna). The toxic effects were quantified as EC₅₀ value (i.e. the concentration (mg/L) of a compound or mixture that is required to produce a 50% change in a toxic response parameter).

• To evaluate the time required to generate the toxic response on V. fischeri and D. magna assays.