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Molecular Mechanisms in Ecotoxicology: an Interplay CHEMISTRY AT EA WAG 915 CHIMIA 5/ (1997) Nr. 12 (Dczcmher) Chimia 51 (1997) 915-921 standing of underlying molecular mecha- © Neue Schweizerische Chemische Gesellschaft nisms and modes of toxic action on differ- lSSN 0009-4293 ent levels of biological organization, while the other trend is aimed at understanding the complex interactions and feedback Molecular Mechanisms in mechanisms in ecosystems disturbed by pollutants. Taken together, these two ap- Ecotoxicology: An Interplay proaches complement each other and will finally enhance the understanding of the between Environmental effects of pollutants on living systems. In this paper, we focus on the mode-of-ac- Chemistry and Biology tion-based molecular approach. C 3 Beate I. Escher3)*, Renata Behrab), Rik I.L. Eggen ), and Karl Fent ) 2. Ecotoxicology at EA WAG Fig. 1 shows an overview of the pro- Abstract. A close collaboration between environmental chemistry and biological cesses considered in the molecular ap- sciences is required for a complete understanding of ecotoxicological effects. Bioavail- proach to ecotoxicology. Emphasis is ability and uptake of pollutants cannot be regarded as isolated chemical or biological placed on the importance of understand- questions. Knowledge of the effective concentrations in the organism or at the target ing the interplay between environmental site(s) is essential to link the fate and effects of a chemical and is a prerequisite for chemistry and toxicology, thereby linking quantitative investigation of the modes of toxic action. These modes of action need to the concepts of bioavailability, effective be unraveled using whole-organism or in vitro systems in order to be able to develop concentration in the organisms or at the specific biomarkers and biosensors that can be applied as early warning systems. Our target sites, and the mechanism of ecotox- mode-of-action-based approaches, in which chemical and biological analytical tools icity. are combined, should improve the understanding of ecotoxicological effects and should Evaluation of the adverse effects of be implemented in the future in risk assessment. pollutants in aquatic ecosystems requires discrimination between the total concen- tration of a chemical, the bioavailable 1. Introduction has led to an accumulation of valuable fraction, and the final concentration at the empirical data on the effects of specific target site(s) (Fig. 1, left). Environmental Ecotoxicology is the science of the pollutants on selected species that are cur- chemistry plays a major role in assessing impact of toxic substances on living or- rently used for regulatory purposes. Much the influence of environmental processes ganisms, encompassing all levels of bio- less emphasis has so far been placed on the on the fate of a substance in the environ- logical organization from single organ- development of general concepts that al- ment [2]. The fate of a chemical is affected isms to ecosystems [1]. Ecotoxicology low assessments of effects on the basis of by its physicochemical properties, the char- integrates environmental chemistry, bio- explanatory principles. acteristics of the environment, and by chemistry, toxicology, and ecology in a In this more conceptually oriented ap- biological processes. As a result, only a multidisciplinary manner. The unifying proach to ecotoxicoJogy, essentially two fraction of the total input into the ecosys- theme of ecotoxicological research is to trends have evolved in the last decade. tem will be available for uptake by organ- provide general concepts to evaluate the One trend is directed towards the under- isms. potential harmfulness of pollutants. This research is the basis for the development of tools that can be used in environmental regulation. bioavailability and uptake toxicity and response Despite major advances in the last de- cade, descriptive studies still make up the majority of ecotoxicological research con- cerned with the effects of chemicals. This *Correspondence: Dr. B.I. Escher Swiss Federal Institute for Environmental Science and Technology (EA WAG) and £'Ie Institute for Aquatic Sciences of the Swiss Federal Institute of Technology (IGW /ETHZ) enVIron· effecti effects at alfeCls al b,oavallable a) Department of Chemistry mental concenlr Ion molecular Inl'v,du(li concenlration b) Department of Limnology fale inorgan M level r vel C) and Department of Microbiology Oberlandstrasse ]33 CH-8600 Diibendorf Fig. 1. The effective concentration of a pollutant in an organism (e.g. fish, daphnia, algae) or at the Tel.: +41 18235068, Fax: +4] 18235471 target site inside the organism is the link between the environmental fate of a pollutant and its toxic E-Mail: [email protected] effect. CHEMISTRY ATEAWAG 916 CHIMIA 51 (1997) Nr. 12 (Dclcmhcr) Properties of a compound that affect of sensitive ecotoxicological endpoints. are used to examine the stress response of its fate include its speciation and the hy- In vitro assays can, to a certain extent, an organism. In the biosensor approach, drophobicity of the different chemical spe- replace toxicity tests on whole organisms the expression of affected genes can be cies. Speciation is important for metals and can be used for the assessment of the linked to artificially introduced marker (see Sect. 3), organometallic compounds toxic potential of chemicals and environ- genes whose products can easily be mea- [3][4] and hydrophobic ionizable organic mental samples. Since they are simplified sured (e.g., luciferase, {J-galactosidase, compounds (HIOC) (see Sect. 4). In case model systems, in vitro systems offer the green-fluorescence protein; 'or arylsulfa- of organotin compounds, charged species opportunity to focus in detail on specific tase) [7]. Biosensors work in both prokary- usually show a lower but still significant modes of actions [3]. From an ethical otic and eukaryotic organisms and may be bioaccumu]ation as compared to the cor- point of view, they are less problematic used for on-line monitoring of the envi- responding neutral species [5]. The freely than animal testing. However, a major ronment. dissolved fraction of a compound is pri- problem of in vitro systems is the difficul- marily available for uptake by organisms. ty to extrapolate the results to whole or- Sorption to minerals and organic matter ganisms. Another disadvantage is shared 3. Uptake and Effects of Metals in Algae reduces the bioavailable concentration. with anima] testing on single species in Even the presence of dissolved organic that they typically cannot account for spe- Most studies on the ecotoxicology of carbon reduces the bioavailability of or- cies-specific differences in sensitivity. metals in the aquatic environment try to ganic compounds [5]. Therefore, species-specific systems have relate biological responses to nominal con- The uptake and effect of chemicals is to be developed. In Sect. 5, different appli- centrations [8]. In aquatic systems, metals not solely determined by the bioavailable cations of a fish-specific in vitro system occur in various chemical forms, the for- concentration of a chemical, but is also are shown, employing a permanent fish mation of which is influenced by the local- influenced by biological factors, particu- hepatoma cell line (PLHC-I). ly prevailing physicochemical conditions, larly differences in lipid content and sen- Further processes whose importance e.g., acidity, salinity, inorganic and organ- sitivity of organisms. Differences in sen- cannot be unraveled by looking only at ic ligands, and the presence of particles sitivity among species or populations of lethality or other observable effect end- [9]. Chemical speciation is an important the same species are related to differences points include defense and repair mecha- determinant of metal uptake and toxicity in morphology, developmental stage, sex, nisms. Organisms have developed protec- although its characterization is not trivial genotype, metabolic activity, and individ- tive mechanisms (such as mobilization of considering the di versity of chemica] con- ual history. Moreover, organisms can various cellular constituents and enzymes stituents of aquatic ecosystems. Moreover, evolve protection mechanisms in response that in a highly coordinated way minimize in many cases, the concentrations of the to continuous exposure to elevated con- disturbances of cellular homeostasis) that free and other bioavailable forms cannot centrations of a chemical. This results in allow them, within certain limits, to resist be directly measured but have to be calcu- an increased tolerance to the chemica], adverse conditions, including the negative lated. Studies of metal-algae interactions which further obscures concentration-ef- effects of anthropogenic chemicals. If the in chemically defined culture media, in fect relationships. The link between total, disturbances become too large or are chron- combination with thermodynamic calcu- bioavailable, and effecti ve concentrations ic, however, organisms react with stress lations of the equilibrium speciation, are is further illustrated in the following sec- responses, which are accompanied by an an approach to gain insights into the influ- tion using the example of uptake and ef- increased production of enzymes that ence of the speciation of a metal on its fects of metals in algae. dampen the deleterious effects of the stres- biological availability [10]. Once inside the organism, the pollut- SOl'S, or by repairing damaged cellular At EA WAG, evaluation of the effects ant may initiate a variety of effects,
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