The BEAM-project: prediction and assessment of mixture toxicities in the aquatic environment

Freshwater and marine ecosystems are exposed to various multi-component mixtures of pollutants. Nevertheless, most ecotoxicological research and chemicals regulation focus on hazard and exposure assessment of individual substances only, the problem of chemical mixtures in the environment is ignored to a large extent. In contrast, the assessment of combination effects has a long tradition in pharmacology, where mixtures of chemicals are specifically designed to develop new products, e.g. human and veterinary drugs or agricultural and non-agricultural pesticides. In this area, two concepts are frequently used and are thought to describe fundamental relationships between single substance and mixture effects: Independent Action (Response Addition) and Concentration Addition. The question, to what extent these concepts may also be applied in an ecotoxicological and regulatory context may be considered a research topic of major importance, as the concepts would allow to make use of already existing single substance toxicity data for the predictive assessment of mixture toxicities. Two critical knowledge gaps are identified: (a) There is a lack of environmental realism, as a huge part of our current knowledge about the applicability of the concepts is restricted to artificial situations with respect to mixture composition or biological effect assessment. (b) The knowledge on what exactly is needed for using the concepts as tools for the predictive mixture toxicity assessment is insufficient. Both gaps seriously hamper the necessary, scientifically sound consideration of mixture toxicities in a regulatory context.In this paper, the two concepts will be briefly introduced, the necessity of considering the toxicities of chemical mixtures in the environment will be demonstrated and the applicability of Independent Action and Concentration Addition as tools for the prediction and assessment of mixture toxicities will be discussed. An overview of the specific aims and approaches of the BEAM project to fill in the identified knowledge gaps is given and first results are outlined.     

Proposal for applying a component-based mixture approach for ecotoxicological assessment of fracturing fluids

Hydraulic fracturing is increasingly being used to produce gas from unconventional resource sites for energy supply. Therefore, concerns about risks of this technology related to human health and the environment have to be addressed. Among the major issues is the potential contamination of surrounding water systems by chemical additives used in fracturing fluids. In this study, the ecotoxicological hazards of fracturing fluids, both, their individual components (chemicals) as well as their mixtures (product) were assessed using a component-based mixture approach. For five exemplary fracturing fluids, 40–90 wt% of the contained substances could unambiguously be defined in their chemical identity. The concentrations used in the applied fluid mixture were considered as (maximum) exposure concentrations. For components with mass fractions between 10 and 74 wt%, the effect concentrations for acute and chronic toxicity of fish, daphnia and algae were retrieved from experimental databases and through predictive modeling. The hazard indices calculated from the ratio of exposure to effect concentration were >1 for all fracturing fluids, using different scenarios. This indicated a hazard from the undiluted fracturing fluids. The assessment framework presented in this study allows for dealing with data gaps and uncertainties in a tiered fashion and in particular accommodates for combined effects resulting from chemical mixtures. It might be employed for ecotoxicological risk assessment of products containing chemical mixtures and optimization of their environmental performance.     

Teasing apart activities of different types of ABC efflux pumps in bivalve gills using the concepts of independent action and concentration addition

Fluorescent dyes and inhibitor compounds are commonly used to detect activity of multixenobiotic resistance (MXR) efflux pumps in marine invertebrates. We here address the question whether compounds acting as specific inhibitors of certain mammalian transporters can be used in dye efflux assays to distinguish different transporter activities in gill tissue from a marine mussel. We quantified effects of PSC833, a specific inhibitor of mammalian P-gp (P-glycoprotein, ABCB1), and MK571, which blocks MRP (Multidrug resistance associated protein, ABCC) type transporters, on calcein-am efflux in gill tissue of Mytilus californianus. Calcein-am acts as a substrate of both P-gp and MRP. Effects of single compounds and mixtures were determined and combined effect models predicting independent action (IA) and concentration addition (CA) of the chemicals were applied. Effect values predicted by IA showed better correspondence with the experimentally obtained data. This indicates that the inhibitor compounds target different mechanisms of calcein-am efflux and points to P-gp and MRP activities in mussel gills. Our approach could be a simple way for identifying the efflux transporter types targeted by chemosensitizers, including environmentally relevant compounds, in native tissues from marine invertebrates.