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As simple as possible: a modelling approach to upscale the relevance of ecotoxicological studies

Final Report Summary - ASAP (As simple as possible: a modelling approach to upscale the relevance of ecotoxicological studies.)

This summary presents the research highlights and conclusions of the project titled “As simple as possible: a modelling approach to upscale the relevance of ecotoxicological studies” (acronym ASAP).
The aim of this project was the application of one simple, but generic modelling framework that integrated lethal and sublethal effects on life-history traits of an invertebrate model organism, the freshwater oligochaete Tubifex tubifex, over its life-cycle and explored the population level consequences of sediment-associated metal contamination. The toxicokinetic-toxicodynamic (TK-TD) General Unified Threshold model of Survival (GUTS) and the DEBkiss model were applied to represent at individual level the effects of three sediment-associated metals, copper, cadmium and nickel. After completing the parameterization, these effects were extrapolated to population level via an IBM.
The proposed objectives of this project were:

a. Generation of time-course lethal and sublethal data sets
b. Determination of the internal metal partitioning analysed over time
c. Parameterization and validation of the TKTD models
d. Identification of the physiological modes of action for each metal
e. Analysis of the population dynamics with a coupled TKTD-IBM

Most consistent outcomes were:
i. The different physiological modes of action that determined a variation in the energy allocated to reproduction depending on the metal. Chemical stress induced by metals altered the energetic parameters of T. tubifex (e.g. decreasing assimilation, increasing cost of maintenance or direct hazard to the embryo) and in turn a measurable quantity such as the reproductive output. These responses, that represented changes in energy allocation, helped to reveal the physiological modes of action of the metals. While the stress induced by copper stimulated the organism to invest more energy into the reproductive effort, represented by the cocoon production, the mode of action of cadmium indicates a different allocation of the energy available. Nickel exposure also showed a slight hormesis effect more oriented towards maintaining a longer reproductive effort. Whatever is the mode of action the final reproductive output, represented by the juveniles production, was significantly affected, at less extent for nickel. This adverse effect could be a direct consequence of the metal stress in the adult that provoked embryo impairment or greater sensitivity of the juveniles to exposure concentrations.

ii. The application of GUTS to model the organism’s survival using Open Model (University of Nottingham, UK) computer platform optimized by Markov Chain Monte Carlo algorithm to parameterise the model revealed that the selection of the dose metric affects the goodness of fit of the model. The comparison of the goodness of fit of models whose dose metrics applied were a) the subcellular copper concentration associated to the metabolically available copper fraction and, b) the total copper bioaccumulated, demonstrated that the application of the metabolically available copper fraction provides a more sensitive model fit.

In conclusion, this was the first study that applied this modelling framework to time series data generated by whole-sediment toxicity tests and also the first one to include the integration of direct measurements of the time course of internal metal concentrations to replace the currently used “scaled internal concentrations”. By the analysis of preliminary results, this method has demonstrated to improve the sensitivity of the model providing a better fit to model organism’ survival. We consider that this result highlights the importance of choosing a dose metric that helps to reduce the model uncertainty associated to indirect measurements and to unravel the mechanisms of toxicity underlying the physiological modes of action. We are confident that this approach can contribute to develop standardized models suitable specifically for chemical risk assessment.

Contact details: Dr. Olivia Campana, Postdoctoral Fellow
University of York, Environment Department, York UK