The current soil risk assessment procedure follows a tiered approach, where a Tier 1 analysis is based on standard laboratory test results and estimates risks for the worst-case environmental scenarios. The exposure experiments carried out under laboratory conditions have a static approach and they do not consider the spatial variability and seasonal dynamics in nature. Therefore, higher tier assessment is used to refine the estimated risk of exposure by conducting field or semi-field studies under locally relevant conditions. Nevertheless, there is a substantial gap between Tier 1 laboratory studies and field studies and uncertainties arise when exposure and effect studies from the lab are extrapolated to different ecosystems. This suggests that intermediate tier assessment, such as mechanistic effect models, can be useful to integrate pesticide effects from laboratory studies to exposure scenarios expected in the field.
Mechanistic effect models like toxicokinetic-toxicodynamic (TK-TD) models offer the possibility to provide more realistic exposure profiles by taking into account spatiotemporal variability of chemicals and linking effects to life-history traits of organisms. TK includes processes of uptake, elimination, internal distribution, and biotransformation of chemicals, whereas TD studies the processes leading to toxic effects. When the uptake rate of a certain chemical exceeds the elimination rate it starts to accumulate in an organism, and it can create toxic effects if the critical body threshold is reached. However, the biotransformation rate at which the parent compound is degraded limits the overall bioaccumulation of the toxicant. So far, few studies have analyzed biotransformation in soil invertebrates, and yet there are no available predictive models to estimate the metabolites of a given chemical in these organisms. Hence, it is important to develop sound predictive TK models to estimate the time needed to start biotransformation and the metabolites that are formed afterward.
Additionally, the TK of chemical residues in soil needs to be studied more in-depth because the bulk concentration in soil is not a precise measure for risk assessment since not all of it might be available to organisms. Uncertainties exist regarding the availability of chemical residues in pore water, residues bound to organic matter, and non-extractable residues. In fact, the bioavailable fraction of chemicals in soil depends on soil characteristics (total organic carbon content, water holding capacity of soil, cation exchange capacity, etc.), physicochemical properties of compounds (hydrophobicity, polarizability, solubility, etc.), and species-specific traits. Understanding how these factors interact with each other and how they affect the uptake of chemicals in organisms allows making predictions not only for bioaccumulation of chemicals but also for the effects that a certain toxicant may have on soil-dwelling organisms.
TD models, on the other hand, are based on the biologically effective dose that causes an effect at the individual level. A well-established framework used in TD modeling is the Dynamic Energy Budget (DEB) theory. DEB-based models study the sub-lethal effects of chemicals on different traits such as growth, development, maturation, and reproduction by changes in the allocation of resources. Thus, a simple TKTD model offers a dynamic approach linking the effects on various traits over the entire life cycle of an organism that can be extrapolated to populations and untested scenarios as well. Furthermore, these models offer the possibility to study effects at non-conventional endpoints.
In this context, the focus of the research project is to assess the long-term effects of pesticides in terrestrial organisms by applying TK-TD models. Earthworms are going to be used as model species and experimental lab work with live organisms is going to be conducted to calibrate model parameters. To describe biotransformation in earthworms, a TK model is going to be developed, and metabolites identified by analytical instruments will explain the main metabolic pathways involved in the degradation of a certain chemical. Furthermore, relevant toxicological endpoints are going to be assessed to link exposure effects on individual organisms with the affected primary Mode of Actions (pMoAs) by applying DEB models. In this respect, the project will aim to:
• Understand the relationship between the main environmental and physicochemical parameters that affect the bioavailability of chemicals in soil.
• Recognize the main metabolites and enzymatic pathways involved on the biotransformation of chemicals in earthworms.
• Elucidate whether the main enzymatic pathways involved in biotransformation in earthworms are conservative within oligochaetes and across different soil dwelling taxa.
• Study the effects of chronic exposure of low dose of pesticides and link effects on endpoints with the affected pMoAs.
The present PhD project is part of an EU funded Horizon 2020 MSCA-ITN programme called CHRONIC. It includes 13 PhD projects aimed at developing tools and approaches to identify relevant nonstandard modes of toxicity for low chronic chemical exposure and integrate these with environmental stressors. Different educational training activities are organized by the CHRONIC consortium, which includes several European universities, research institutions, environmental agencies, companies and ONGs. The activities will be carried out every six months during an internal conference where in-depth topic related courses will be taught, as well as more general sessions in which the student will gain soft and transferable skills.