Evaluating the impact of experimental duration on toxicokinetic parameters and bioaccumulation factors in earthworm (<i>Eisenia fetida</i>) studies

Earthworms contribute to soil health and nutrient cycling and serve as a crucial food source for higher trophic organisms. The Organisation for Economic Co-operation and Development (OECD) 317 bioaccumulation test investigates the potential for chemicals to bioaccumulate in earthworms, where a kinetic bioaccumulation factor (BAF) is calculated as the ratio between the uptake and elimination rate constants, assuming first-order kinetics. This implies that steady state conditions are not necessary, suggesting independence of experimental durations. If valid, the kinetic BAF could be estimated within a shorter timeframe than recommended by the 6-week OECD guideline. However, there is limited data to validate this. This study aimed to determine whether toxicokinetic (TK) rate constants and BAF depend on experimental duration. Individual Eisenia fetida were exposed to three organic compounds in two parallel experiments labeled "short" and "long." The short experiment involved a 7-day exposure and 14-day elimination phase, and the long experiment entailed a 21-day exposure and 14-day elimination phase. A one-compartment TK model estimated uptake and elimination rate constants from exposure to bulk soil and water extractable fraction, from which kinetic BAFs were derived. Our findings indicated fast kinetics for all compounds. Importantly, in five out of six cases, kinetic rate constants from the short experiments fell within the confidence intervals of those from the long experiment, suggesting comparable results. Three of the six kinetic BAF values in the short treatment were within the 95% confidence interval of the long treatment. In conclusion, a 3-week bioaccumulation study can predict the outcome of a 5-week study. However, additional studies are needed before recommendations can be made regarding the applicability of shorter experimental approaches for earthworm bioaccumulation studies.