In ecotoxicity assessment, the ambient exposure concentration is typically applied to quantify the toxic potential of xenobiotic substances. However, exposure and organism-related differences in bioconcentration often cause a considerable variability of toxicity data. This can be minimized by using the internal organism concentration, because toxicokinetic modifying factors are considered implicitly. In the present study, the relationship between ambient and internal concentration-time profiles was investigated for zebrafish (Danio rerio) embryos. The aim was to gain a better understanding and interpretation of exposure-based methods using this model organism. For this purpose, a simple and effective approach to determine the internal concentration was developed. Embryos were exposed to a series of 4 neutral organic substances (naphthalene, fluorene, fluoranthene, benz[a]anthracene) of different hydrophobicity for 72 h. The internal and ambient concentrations were measured at 8 to 9 time points. Kinetics of uptake and elimination were modeled using a first-order 1-compartment model. Biotransformation processes appeared to influence the internal concentrations of fluoranthene and benz[a]anthracene after 48 h. The bioconcentration factors (BCFs) obtained are in excellent agreement with those determined in previous studies using radiolabeled substances. The method demonstrated in the present study is a further step toward a refined ecotoxicity assessment using fish embryos, which links toxicity to the chemical concentration within the organism. This system may also be considered as an alternative to animal testing for BCF determination.

The toxic potency of chemicals is determined by using the internal effect concentration by accounting for differences in toxicokinetic processes and mechanisms of toxic action. The present study examines toxicokinetics of specifically acting and reactive chemicals in the green algae Scenedesmus vacuolatus by using an indirect method. Concentration depletion in the exposure medium was measured for chemicals of lower (log KOW < 3: isoproturon, metazachlor, paraquat) and moderate (log KOW 4-5: irgarol, triclosan, N-phenyl-2-naphthylamine) hydrophobicity at 7 to 8 time points over 240 min or 360 min. Uptake and overall elimination rates were estimated by fitting a toxicokinetic model to the observed concentration depletions. The equilibrium of exposure concentrations was reached within minutes to hours or was even not observed within the exposure time. The kinetics of bioconcentration cannot be explained by the chemical's hydrophobicity only, but influential factors such as ionization of chemicals, the ion trapping mechanism, or the potential susceptibility for biotransformation are discussed. Internal effect concentrations associated with 50% inhibition of S. vacuolatus reproduction were predicted by linking the bioconcentration kinetics to the effect concentrations and ranged from 0.0480 mmol/kg wet weight to 7.61 mmol/kg wet weight for specifically acting and reactive chemicals. Knowing the time-course of the internal effect concentration may promote an understanding of toxicity processes such as delayed toxicity, carry-over toxicity, or mixture toxicity in future studies.

The results indicate that WC is able to modulate the enzymes SOD and GPX in blood cells in vitro and in vivo, and suggest that the capacity of moderate intake of CV to induce detoxification is dependent in part on the GSTM1 genotype.