Toxicokinetics and oxidative stress in Tubifex tubifex exposed to hexachlorocyclohexane isomers

In this study, Tubifex tubifex (Oligochaeta, Tubificida) was exposed to hexachlorocyclohexane isomers (alpha-HCH, beta-HCH and gamma-HCH) in an aquatic environment to study the bioaccumulation and elimination of these chemicals in this organism. Rapid and linear accumulation processes with high accumulation factor values (AFs) were found during the 14 d water exposure, and there were no significant differences among the concentrations of HCH isomers. The unsteady state (linear shape) might be due to the change of feeding physiology of tubifex. The AFs of alpha-HCH were significantly different from those of beta-HCH, which may be due to different physicochemical properties. Furthermore, no enantioselective bioaccumulation of alpha-HCH enantiomers was found in tubifex tissue. The data from elimination experiments fitted a typical first-order kinetics model. The elimination rates (k(e)) decreased with increasing hydrophobicity (K-ow) for HCHs. If evaporation was not considered, the majority of HCHs were metabolized by tubifex. So the existence of worms and their biotransformation ability could change the distribution of HCHs in the aquatic environment and reduce the surrounding HCHs levels, which is significant in environmental pollution control. This study also assessed the responses of biochemical biomarkers. Increased superoxide dismutase (SOD) and malondialdehyde (MDA) levels demonstrated the presence of oxidative stress in worms. The different changes of catalase (CAT) activities depend on many factors, and there are other enzymes involved in H2O2 catabolism. Additionally a reduction of glutathione-S-transferase (GST) was observed indicating the toxic impacts of HCHs. Our results suggested that bioaccumulation of HCHs in T. tubifex can cause sublethal toxicity. T. tubifex could reflect the HCHs contaminant concentrations in the environment to some extent, and the worms may be proposed as biomarkers to characterize water contamination.