Differentiation between bioavailable and total hazard potential of sediment-induced DNA fragmentation as measured by the comet assay with zebrafish embryos

Goals, Scope and Background. While water quality strongly improved over decades in the Rhine River, sediments still reflect elapsed contaminations of organic pollutants and heavy metals. In comparing genotoxic effects induced by both sediment extracts and whole sediments, a ratio of bioavailable toxicity and total extractable toxicity is obtained. Since contaminated sites whose contaminants are toxic and as well bioavailable present an elevated risk to the ecosystem, such ratios may be used as a warning signal to identify sites of primary concern. Methods. Accordingly, two different exposure scenarios were compared to reveal the genotoxic potential of 18 sediment samples derived from 9 sample sites along the River Rhine. For assessment of effects on genome integrity, DNA fragmentation was measured using the comet assay with primary cells isolated from zebrafish embryos previously exposed to either organic sediment extracts or freeze-dried sediments at sublethal concentrations. Additionally, chemical data were used to determine responsible pollutants and correlate them with biological effects. Results. Whereas 17 out of 18 sediment extracts caused significant DNA damage to the embryo cells, only 4 native sediments showed a genotoxic potential. Thus, under field-like exposure conditions, a major part of potentially genotoxic compounds seem to remain particle-bound and ineffective, as shown for whole sediment exposure. Conversely, the organic extracts seem to contain enriched concentrations even of hardly soluble substances. Hence, organic extracts may be used as a screening tool to address potentially polluted sites, even though the relevance of these results for the field situation may be questionable. Investigations on native sediments determined few sites with bioavailable and therefore ecologically most relevant genotoxic sediment compounds. Discussion. However, these results may underestimate the total hazard potential of sample sites with hardly bioavailable substances. Chemical data revealed a variety of anthropogenic pollutants, ranging from PAHs to heavy metals. Nevertheless, chemical data on the measured priority pollutants did not fully explain the pollution pattern of the bioassays but clearly determined substances of concern (e.g., HCB, heavy metals) in particular sample sites. Conclusions. There is a striking advantage in assessing the genotoxicity by means of different exposure scenarios that focus on either bioavailable or extractable fractions, as the combination of the results allows obtaining information on specific properties of the genotoxicants and their bioavailability. An additional correlation with chemical data should be required to identify priority pollutants, as long as the responsible contaminant is known a priori. As many studies revealed inherent failures of such a correlation, an effect-driven analysis of pollutants is recommended as a promising tool to identify even non-priority pollutants by means of their ecotoxicological effectiveness.