Abstract
Molecular responses to acute toxicant exposure can be effective biomarkers, however responses to chronic exposure are less well characterised. The aim of this study was to determine chronic molecular responses to environmental mixtures in a controlled laboratory setting, free from the additional variability encountered with environmental sampling of wild organisms. Flounder fish were exposed in mesocosms for seven months to a contaminated estuarine sediment made by mixing material from the Forth (high organics) and Tyne (high metals and tributyltin) estuaries (FT) or a reference sediment from the Ythan estuary (Y). Chemical analyses demonstrated that FT sediment contained significantly higher concentrations of key environmental pollutants (including polycyclic aromatic hydrocarbons (PAHs), chlorinated biphenyls and heavy metals) than Y sediment, but that chronically exposed flounder showed a lack of differential accumulation of contaminants, including heavy metals. Biliary 1-hydroxypyrene concentration and erythrocyte DNA damage increased in FT-exposed fish. Transcriptomic and 1H NMR metabolomic analyses of liver tissues detected small but statistically significant alterations between fish exposed to different sediments. These highlighted perturbance of immune response and apoptotic pathways, but there was a lack of response from traditional biomarker genes. Gene-chemical association annotation enrichment analyses suggested that polycyclic aromatic hydrocarbons were a major class of toxicants affecting the molecular responses of the exposed fish. This demonstrated that molecular responses of sentinel organisms can be detected after chronic mixed toxicant exposure and that these can be informative of key components of the mixture.
Highlights
In a study described previously (Leaver et al, 2010) we reported that hepatocytes isolated from flounders that had been exposed in mesocosms for 7 months to relatively clean, or multiply-polluted, sediments did show differences in gene expression
ICES recommend a Background Assessment Criteria (BAC) value for the Comet assay of 5% DNA in tail to assess whether exposure to genotoxins has occurred in fish; the values here of 3.8% in the Y fish and 7.8% in the FT fish are consistent with the latter being exposed to genotoxins and the former not being highly exposed
Chemical analyses of flounder chronically exposed to more highly contaminated Tyne/Forth estuarine sediment demonstrated a lack of contaminant accumulation in fish tissues
Summary
Williams et al / Chemosphere 108 (2014) 152–158 useful for determining toxicant modes of action, but relating these results to field studies is not straightforward, due to the differences outlined above and the non-pollutant variation inevitably encountered in the environment Certain biomarker genes, such as cytochrome P450 1A (CYP1A) and vitellogenins (VTG), have been found to be useful in revealing the classes of contaminants to which wild animals have been exposed (van der Oost et al, 2003). We aimed to determine if the different sediment types elicited different transcriptional and metabolic responses from the flounders, whether there was evidence of DNA damage and whether there was accumulation of contaminants and their metabolites in flounder tissue and bile This would allow further validation of the results from the previous hepatocyte study, and contribute to understanding the differences between chronic and acute chemical exposures
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