Tissue differences, dose–response relationship and persistence of DNA adducts in blue mussels ( Mytilus edulis L.) exposed to benzo[ a]pyrene

Abstract

Baltic Sea blue mussels ( Mytilus edulis) were experimentally exposed to the genotoxic model substance benzo[ a]pyrene (B[ a]P) to study DNA adduct formation. The specific aims were (a) to examine where in the mussels the DNA adducts were formed, in gills or digestive gland; (b) to study the dose–response relationship between B[ a]P exposure and DNA adduct formation; and (c) to examine the persistence of the formed adducts. A Scope for growth (SFG) study was also run to compare physiological responses of the mussels with the degree of DNA adduct formation. In an initial dose–response experiment, the mussels were exposed to 0, 5, 50, and 100 μg/l of tritium labelled B[ a]P under semi-static conditions for 4 days, and thereafter the bioaccumulation of B[ a]P and DNA adduct formation in different tissues was determined using liquid scintillation counting and 32P-postlabelling analysis, respectively. In a following exposure–depuration experiment, mussels were exposed to 17 μg/l of radiolabelled B[ a]P under semi-static conditions for 6 days. B[ a]P accumulation and DNA adduct formation were determined during the exposure, and B[ a]P elimination and persistence of DNA adducts were studied during 28 days of depuration in uncontaminated water. The results revealed large tissue differences in DNA adduct formation. DNA adduct levels were not elevated in the digestive gland of the mussels at any exposure concentration (0–100 μg/l), even though the highest B[ a]P tissue concentrations were found in the digestive gland (1.0±0.1 mg B[ a]P/g tissue dry wt at 100 μg/l, mean±SE, n=12). DNA adducts were on the other hand formed in the gills, with the highest levels found in mussels exposed to 50 and 100 μg B[ a]P/l, and a dose dependent increase in adduct levels (from 1.6 to 5.9 nmol adducts/mol nucleotides) from 0 to 50 μg B[ a]P/l. In gills, DNA adduct levels increased with time during the 6-day exposure period in the exposure–depuration experiment, and then persisted for at least 2 weeks after exposure cessation while B[ a]P tissue levels exhibited a rapid decrease (half-life of 8 days). No significant differences were observed in SFG between the control and exposed groups. Since DNA adducts exhibited a relatively high persistence in gills compared to B[ a]P tissue concentrations, they seem to be a more integrated measure of genotoxic exposure than only chemical analysis of the contaminant bioaccumulation. The results also suggest that if using analysis of DNA adducts in M. edulis for monitoring purposes, analysis of gills in addition to the more commonly used digestive gland should be taken into consideration.