William H.L. Siu

DNA ADDUCT FORMATION AND DNA STRAND BREAKS IN GREEN LIPPED MUSSELS (PERNA VIRIDIS) EXPOSED TO BENZO[A]PYRENE: DOSE AND TIME DEPENDENT RELATIONSHIPS

Green-lipped mussels, Perna viridis, were exposed to 0, 0.3, 3 and 30 micrograms l-1 (nominal concentrations) B[a]P under laboratory conditions over a period of 24 days. Mussels were collected on day 0, 1, 3, 6, 12, 18 and 24, and the levels of DNA adducts and DNA strand breaks in their hepatopancreas tissues monitored. Mussels exposed to 0.3 and 3 micrograms l-1 B[a]P showed marked increases in strand breaks after 1 day of exposure. DNA strand break levels in these mussels remained high and significantly different from the control values until day 3 for the 0.3 microgram l-1 treatment group, and day 6 for the 3 micrograms l-1 treatment group. This was followed by a gradual reduction in strand breaks. After 12 days, the levels of both groups had returned to the same level as that of the control. No increase in DNA strand breaks was observable in mussels exposed to 30 micrograms l-1 B[a]P in the first 12 days of exposure, but a significant increase was observed from day 12 to day 24. Increasing B[a]P concentrations resulted in elevated DNA adduct levels after 3-6 days of exposure, but this pattern of dose-related increase disappeared after 12 days. These results indicate that a better understanding of the complex interactions between exposure levels and durations is crucially important before DNA adduct levels and DNA strand breaks in P. viridis can be used as effective biomarkers for monitoring genotoxicants in marine waters.

Exposure and time dependent DNA strand breakage in hepatopancreas of green-lipped mussels (Perna viridis) exposed to Aroclor 1254, and mixtures of B[a]P and Aroclor 1254

Green-lipped mussels (Perna viridis) were exposed to Aroclor 1254 (0.5, 5 and 50 microgl(-1)) and a mixture of benzo[a]pyrene (B[a]P) and Aroclor 1254 (0.3+0.5 and 3+5 microgl(-1)) for 12 days. On day 0, 1, 3, 6 and 12, the levels of DNA strand breaks in the mussel hepatopancreas were monitored using an alkaline unwinding assay. The results were compared to the findings of a previous study in which the levels of DNA strand breakage in the same species were measured following exposure to various concentrations of B[a]P (0.3, 3 and 30 microgl(-1)). The results indicated that Aroclor 1254 at a concentration </=50 microgl(-1) did not cause any increase in DNA strand breaks while B[a]P at concentrations ranging from 0.3 to 3 microgl(-1) caused an increase in strand breaks after one day of exposure, followed by an apparent rapid recovery. In contrast, exposure to 30 microgl(-1) B[a]P caused no increase in DNA strand breaks over the exposure period. This was postulated to be due to an early elicitation of the DNA repair system by the relatively high exposure level of B[a]P. This hypothesis was tested in the present study, and the results suggest that exposure to the high B[a]P concentration might have elicited the defense mechanism within the mussels, resulting in no observed increase in DNA strand breaks. An increase in strand breaks was, however, evident when the mussels were exposed to lower B[a]P levels. The levels of DNA strand breaks were correlated with the body burden of B[a]P and Aroclor 1254 but no significant relationship was observed, possibly owing to the rapid metabolism of the toxicant and/or an effective DNA repair mechanism. As a result, DNA strand breakage in the hepatopancreas of green-lipped mussels may not be a suitable biomarker of exposure to the above toxicants in the marine environment. Our findings also suggest that it would be instructive to investigate (1) the role of DNA repair enzymes in the exposed mussels; and (2) the correlation between the activity of these enzymes and the body burden of specific toxicants.