The effects of UV radiation on the acute toxicity of
retene (7-isopropyl-1-methylphenanthrene) to Daphnia magna Straus were studied.
Dehydroabietic acid (DHAA) from which
retene is formed in the vicinity of pulp and paper industry was also studied.
Pyrene,
anthracene, and
phenanthrene were used as model PAH compounds. The time taken for immobilization (ET50) was monitored under biologically effective UV-B dose rates of 240, 365, 565, and 650 mW m(-2) (UV-A and visible light also present). Median effective concentrations (EC50) were determined after a 15-min UV exposure (565 mW m(-2)) followed by 24 h in the dark.
Retene (10-320 microg l(-1)) was not acutely toxic in the dark. The induction of
phototoxicity was in agreement with the absorption properties of the compounds (absorption peak of
retene at around 300 nm). Photoinduced toxicity followed an order
pyrene >
anthracene >
retene.
Phenanthrene and DHAA were not acutely phototoxic. Accumulation of the compound in Daphnia before UV exposure was essential. Some changes in the absorption spectra of the compounds were seen after a 5-h UV irradiation (565 mW m(-2)), but none of the irradiated compounds were acutely toxic without further UV exposure. Therefore, the enhanced acute toxicity was primarily due to internal
photosensitization reactions rather than photomodification. The dissolved fraction of 25% pulp and paper mill effluent reduced
phototoxicity by attenuating UV radiation. The
phototoxicity of
retene was a function of both the exposure concentration and the UV-B dose rate, but relatively high UV-B dose rates and concentrations were needed for the acute photoinduced toxicity.