INCREASED POLYCYCLIC AROMATIC HYDROCARBON TOXICITY FOLLOWING THEIR PHOTOMODIFICATION IN NATURAL SUNLIGHT - IMPACTS ON THE DUCKWEED LEMNA-GIBBA L G-3

The authors previously demonstrated that simulated solar radiation (SSR), with a fluence rate of only 40 mu mol m(-2) sec(-1) increased polycyclic aromatic hydrocarbon (PAH) toxicity to the duckweed Lemna gibba and that PAHs photomodified in SSR (generally oxygenation of the ring system) are more toxic than the parent compounds (Huang et at, Environ. Toxicol. Chem, 1993, 12, 1067-1077). It is not known, however, to what extent toxicity of PAHs can increase due to photomodification. Thus, natural sunlight, which has a high fluence rate (approximately 2000 mu mol m(-2) sec(-1)), was used to photomodify anthracene, benzo[a]pyrene, fluoranthene, phenanthrene, and pyrene. Toxicity was based on growth inhibition of L. gibba, measured as the rate of production of new leaves over an 8-day period, Initially, the toxicity of the PAHs applied in intact form was probed, with the compounds demonstrating greater toxicity in sunlight than in SSR, Next the PAHs were photomodified in sunlight prior to incubation with the plants. The half-lives of the PAHs in sunlight ranged from 12 min to 30 hr, Although most of the products of PAH photomodification are not yet identified, the degree that PAH toxicity increased following photomodification in sunlight could still be probed, The mixtures of photomodified chemicals that were derived from each PAH in sunlight were applied of L. gibba and growth inhibition under 100 mu mol m(-2) sec(-1) of SSR was determined, The LC(50)s for the PAH photoproducts generated in sunlight were an order of magnitude lower than the LC(50)s for the PAHs applied in intact form. (C) 1995 Academic Press, Inc.