Environmental photooxidation and mixture toxicity of polycyclic aromatic hydrocarbons

Exposure of polycyclic aromatic hydrocarbons (PAHs) to light has been shown to result in an increase in toxicity of PAHs to aquatic organisms. The observed increase in toxicity is a general phenomenon, and has been observed in bacteria, plants, invertebrates and fish. Two mechanisms of action for this increase in toxicity are photosensitization (production of singlet oxygen) and photooxidation, whereby the oxidation products are more toxic than the parent PAHs. Either may have the greater effect, depending on the organism and the exposure history. Previously, specific toxic products of PAH photooxidation have not been well characterized. Furthermore, PAHs and PAH photoproducts exist almost exclusively as complex mixtures in environmental compartments, and methods for addressing mixture toxicity are required.; PAH photoproducts were generated by exposing PAHs, either in solution or in bound phase, to natural sunlight or a light source mimicking solar radiation. The generated photooxidation products were identified by GC/MS, HPLC/diode array, and/or by comparison with authentic standards. Photooxidation experiments were conducted with two to five ring PAHs, specifically naphthalene, phenanthrene, fluoranthene, pyrene, benzo(a)anthracene, benzo(b)anthracene, chrysene, and benzo(a)pyrene. The toxicity of observed photoproducts was addressed either by the use of toxicity assays, or reference to published toxicity data. The primary organism used to estimate the relative toxicity of PAH photoproducts and intact PAHs was the luminescent marine bacteria, Photobacterium phosphoreum. It was found in many cases that the products of PAH photooxidation were more toxic than the intact PAHs. The observed toxicity was further corroborated by toxicity assessment using other species as test organisms. PAH quinones were identified as a class of compounds frequently having greater toxicity than the intact PAHs. Some observed photoproducts were also previously identified as toxicants or mutagens, resulting from metabolism of PAHs in mammalian systems.; Two factors influencing the photodegradation rates of PAHs were identified. The ability of PAHs to generate singlet oxygen by photosensitization reactions was related to PAH reaction rates, as was the change in the delocalization energy of PAH electronic orbitals on oxidation. Changes in delocalization energy were also used to predict thermodynamically favoured oxidation products.; PAHs occur in environmental compartments as complex mixtures, and each PAH may produce several major photoproducts. Thus, a means to assess the toxicity of a mixture of PAHs and PAH photoproducts is needed. ‘Factional simplex designs’ were generated to address the toxicity of complex mixtures. These designs provide a means to screen for interactions and to investigate the behaviour of many-component mixtures as a whole, without extensive data requirements. One of these designs was used to investigate interactions within a group of PAHs and PAH photoproducts, using the P. phosphoreum toxicity assay. For the given mixture of chemicals and the bacterial toxicity assay, a concentration additive model of interaction was found to be a good descriptor of the data set, though a trend to slightly greater than additive interaction was observed for some mixtures.