Short- and long-term photoinduced toxicity of polycyclic aromatic hydrocarbons to luminescent bacteria

Toxicity of most polycyclic aromatic hydrocarbons (PAHs) to aquatic organisms can be greatly enhanced upon exposure of the target organism and/or the chemicals to the ultraviolet (UV) radiation present in sunlight. There are two major mechanisms involved in the photoinduced toxicity of PAHs: photosensitization and photomodification. In the former, production of singlet oxygen leads to cellular damage. In the latter, photooxidation of PAHs results in new compounds (usually oxygenated PAHs) that are often more toxic than their Parent PAHs. In an effort to examine the photomodification and photosensitization processes of PAHs, microbial toxicity assays were developed to measure short- and long-term photoinduced toxicity. The test organism was the luminescent bacterium Vibrio fischeri (strain NRRL B-11177). Two physiological characteristics of this test organism that make it attractive for toxicity testing are a short division cycle and an inducible luciferase pathway. The bioassay methods were based on inhibition of luminescence and growth of Vibrio fischeri . The short-term assay developed was based on inhibition of luminescence after a 15 minute incubation with a test chemical. The long-term assay involved returning the cells to the incubator after the short-term endpoint was measured and growing them for 18 hours with the test chemical. The sensitivities of the assays were found to correlate well with other bioassays and they were effective at screening a large number of compounds. Both assays could be preformed in darkness or simulated solar radiation (SSR) to examine the effects of light on PAH toxicity. The short-term and long-term assays were tested with representative intact PAHs and modified PAHs. With the short-term assay the toxicity of all the chemicals was the same in SSR or darkness. This means photoinduced toxicity is not apparent in a short-term exposure. However, with the long-term assay, SSR did enhance PAH toxicity. Thus, photoinduced toxicity could be observed under appropriate conditions. Strikingly, ANT, one of the most phototoxic PAHs, was not toxic in SSR or darkness. It was thought that the reduced carbon in the medium might be limiting ANT bioavailability. The long-term assay was thus modified by incorporating an 8 hour pre-incubation period in minimal medium. This was found to be effective, as the photoinduced toxicity of ANT and other PAHs was readily observed if a pre-incubation in minimal medium was employed. Having developed the V. fischeri assay, a quantitative structure-activity relationship (QSAR) previously developed for the aquatic plant Lemna gibba was applied to the bacteria. Summing two factors, one for photosensitization and one for photomodification resulted in predictive values that showed strong correlation to the V. fischeri toxicity data. Thus, a QSAR model derived for plants accurately described the toxicity of PAHs to a bacterial species. This indicates that the bipartite mechanism of PAH photoinduced toxicity is broadly applicable. The V. fischeri short- and long-term assays were finally applied to assessment of PAH-contaminated sediments. The sediments were collected from Hamilton Harbor, ON, and Mohawk Lake, Brantford, ON. They were fractionated and found to contain PAHs and oxyPAHs. Strikingly, the oxyPAH fractions were observed to be the most toxic samples. Thus, oxyPAHs in the environment have a hazard potential.