Impacts of structural photomodification of anthracene derivatives on the aquatic higher plant Lemna gibba (Lemnaceae) under actinic radiation and modeling of toxicological risk

The toxicity of polycyclic aromatic hydrocarbons (PAHs) is enhanced by light via photosensitization reactions (production of active oxygen) and photo-modification of the chemicals (e.g., oxidation) to more toxic compounds. Anthracene (ANT) toxicity in particular increases dramatically following photo-modification. The objective of this study was to identify the photooxidation products of ANT and assess the toxicity of selected photoproducts. It was found that PAH photooxidation products likely exist as complex, dynamically changing mixtures in PAH contaminated aquatic environments. High Performance Liquid Chromatography analysis of anthracene photooxidation revealed a complex array of oxidation products; prevalent among these were anthraquinone (ATQ) and hydroxy-anthraquinones (hATQs). Thirteen compounds (ANT, ATQ, a hANT and 10 hATQs) were tested for toxicity reflected by growth inhibition of the duckweed Lemna gibba L. G-3. Almost all the ANT photoproducts were directly toxic to L. gibba. Light was not a requirement for toxicity of most of the ANT photoproducts. However, in about half the cases, the ATQ compounds were rapidly photooxidized and the resultant photoproducts were more toxic than the parent compounds. Photosynthetic activity was monitored both in vivo and in vitro by measuring chlorophyll a (Chl a) fluorescence. Impacts on Chl a fluorescence were found to correlate with whole organism toxicity for the thirteen compounds used in this study. One Chl a fluorescence end point (FV/F M, maximal PSII activity) was found to be a measure of acute toxicity. Fluorescence quenching (FQ/FM) was found to be a measure of chronic toxicity. Thus, Chl a fluorescence was validated as a bioindicator of toxicity of photomodified ANT. Finally, computer analysis was used to determine which shape features of ANT and its photoproducts best correlated with toxicity. It was found that a QSAR could be generated based on computer generated electron density shape maps of the molecules.