TOXICITY OF A PAH PHOTOOXIDATION PRODUCT TO THE BACTERIA PHOTOBACTERIUM-PHOSPHOREUM AND THE DUCKWEED LEMNA-GIBBA - EFFECTS OF PHENANTHRENE AND ITS PRIMARY PHOTOPRODUCT, PHENANTHRENEQUINONE

Phenanthrene (PHE) undergoes a significant increase in toxicity after exposure to simulated or natural sunlight in aqueous media, coincident with the appearance of PHE photoproducts. To investigate whether the primary photoproduct of PHE, 9,10-phenanthrenequinone (PHEQ), contribu tes to the increased hazards of solutions containing photomodified PHE ,toxicity assays were conducted using the marine bacteria Photobacteri um phosphoreum and the aquatic plant Lemna gibba (duckweed). Photo-bac terium phosphoreum was exposed to PHE, PHEQ, a photomodified PHE mixtu re containing known amounts of PHE and PHEQ (pmPHE), and a mixture mim icking the amounts of PHE and PHEQ in the pmPHE mixture. The bacteria were found to be equally sensitive to PHE in simulated solar radiation (SSR, a light source with a visible light:UVA:UVB ratio similar to th at of sunlight) or darkness, with an EC50 of 0.53 mg/L. In both darkne ss or SSR, solutions containing PHEQ (with or without PHE) all exhibit ed an EC50 of 0.06 to 0.10 mg/L based on PHEQ concentrations, indicati ng that PHEQ was the primary active component of the pmPHE mixture. Le mna gibba was tested in SSR and visible light with PHE, PHEQ, and the pmPHE mixture. The calculated EC50 for PHE was 3.5 mg/L in SSR and 10. 8 mg/L in visible light, showing that the presence of UV radiation in the SSR source increased the phytotoxicity of PHE. Strikingly, PHEQ wa s much more toxic to L. gibba than PHE ina light-independent manner (a n EC50 of 0.53 and 0.57 mg/L PHEQ in dark and SSR, respectively). Thus , for both P. phosphoreum and L. gibba the major photooxidation produc t of PHE in SSR, PHEQ, is the more toxic of the two chemicals.