BIOTRANSFORMATION OF BENZO[A]PYRENE AND OTHER POLYCYCLIC AROMATIC-HYDROCARBONS AND HETEROCYCLIC-ANALOGS BY SEVERAL GREEN-ALGAE AND OTHER ALGAL SPECIES UNDER GOLD AND WHITE-LIGHT

This laboratory has shown that the metabolism of benzo[a]pyrene (BaP), a carcinogenic polycyclic aromatic hydrocarbon (PAH), by a freshwater green alga, Selenastrum capricornutum, under gold light proceeds thro ugh a dioxygenase pathway with subsequent conjugation and excretion. T his study was undertaken to determine: (1) the effects of different li ght sources on the enzymatic or photochemical processes involved in th e biotransformation of BaP over a dose range of 5-1200 mg/l; (2) the p hototoxicity of carcinogenic PAHs and muta. genic quinones to a green alga; (3) the ability of other algal systems to metabolize BaP. Cultur es were exposed to different doses of BaP for 2 days at 23 degrees C u nder gold,-white or UV-fluorescent lights on a diurnal cycle of 16 h l ight, 8 h dark. Under gold light, metabolites of BaP produced by Selen astrum capricornutum were the dihydrodiols of which the 11,12-dihydrod iol was the major metabolite. Under white light, at low doses, the maj or metabolite was the 9,10-dihydrodiol. With increasing dose, the rati o of dihydrodiols to quinones decreased to less than two. With increas ing light energy output, from gold to white to UV-A in the PAH absorbi ng region, BaP quinone production increased. Of other carcinogenic PAH s studied, only 7H-dibenz[c,g]carbazole was as phototoxic as BaP while 7,12-dimethylbenz[a]anthracene, dibenz[a,j]acridine and non-carcinoge nic PAHs, anthracene and pyrene, were not phototoxic. The 3,6-quinone of BaP was found to be highly phototoxic while quinones that included menadione, danthron, phenanthrene-quinone and hydroquinone were not. T he data suggest that the phototoxicity of BaP is due to photochemical production of quinones; the 3,6-quinone of BaP is phototoxic and is pr obably the result of the production of short lived cyclic reactive int ermediates by the interaction of light with the quinone. Lastly, only the green algae, Selenastrum capricornutum, Scenedesmus acutus and Ank istrodesmus braunii almost completely metabolized BaP to dihydrodiols. The green alga Chlamydomonas reinhardtii, the yellow alga Ochromonas malhamensis, the blue green algae Anabaena flosaquae and euglenoid Eug lena gracilis did not metabolize BaP to any extent. The data indicate that algae are important in their ability to degrade PAHs but the degr adation is dependent on the dose of light energy emitted and absorbed, the dose of PAHs to which the algae are exposed, the phototoxicity of PAHs and their metabolite(s) and the species and strain of algae invo lved. All of these factors will be important in assessing the degradat ion and detoxification pathways of recalcitrant PAHs by algae.