Phototoxicity of azaarene isomers to the marine flagellate Dunaliella tertiolecta

Many studies have shown that narcosis or baseline toxicity of polycyclic ar omatic hydrocarbons (PAHs) is strongly related to their lipophilicity. For azaarenes, such relationships have also been demonstrated, but for some com pounds, deviations from these relationships have been observed, even for cl osely related compounds such as isomers. In the present study, the toxicity of four azaarene isomer pairs to the marine flagellate Dunaliella tertiole cta was determined. For quinoline, isoquinoline. acridine, phenanthridine. benz[a]acridine. and benz[c]acridine. the 72-h median effective concentrati ons for growth were 571, 464, 2.10, 14.7, 0.50, and 0.11 muM, respectively. For the five-ringed isomers dibenz[a,i]acridine and dibenz[c,h]acridine, n o effects were observed at the highest concentration tested (0.1 and 0.005 muM, respectively). Growth inhibition by the two-, three-, and four-ringed isomer pairs to D. tertiolecta was well described by molecular volume and l og K-ow, indicating a narcotic mode of action. However, the toxicity of acr idine and benz[c]acridine was much higher than that of their respective iso mers, phenanthridine and benz[a]acridine. suggesting an additional specific mode of action. Based on the differences in energies between the highest o ccupied molecular orbital and the lowest unoccupied molecular orbital, acri dine and benz[c]acridine are susceptible to ultraviolet (UV) radiation, in contrast to the other tested compounds. Because UV was present, it is there fore argued that the toxicity of both compounds was photoenhanced. Photoenh anced toxicity may increase the environmental hazard of azaarenes, indicati ng the importance of enlarging the present monitoring of PAHs with phototox ic N-heterocyclic PAHs and incorporating this mode of action in water-quali ty criteria.