Toxicity cutoff of aromatic hydrocarbons for luminescence inhibition of Vibrio fischeri

• Apparent toxicity cutoff for hydrophobic chemicals for Vibrio fischeri test was investigated.
• A critical chemical activity model explained the apparent toxicity cutoff very well.
• Toxic effects were not observed for a PAH with log Kow>5 or melting point >100 °C.
• A QSAR was derived for petroleum hydrocarbons between log (1/EC50) and log Kow.

Effects of individual petroleum hydrocarbons on the luminescence inhibition of Vibrio fischeri were evaluated according to a standard protocol to develop a quantitative structure−activity relationship and identify the apparent toxicity cutoff. Eighteen aromatic hydrocarbons, including benzene and its derivatives and polycyclic aromatic hydrocarbons (PAHs), were chosen as model compounds with their log Kow values between 2.7 and 6.4. The obtained values of 50 percent luminescence inhibition (EC50) showed a good linear correlation with log Kow up to ~5. However, toxic effects were not observed for more hydrophobic chemicals with log Kow value >5. The calculated chemical activities that caused EC50 were mostly between 0.01 and 0.1. This agrees with an earlier hypothesis concerning a chemical activity resulting the critical membrane concentration of aromatic hydrocarbons. The highest chemical activities for aromatic hydrocarbons with log Kow value >5 or melting point >100 °C are <0.01 when they are spiked at their water solubility level according to the standard test protocol; this occurs for two primary reasons: (1) partitioning between organism and the test solution and (2) decreasing fugacity ratio with increasing melting point. Accordingly, luminescence inhibition by petroleum hydrocarbons is well explained by the baseline toxicity model. However, the apparent toxicity cutoff observed for single chemicals is not necessarily valid in a complex mixture, because baseline toxicity is regarded concentration additive.

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