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Glycolipids produced by Rhodococcus species H13-A and a representative synthetic surfactant Tween-80 (polyoxyethylene sorbitan monooleate) were used to demonstrate enhanced substrate “solubility” (aqueous-plus-micellar phase) in the presence of surfactants. Nascent concentrations of naphthalene and its methyl-substituted derivatives in crude oil were used as representative polycyclic aromatic hydrocarbons for the study. Both biosurfactant glycolipids from H13-A and Tween-80 lowered the surface tension of aqueous solutions from 72 to ∼30 dyn/cm. The two-ring aromatics showed a substantial increase in their apparent solubilities in the presence of surfactants; the increase being significantly greater for the biosurfactant as compared to the synthetic surfactant. The aqueous phase solubility enhancement was greater for the highly substituted derivatives as compared to the lesser substituted compounds. Higher toxicity levels, as seen by the lower EC50 values, of the surfactant mixtures indicated enhanced partitioning of the petroleum contaminants in the aqueous phase. Higher initial EC50 values for the biosurfactant meant that they exhibit lesser aqueous toxicity as compared to the synthetic surfactant. When compared on a toxicity per mass of PAH basis, the end point Tween-80 system was approximately 50% more toxic than the biosurfactant system.

Dispersants are a preapproved chemical response agent for oil spills off portions of the U.S. coastline, including the Texas-Louisiana coast. However, questions persist regarding potential environmental risks of dispersant applications in nearshore regions (within three nautical miles of the shoreline) that support dense populations of marine organisms and are prone to spills resulting from human activities. To address these questions, a study was conducted to evaluate the relative toxicity of test media prepared with dispersant, weathered crude oil, and weathered crude oil plus dispersant. Two fish species, Cyprinodon variegatus and Menidia beryllina, and one shrimp species, Americamysis bahia (formerly Mysidopsis bahia), were used to evaluate the relative toxicity of the different media under declining and continuous exposure regimes. Microbial toxicity was evaluated using the luminescent bacteria Vibrio fisheri. The data suggested that oil media prepared with a chemical dispersant was equal to or less toxic than the oil-only test medium. Data also indicated that continuous exposures to the test media were generally more toxic than declining exposures. The toxicity of unweathered crude oil with and without dispersant was also evaluated using Menidia beryllina under declining exposure conditions. Unweathered oil-only media were dominated by soluble hydrocarbon fractions and found to be more toxic than weathered oil-only media in which colloidal oil fractions dominated. Total concentrations of petroleum hydrocarbons in oil-plus-dispersant media prepared with weathered and unweathered crude oil were both dominated by colloidal oil and showed no significant difference in toxicity. Analysis of the toxicity data suggests that the observed toxicity was a function of the soluble crude oil components and not the colloidal oil.

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