The Biodegradation of Dispersed Oil Does Not Induce Toxicity at Environmentally-Relevant Concentrations

The acute toxicity of dispersed oil is well understood, and oil dilutes to levels below those of acute concern within hours of dispersion whether oil is dispersed by waves or by lower energy turbulence in the presence of chemical dispersants. Once dispersed, the hydrocarbon components of the spilled oil are degraded promptly by the native microbes in seawater, typically with an apparent half-life of 7–30 days even under Arctic conditions. Nevertheless, concern has been raised that this biodegradation might increase the oil's acute toxicity by generating and releasing toxic by-products. We show here, using Americamysis bahia as the test species, that this does not occur when dispersed oil is present at environmentally-relevant concentrations (initially 3 ppm oil dispersed with Corexit 9500 at a dispersant to oil ratio of 1:20). The guidelines for this toxicity test mandate a temperature of 26 ± 1C, rather warmer than the temperature of collection of the seawater from the New Jersey shore that we used as our experimental medium, so it is not surprising that biodegradation was especially rapid with a half-life for the loss of detectable hydrocarbons of approximately 4 days. We conducted sequential 4-day acute toxicity tests for 20 days, by which time the indigenous microorganisms had removed almost 80% of the detectable hydrocarbons in the lightly weathered crude oil. We saw no mortality in any of the five sequential tests.