Natural “weathering” processes altered the composition and presentation of the oil during transport from greater depths and from the Deep Horizon spill site to the coast. In addition, remediation efforts with the use of dispersants as well as in situburns modified available and affected “crude” at the waters surface. The net result was that coastal zones serving as nurseries for sensitive early life stages of fish were exposed to petrochemical mixtures significantly altered in content and form from the South Louisiana Light Crude released at the DH wellhead.
The question was: Do these weathered oil products, use of surfactants and combustion present a danger to developing fish in coastal Louisiana? Research efforts focused on elucidating biologically significant exposure modalities as well as quantifying and characterizing developmental toxicity and molecular responses of developing fish to the modified and transported weathered DH oil emulsions. These studies examined weathered oil with and without the dispersant, Corexit, as well as the products resulting from in situcombustion of crude oil.
Using zebrafish as a developmental and genomic model species, the semi-solid weathered oil emulsions found on the coast and vegetation (Deep Horizon mousse) were found to elicit a high level of developmental toxicity. Toxicity was overtly observed as deformities and mortalities of fish embryos as well as gene expression changes. Affected embryos exhibited cardiac edema, yolk sac edema, axial changes, and lack of GI/ swim bladder development and functionality. Developmental abnormalities (40-60%) and mortality were most evident with direct contact of developing fish embryos with the non-smothering water-in–oil emulsions as compared to non-contact waterborne toxicity at the same dose Direct contact developmental toxicity with the oil emulsion followed an attenuated non-stoichiometric dose response relationship (low doses were proportionally more toxic than high dosages).
Corexit, alone did not elicit developmental toxicity, but at high concentrations enhanced the developmental toxicity of oil emulsions, especially for non-contact exposures. Embryos exhibited much greater developmental toxicity when exposed by direct contact to the oil emulsion on surfaces during the first 0-48 hr post-fertilization developmental interval than later intervals suggesting that interval of exposure may play a critical role in outcome. Air-exposed and aged water-in-oil emulsions retained their developmental toxicity as compared to a freshly exposed surface of the same material (implications for the ebb and flow of the tides and persistence of toxicity with time, sunlight and washing), but continual washing with water of a thin emulsion layer over months decreased the developmental toxicity.
Gene expression alterations were evident globally on gene arrays and qRT-PCR. Many genes were up and down regulated with exposure to oil emulsions. Genes operative in sensory and neurologic systems as well as those operative in the response to abiotic and chemical stimuli were affected. Products resulting from combustion of crude also elicited toxicity. Collectively, these data indicate that it is pausible that the oil emulsions and remediation products reaching Louisiana’s coastal areas elicited deleterious effects in exposed developing fish in coastal ecosystems, perhaps with special significance for those with select life histories.