The Decade-long Evolution and Toxicity of Oil Transformation Compounds in Louisiana Salt Marsh Sediments Revealed by FT-ICR Mass Spectrometry

Abstract

The 2010 explosion of the Deepwater Horizon (DWH) drilling rig provides a unique opportunity to advance the understanding of the evolution of lingering oil in Louisiana salt marshes for nearly a decade after the initial spill. After an oil spill, petroleum hydrocarbons undergo physical and chemical processes that transform the native oil composition to a polyfunctional oxygenated hydrocarbon matrix that contains ketones, hydroxyl, and carboxylic acid functional groups. Preliminary analysis of saltmarsh sediments impacted by the DWH spill between 2010–2014 identifies highly polar oxygenated hydrocarbons with carboxylic acid functionalities. These highly polar, high molecular weight compounds are not detectable by conventional gas chromatography (GC) based techniques. Here, we employ advanced analytical technologies such as targeted fractionation and isolation methods with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to document and catalog oil transformation compounds detected in oiled saltmarsh sediments. The toxicities of these environmental transformation products are determined by commercial Microtox bioassay. Samples have been continuously collected from 21 previously established sampling stations located in northern Barataria Bay for the past ten years post-spill. The concentration of oil contamination from heavily and moderately oiled sites exceed non-oiled reference sites for years after the spill. Combining chromatographic fractionation, conventional GC MS, and ultrahigh-resolution FT-ICR Mass Spectrometry analyses, we identified tens-of-thousands of biotic and abiotic crude oil transformation products that remain persistent and keep evolving in the environment. Heavily oxygenated transformation products, which are mostly only accessible by FT-ICR MS, span a wide range of chemical functionalities. Toxicity of the non-GC amenable fraction of oil contaminants was measured to determine the health risk of weathered oil residues left in the salt marsh environment. The current study creates a decade-long assessment record, and a comprehensive understanding of petroleum compounds evolution of oil-impacted wetlands ecosystem.