Resolving Biodegradation Patterns of Persistent Saturated Hydrocarbons in Weathered Oil Samples from the Deepwater Horizon Disaster

Abstract

Biodegradation plays a major role in the natural attenuation of oil spills. However, limited information is available about biodegradation of different saturated hydrocarbon classes in surface environments, despite that oils are composed mostly of saturates, due to the limited ability of conventional gas chromatography (GC) to resolve this compound group. We studied eight weathered oil samples collected from four Gulf of Mexico beaches 12-19 months after the Deepwater Horizon disaster. Using comprehensive two-dimensional gas chromatography (GC × GC), we successfully separated, identified, and quantified several distinct saturates classes in these samples. We find that saturated hydrocarbons eluting after n-C22 dominate the GC-amenable fraction of these weathered samples. This compound group represented 8-10%, or 38-68 thousand metric tons, of the oil originally released from Macondo well. Saturates in the n-C22 to n-C29 elution range were found to be partly biodegraded, but to different relative extents, with ease of biodegradation decreasing in the following order: n-alkanes > methylalkanes and alkylcyclopentanes+alkylcyclohexanes > cyclic and acyclic isoprenoids. We developed a new quantitative index designed to characterize biodegradation of >n-C22 saturates. These results shed new light onto the environmental fate of these persistent, hydrophobic, and mostly overlooked compounds in the unresolved complex mixtures (UCM) of weathered oils.

Impact of Photooxidation and Biodegradation on the Fate of Oil Spilled During the Deepwater Horizon Incident: Advanced Stages of Weathering

“…15,18,20,23 The environmental fate of the sand patties was explored and documented in Chapter 1. It was found that photooxidation and aerobic biodegradation were significant advanced weathering processes impacting the stability of these structures.…”

“…Since sand patties originate from spilled oil and contain a variety of hydrophobic compounds 15,18,21,23 there is a tendency to assume that these structures resist the 4 and heat maps were generated using the associated imaging software.…”

“…10,12,14-19 Not surprisingly, the environmental fate of the oil was influenced by many well documented weathering processes. [20][21][22][23][24][25] These processes resulted in the massive transformation of the oil components and a chemical footprint that differed substantively from the crude oil released at the wellhead. Most notably, the transformation of the crude Macondo 252 oil resulted in the formation of a group of partially oxidized organic compounds collectively termed 'oxyhydrocarbons.'…”

“…[1][2][3] This crude oil was transformed by a number of weathering processes and eventually converted into sand patties that are largely composed of partially oxidized organic compounds termed 'oxyhydrocarbons'. 20,21,25,54 The weathered sand patties may remain on the seafloor or be transported to beaches where they can settle in the swash zone. 15,18,55,56 where they are exposed to direct sunlight, continual wave action and the ambient microbial communities.…”

Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon

“…For sparingly soluble hydrocarbons, biodegradation is expected to serve as the primary cause of weathering in the deep ocean because other key weathering processes (e.g., evaporation, photooxidation) depend on atmospheric and solar exposure. Although hydrocarbon recalcitrance to biodegradation is expected to scale roughly with molecular mass and steric complexity (32), the rates at which specific hydrocarbons are metabolized vary based on myriad environmental variables: temperature, salinity, pressure, oxygen concentration, pH, solar exposure, availability of nutrients and other sources of organic matter (33), water availability, access to substrate, solid-phase interactions, competition, predation, and inhibition (34). Although early reports addressed the degradation of some low-to moderate-molecular-mass compounds in the water column (12,14,16), the degradation rates of the petroleum hydrocarbons constituting Macondo oil after seafloor deposition are unknown.…”