Oceanographic controls of hydrocarbon degradation in the Gulf of Mexico

Xiaoxu Sun
Tuesday, March 13, 2018 - 2:00pm
Ford ES&T Bldg, Room 1229
Dr. Joel Kostka (advisor), Dr. Martial Taillefert, Dr. Ellery Ingall, Dr. Yuanzhi Tang, and Dr. David Hollander (College of Marine Sciences, University of South Florida)

The risk of an oil spill accident is increasing in pristine regions of the world’s oceans due
to the development and transport of crude oil resources. The ability to predict the trajectory of
spilled oil is critical for the improvement of emergency response strategies. Although the controls of
petroleum hydrocarbon biodegradation have been studied in the ocean for years, there is as yet no
consensus on the results for predictive modeling. One of the reasons is the complexity of the
oceanographic controls of the hydrocarbon degradation process. Thus, the objective of this
dissertation was to quantify the potential for hydrocarbon biodegradation under the impact of
various environmental controls, including dispersant application, mixing energy, temperature,
nutrient availability, pressure, and microbial community composition. Mixing energy was shown to be
a critical variable during dispersant application. Under completely dispersed conditions,
biodegradation was substantially enhanced, decreasing the overall half-life of the total petroleum
hydrocarbon by 43%. Very different microbial populations capable of hydrocarbon degradation were
enriched when dispersant was added to oil-contaminated surface seawater, which might lead to
changes in ecological function, such as the stimulation of nitrogen fixation with dispersant
application. While nutrient and temperature were both crucial factors, degradation potentials were
shown to be strongly site-specific, which may be attributed to in situ microbial community
composition. Pressure is the most understudied of oceanographic controls, due to methodological
constraints and the difficulty to obtain samples from the deepsea at high integrity. Deepsea water
and sediment samples from a range of sites in the Gulf of Mexico were incubated in specially
designed high pressure chambers. The results revealed an enhancement of hydrocarbon carbon
degradation rates at ambient pressure in comparison to atmospheric pressure and the enrichment of
microbial populations distinct to high pressure conditions. These observations suggest that pressure
may have a substantial impact on the degradation of natural organic matter as well as on petroleum
hydrocarbons. Taken together, the results from this dissertation show that the rate and extent of
hydrocarbon degradation are determined by the complex interplay between all of the
aforementioned parameters.