COLLEGE OF SCIENCES

SCHOOL OF EARTH AND ATMOSPHERIC SCIENCES

EAS Ph.D. Defense

Xiaoxu Sun

March 13, 2018

2:00 PM

Earth and Atmospheric Sciences

Ford Environmental Science & Technology (ES&T)311 Ferst Drive, ES&TAtlanta, GA 30332-0340Web: eas.gatech.edu

ES&T

1229

Title:Oceanographic controls of hydrocarbon degradation in the Gulf of Mexico

Committee members: Dr.Joel Kostka (advisor), Dr.MartialTaillefert, Dr.Ellery Ingall, Dr.Yuanzhi Tang, andDr. David Hollander (College of Marine Sciences, University of South Florida)

Abstract: 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 situmicrobial community composition. Pressure is the most understudied of oceanographic controls, due to methodological constraints and the difficulty to obtain samples from the deepseaat high integrity. Deepseawater 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.