Will Overholt Thesis Defense

On March 14, 2018, Will Overholt successfully defended his PhD Thesis: "The response of marine benthic microbial populations to the Deepwater Horizon oil spill." Abstract: The Deepwater Horizon (DWH) oil spill was the largest accidental oil spill in history, the first large spill that occurred in the deepsea, and is unique in the unparalleled volumes of chemical dispersant that were applied during emergency response efforts. Microbial biodegradation ultimately removes most of the hydrocarbons discharged during oil spills, which allows the system to recover. However, the environmental controls that regulate this process are poorly understood. Furthermore, benthic environments are understudied relative to their pelagic counterparts, and were contaminated with approximately 20 % of the released oil after the DWH disaster. Aside from the emergence of hydrocarbon-degrading bacterial populations, oil contamination may impact sensitive, benthic microbial groups and disrupt critical biogeochemical cycles, causing far-reaching and largely unknown ecosystem level consequences. The work presented in this dissertation addresses knowledge gaps associated with the environmental controls of the structure and function of benthic microbial communities across the Gulf of Mexico as well as their response to major perturbations such as oil contamination. Specifically, the overall goal was to advance our understanding of the fate and consequences of deposited DWH crude oil to benthic ecosystems and the in situ microbial community. Objectives were to: (1) determine the impact of chemical dispersant on individual oil-degrading microbial populations and the consequences to oil ecotoxicity, (2) interrogate the natural or baseline state of benthic microbial communities throughout oligotrophic sediments in Gulf of Mexico, and (3) quantify the controls on biodegradation and microbial populations in sandy coastal ecosystems. The results improve our mechanistic understanding of the constraints on the rates and pathways of oil biodegradation. Moreover, the most significant findings from this study demonstrate the occurrence of large-scale disruptions to the marine nitrogen cycle in subtidal sands in response to oil contamination through the inhibition of nitrification. This disruption can be linked to the microbial populations that mediate nitrification along with other nitrogen cycling processes, offering direction for environmental monitoring programs to assess ecosystem health and recovery. The results from this dissertation can be directly incorporated into predictive models to forecast recovery pathways for future spills as well as hindcast the fate of remaining DWH oil.