Master of Science (MS)


Environmental Sciences

Document Type



Although the impacts of the Deepwater Horizon (DWH) oil spill on the bacterial communities in several Gulf of Mexico habitats have been described, little if any research has been published evaluating the impacts of the spill on bacterial populations in the most severely oiled salt marsh soils of Barataria Bay, Louisiana. The aim of this study was to define the effects of the spill on the total and oil-degrading bacterial communities in these marshes. Using quantitative real-time PCR (qPCR) and traditional cultivation methods bacterial communities from contaminated marsh soils were examined thirteen, twenty-two, and twenty-nine months after the initial oiling event. Overall, the relative abundance of the n-alkane degrading bacteria decreased over time while that of the polycyclic aromatic hydrocarbon (PAH) degrading bacteria increased. The oil-degrading communities were increasingly dominated by gram-positive PAH-degrading bacteria over time and in the last sampling more than 99% of the oil degrading genes detected were from gram-positive PAH-degraders. Supplemental 16S rRNA sequencing of oil-degrading bacterial strains isolated from samples taken thirty months after the spill revealed that 66% of the isolates were gram-positive with nearly half of the gram-positives belonged to the genus Bacillus. The observed shift in microbial populations over time and the dominance of gram-positive PAH-degraders is indicative of a later stage of the biologically-driven attenuation of hydrocarbon-contaminated soils. Negative correlations between populations (total and oil-degrading) and TPH levels were observed on some occasions (i.e. the 16S rRNA gene count at thirteen months and PAH-degrading genes at twenty two months), indicating adverse effects of high TPH levels on bacteria. The increased abundance of oil-degraders at the lightly oiled reference sites was likely due to increased vegetative cover and lower levels of toxic hydrocarbons. Differences in the culture-based and molecular genetic data suggest that some of the oil-degraders in the marsh sediments may be nonculturable in a laboratory setting. Based on this study, twenty-nine months after the DWH oil spill the oiled Louisiana salt marsh soil has come into its latter stages of naturally microbial attenuation. Also, extremely high levels of oil residuals in the marsh soil possibly have inhibited proliferation of certain microbes including oil-degrading bacteria, potentially impairing the marsh’s efficiency of oil degradation and other biogeochemical processes.



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Committee Chair

Hou, Aixin