BIODEGRADATION OF DEEPWATER HORIZON PETROLEUM HYDROCARBONS IN BARATARIA BAY MARSHES: GEOMICROBIOLOGY AND CLAY MINERAL ENHANCEMENT

Several Barataria Bay marshes remain coated with a ~1cm-thick weathered petroleum layer deposited following the Deepwater Horizon disaster. Thick deposits are mostly within the ~10m edge of Spartina marsh, persisting as a source of petroleum hydrocarbons in local waters. A resistant weathering rind has formed on the surface of the deposit, slowing aeration and inhibiting aerobic microbiota. Microbial metabolism will likely be the primary means of degradation of contaminant Deepwater Horizon crude. Lab experiments with high layer charge montmorillonite show a potential to enhance biodegradation by facilitating nutrient exchange near the mineral-cell wall interface (Warr et al., 2009), but this has never been tested in the field.

Repeated sampling is now underway to 1) characterize petroleum hydrocarbon biodegradation in the marshes; 2) characterize both aerobic and anaerobic microbial function; and 3) test the hypothesis that clays may enhance biodegradation of Deepwater Horizon petroleum hydrocarbons. Experimental plots were established in September, 2010, several months after emplacement of the contaminants, and amended with high-charge montmorillonite; control areas were also established. Serial analyses include petroleum hydrocarbons by GC/MS, clay mineralogy by XRD, microbial composition and function by analyses of functional gene transcripts using RT-PCR, quantitative PCR, and clone library. Experimental microcosms have also been initiated to model biodegradation with varying clay content.

Natural marsh clay mineralogy is heterogeneous, with kaolinite, illite, and smectite, consistent with Mississippi Basin drainage. Experimental sites show enhanced smectite:kaolinite ratios by XRD, providing a basis for documenting clay amendment. GC/MS analyses show the expected dominance of n-alkane components of petroleum hydrcarbons; detailed forensic biomarker fingerprinting and time-series compound ratio analyses are forthcoming. Transcript analyses of functional genes indicate Fe-reducing, sulfate-reducing, and methanogenic prokaryotic communities are metabolically active at both control and experimental plots. Such communities have been linked to petroleum hydrocarbon biodegradation in previous studies, and likely represent the main agents of biodegradation in the Barataria Bay marshes.