Laboratory incubations of Macondo oil-derived hydrocarbons in Alabama salt marsh sediments and water

Abstract

In order to better understand the impact of the BP Deepwater Horizon and future oil spills on the Gulf of Mexico coast this study assesses the variation of alkanes and polycyclic aromatic hydrocarbons (PAH) in coastal sediments and water. 500g of Sediment and water from Bayou La Batre, Alabama was spiked with 10g Macondo oil for a time series experiment with sampling points at 0, 6, 12, 24, 48, 168, and 336 hours. Sediment and water were also spiked with 0.2g, 2g, 10g, 20g and 50g Macondo oil and were incubated for 21 days for a concentration variation experiment. The composition and concentrations of alkanes and PAHs in the sediments and the concentrations of dissolved inorganic nitrogen and phosphorous and dissolved organic carbon were characterized. Results from the time series experiment show 54.6% total alkanes in sediments were lost within the first six hours and 71.5% were lost after 14 days. Total PAHs decreased by 90.1% within the first six hours and did not show apparent decreases afterwards. The loss of hydrocarbons in the sediments may be attributed to microbial degradation. Low molecular weight (≤ C17) alkanes were preferentially degraded over high molecular weight (> C17) alkanes during the first 24 hours, whereas normal alkanes were not preferentially degraded over isoprenoid alkanes. The high degradation rates of hydrocarbons in the first 12 hours were attributed to aerobic microbial degradation rates of hydrocarbons; the decreases in the loss rates after hour 12 were perhaps due to oxygen depletion in the microcosms. The oxygen depletion was supported by the elevated iron concentration in seawater after 168 hours that indicated anaerobic microbial respiration using Fe (III) as an electron acceptor. However, the concentration of dissolved nitrate and ammonium did not show evident patterns over the course of the incubation, providing no evidence that nitrate was used by microbes as an electron acceptor during anaerobic microbial respiration. Dissolved organic carbon (DOC) concentration continuously decreased until reaching the ambient seawater concentration, indicating an active microbial degradation of oil-derived hydrocarbons that were dissolved in the seawater. In the concentration variation experiment, sterilized controls with 0.2g and 20g of oil treatment had much higher concentrations of saturated alkanes in the sediments than corresponding non-sterilized microcosms with in situ microbial community left intact. However, the microcosms with 10g and 50g of oil treatment did not show reduced concentration of hydrocarbons in the sediments relative to their non-sterilized counterparts. Similarly, variable patterns appeared from the comparison of the dissolved organic carbon concentrations between non-sterilized microcosms vs. sterile microcosms treated with the same amount of oil. Therefore, data from the concentration variation experiment provided inconclusive evidence that the in situ microbial community degraded oil-derived alkanes in sediments and seawater of the microcosms.