Continued Bioremediation of Deepwater Horizon Oil Pollution Almost a Decade On

Oil spills can be catastrophic for affected marine ecosystems. Amongst the most devastating of these events was the Deepwater Horizon disaster which began on April 20^th, 2010. Damage to the Macondo Well led to the discharge of an estimated 779 million litres of oil and gas from the sea floor into the Gulf of Mexico. Much of the oil initially deposited onto the nearby sandy coastline was manually cleaned up soon, however, oiled-sand patties continue to be washed ashore to this day. Almost a decade on from the spill, these sand patties provide the most accessible samples of residual Macondo Well oil.   

 There is a natural attenuation of oil in the environment over time, largely through microbial degradation. Researchers found that the Macondo Well oil within deposited sand patties became heavily degraded only two years after the spill, with many residues dominated by oxygenated hydrocarbon degradation products. These oxygenated hydrocarbon products have previously been hypothesised as resistant to decomposition. If so, remaining Macondo Well oil pollution could persist for a long time to come. Consequently, one recent study attempted to identify whether or not microbes were continuing to degrade the remaining, highly weathered Macondo Well oil deposits on affected beaches.

 Bostic et al. collected both oiled and non-oiled sand patties for analysis from intertidal and supratidal zones of three previously studied beaches on the coasts of Mississippi, Alabama and Florida. Initially, samples were solvent extracted and tests were carried out to ensure that acquired oil residues had originated from the Macondo Well. Moreover, the samples were characterised by gas chromatography coupled with a mass spectrometer and a flame ionisation detector. Results confirmed that samples were both Macondo Well-derived and heavily weathered in concordance with previous studies. Furthermore, analysis via thin layer chromatography coupled with a flame ionisation detector revealed that most of the solvent extractable material within the oiled-sand patties comprised oxygenated hydrocarbons (53-69%).     

 Subsequently, microbial phospholipid fatty acids (PLFA) were extracted from both oiled and non-oiled sand patties for analysis of natural abundance radiocarbon (¹⁴C) content. PLFA degrade rapidly following cell death so offer valuable insight into the viable microbial community and their C sources at the time of sampling. By measuring ¹⁴C content within the PLFA, it is possible to identify microbial C sources as either petroleum or as modern, photosynthesis-derived organic matter; petroleum contains significantly depleted ¹⁴C compared with modern organic matter.

 ¹⁴C content measurements revealed that microbial PLFA extracted from oiled-sand patties primarily contained carbon which was petroleum-derived whereas microbial PLFA extracted from non-oiled-sand patties contained carbon consistent with modern organic matter. Accordingly, it is clear that microbial communities within the oiled-patties are utilising different carbon sources to the communities residing within the non-oiled-patties. Additionally, there was no significant difference in ¹⁴C measurements between intertidal and supratidal oiled-patties, suggesting that proximity to fresh carbon sources and moisture had no influence on the ¹⁴C content of microbial PLFA. Therefore, Bostic et al. were able to conclude that petroleum carbon is the primary source of carbon for the microbes inhabiting all studied oiled-sand patties.  

 The results of this study are promising, highlighting the potential for further bioremediation of Deepwater Horizon residual oil pollution, contrary to previous suggestions that such heavily weathered oil could be resistant to degradation. Nevertheless, this study delivers only a snapshot of weathered oil degradation, providing no insight into the degradation rates of the remaining oil. As such, further research should utilise incubation techniques in an attempt to quantify these degradation rates, permitting improved estimates to be made of when affected ecosystems will return to their pre-Deepwater Horizon states. Additionally, employing metagenomics could identify key OTUs in the degradation process of weathered oil, highlighting shifts in oil degrading bacterial communities as oil becomes more heavily degraded over time.

Reviewed Paper:

Bostic, J. T., Aeppli, C., Swarthout, R. F., Reddy, C. M. & Ziolkowski, L. A. (2018). Ongoing biodegradation of Deepwater Horizon oil in beach sands: Insights from tracing petroleum carbon into microbial biomass. Marine Pollution Bulletin, 126: 130-136