Response of microbial communities to the Deepwater Horizon oil spill

Oil spills occur naturally and frequently in the Gulf of Mexico, but never on the scale of the 2010 Deepwater Horizon oil spill which released approximately 4.9 million barrels of oil into the ocean over a period in the region of 3 months with an oil plume at a depth of ~1000-1300m. This deep sea oil plume contained gaseous as well as non-gaseous components including saturated hydrocarbons, aromatic hydrocarbons and polar compounds. The composition of the microbial communities found at the site of the plume has previously been studied, but there was no information about which microorganisms were active or which functional genes were expressed in the plume. Mason et al. (2012) aimed to determine this by analysing sequenced total RNA metatranscriptomically and the functional gene repertoire from metagenomic samples. One proximal sample (1.5km from the wellhead) and one distal sample (11km from the wellhead) were taken as well as one uncontaminated sample from plume depth (40km from the wellhead), in which no hydrocarbons were detected, at the end of May 2010.

Plume samples had a lower microbial diversity with a dominance of Oceanospirillales not seen in the uncontaminated sample (>60% and 5% respectively, when analyzed metagenomically). This bloom of Oceanospirillales coincided with an increase in bacterial cell densities by 2 orders of magnitude. Further to this, it was found that dominant members of the community were also active, with a relative abundance of transcripts of 46% in the proximal site and 69% in the distal site.

Hydrocarbon degradation genes were found to have a higher abundance in the plume and the entire pathway for degradation of n-alkanes as well as the nearly complete pathway for cyclohexane degradation were both present and abundant in the plume samples, which suggests that the plume was enriched with communities with the capacity for degradation of alkanes. This was likely carried out anaerobically as oxygen was not significantly depleted inside of the plume. Comparison with other studies shows a shift in the natural microbial community composition over time with a succession of bacteria from a community dominated by Oceanospirillales to domination by Colwellia and Cycloclasticus and eventually to methylotrophic bacteria.  Genes for chemotaxis proteins, flagella, pili and signal transduction were also found, providing evidence that bacterial cells may have been actively attracted to and interacting with the oil plume and this may explain how the shifts in community structure occurred.

I believe that the authors have successfully confirmed the views portrayed by many other scientists and studies in demonstrating that the bacteria naturally present in the oceans play an important role in the cleanup of large scale oil spills. This has been proven previously however this study took the approach of looking at the active organisms and gene transcripts rather than the process itself.

Mason, O., Hazen, T., Borglin, S., Chain, P., Dubinsky, P., Fortney, J., Han, J. et al. (2012) Metagenome, metatranscriptome and single-cell sequencing reveal microbial response to Deepwater Horizon oil spill. The ISME Journal. 6, 1715-1727
http://www.nature.com/ismej/journal/v6/n9/pdf/ismej201259a.pdf