Microbes, Corals and Oil: The deep dark secret to cleaning up spills

The Deepwater Horizon oil spill (DWH) spilled an estimated 210 gallons of oil and gas into the Gulf of Mexico in April 2010. Many studies have looked at the detrimental impacts on megafauna, however few have looked at how the release of oil and gas has influenced the microbial community structures, particularly in the deep sea where many corals thrive. Oil degrading microbes, which include γ-Proteobacteria, have been linked via meta-genomic, -transcriptomic and metabolomics studies to the degradation of the aliphatic components of oil and aromatic components of oil. Previous studies have shown varying microbial community when comparing initial responses to 4-5 months after the spill. This study by Simiser et al. (2015) focuses on providing a further insight into the microbial community structure and the diversity of microbial genes involved in oil-degradation pathways in the deep-sea. 

Samples, including flocculent material from coral surfaces (floc) and surface-sediment, were collected at 1370m depth from a site of the Macondo well blowout, known to be impacted by the DWH oil spill. These samples were tested using molecular techniques such as 16S rRNA sequencing which involved constructing and analyzing bacteria and archaea clone libraries from full-length 16S rRNA sequences, Illumina MiSeq 16S rRNA amplicon sequencing and analysis and functional gene sequencing (constructing and analyzing genes associated with oil-compound degradation processes). Functional genes included those for alkane hydroxylase (alkB) and alkylsuccinate synthase/benzylsuccinate synthase (assA/bssA).

The results from the Archaeal 16S rRNA Illumina sequencing were dominated by Thaumarchaeota, however in both sediment samples and coral floc, majority of full length 16S rRNA archaeal sequences are affiliated to the ammonia oxidizing archaea (AOA), which does not correspond to laboratory cultures where ammonia oxidation and nitrification were inhibited by oil, suggesting further study needs to be done to fully understand the impact of oil on archaeal diversity, community structure and function. The results for most abundant bacterial and archaeal OTUs showed evidence of oil degrading species being dominant in sediment samples, such as Cycloclasticus sp. that were enriched in the subsurface plume and surface slick samples. They are also known degraders of petroleum hydrocarbons including PAHs. The next most abundant OTUs for both sediment samples are affiliated within the δ-Proteobacteria, particularly to sulfate reducing bacteria (SRBs). At hydrocarbon sites in the GoM, high abundances of these bacteria are found and can be directly involved in the anaerobic degradation of propane and butane. Sequencing results for functional genes involved in oil degradation exhibited broad diversity within the Proteobacteria, with many OTUs aligned closely to γ-Proteobacteria. The functional gene for aerobic degradation of oil (alkB) were detected in all samples, with the functional gene for anaerobic degradation of oil (assA/bssA) detected in sediment samples. This suggests aerobic oil degradation dominates in the floc.

Overall, this study presents a microbial approach using genetic techniques to look at the impacts of oil spills on the marine environment, and the response by which bacteria can potentially degrade various compounds in oil. This could potentially lead to further studies and monitoring in the microbial community, especially if similar events to the DWH occur in the future. The DWH oil spill has presented an in situ study where data collected now, can be compared to pre-spill data (though this is limited and poses challenges) and data found at deep-sea sediments, natural hydrocarbon seep sites and sediments. 

Reference: 

Simister, R. L., Antzis, E. W. & White, H. K., 2015. Examining the diversity of microbes in a deep-sea coral community impacted by the Deepwater Horizon oil spill. Deep Sea Research II.

http://www.sciencedirect.com/science/article/pii/S0967064515000211