The Deepwater Horizon (DH) oil spill in 2010 resulted in huge amounts of oil and gas being spilled into the Gulf of Mexico, a large quantity of which reached the surface water and was subsequently transported to the shoreline. Once oil reaches the shoreline it is likely to have negative effects on the economy and the environment.
Sands in the gulf are covered in a diverse biofilm of bacteria and the number of bacteria in the sand is much higher than in the sea water itself. The sand is also highly permeable and very important to nutrient cycling in shallow waters. The majority of oil hydrocarbons that enter the environment will be broken down by indigenous bacteria. There is little information available on the specific bacteria that catalyze the breakdown of oil in the environment at a spill site and the environmental parameters that control the activity of these bacteria therefore it is difficult to predict the activity of oil degrading bacteria in situ. Kostka et al (2011) had two objectives for this study; firstly to characterize the main oil-degrading taxa that can be used as model hydrocarbon degraders or microbial indicators of contamination and secondly to the environmental response of indigenous bacteria to contamination of the ecosystem.
The study was conducted on samples collected from Pensacola Beach, Florida which was exposed to heavy oil pollution after the Horizon oil spill. Samples were collected approximately 18 weeks after the start of the spill. Visual observations of oil contamination levels were carried out and hydrocarbon levels verified using gas chromatography –mass spectrometry. Results were compared to source oil from DH wellhead to determine depletion levels. These samples were also used to carry out most probable number counts (MPN) of cultivatable hydrocarbon-degrading bacteria. Bacteria capable of using oil as the only source of carbon were isolated under aerobic and anaerobic conditions and genes sequencing and phenotypic characterization were carried out to identify the bacterial strains.
Hydrocarbon concentrations were found to range from 3.1 to 4,500 mg kg-1 in beach sands and comparison to carbon concentrations found at the wellhead showed substantial degradation had occurred; those remaining hydrocarbon compounds were found to mainly be large aliphatic and aromatic compounds which have previously been shown to degrade more slowly. Smaller hydrocarbons had been reduced to near the detection limit. MNP counts showed samples taken from oil contaminated sands were 3-4 orders of magnitude higher than those taken from clean sands and rRNA analysis backed up MNP counts showing bacterial rRNA gene abundance was 10 times higher in oiled than non-oiled sands. 24 bacterial strains from 14 genera were isolated and confirmed as oil-degrading microorganisms. Strains were mainly found to be Gammaproteobacteria, including bacteria from genera Alcanivorax, Marinobacter, Pseudomonas, and Acinetobacter which are known oil degraders therefore a clear response was seen to oil contamination in the form of a shift in bacterial community structure.
The authors concluded that oil contamination from the DH spill had a marked impact on the abundance and community composition of indigenous bacteria in beach sands. Results from this study indicate that Gammaproteobacteria (Alcanivorax, Marinobacter) and Alphaproteobacteria (Rhodobacteraceae) are some of the most important bacteria to the degradation of oil in this region.
This study is important as few previous studies have used quantitative molecular techniques in situ to assess numbers of hydrocarbon degrading bacteria. The authors also propose Alcanivorax spp. of the Gammaproteobacteria as indicators of the early stages of hydrocarbon degradation and members of the Alphaproteobacteria as indicators of later stages. Future studies are needed to fully identify the specific role of each. This study also produced some pure cultures of these strains to be used as model organisms for further work.
Hi Georgia,
ReplyDeleteReally interesting post, I like how the reviewed paper focusses on shoreline communities rather than the well head itself. Do you remember the baseline work Graham told us about in one of our lectures; where he compared a non-polluted beach to a beach which had suffered from an oil spill? He found that there was a significant background level of hydrocarbon degraders and that population levels naturally fluctuate overtime. How do you think the results from your blog contribute to the work done by Graham some years ago?
Thanks,
Vicky
Hi Vicky,
ReplyDeleteSorry for the late reply. I this study is slightly different to the work carried out by Graham as the clean and polluted site are on the same beach but one was visually uncontaminated.I think that this study contributes to the work Graham did because it shows that the levels in the uncontaminated area are much lower than those which were in the highly contaminated. I think it would have been interesting for this study to be carried out over a longer period of time (rather than 2 months) so that the population fluctuation over time could be assessed and the levels of bacteria in the visually contaminated regions could be compared to the uncontaminated after degradation of the oil had occurred. It would possibly make for a better insight into the population dynamics after the beach had recovered.
Georgia