The Deep Water Horizon Spill and its Microbial Companions.

Hydrocarbons enter the marine environment through both natural and anthropogenic pathways, however the local hydrocarbon input of natural sources pales in comparison to the amount discharged during an event such as the Deepwater Horizon Accident (DWH), which released at least 4 million barrels of oil into the environment. The DWH spill resulted in a unique microbial response due to the release of both oil and natural gas at such significant depths, these conditions drastically altered the diversity and distribution of hydrocarbon degrading bacteria.

It is well documented that microbes aid in the degradation and remediation of hydrocarbons, however due to the rare occurrence of such a massive influx of oil, studies of bacteria communities’ in-situ are rare. Most published studies are carried out in mesocosms which focus on sediment, beach sand, or surface water, while the pelagic microbial response is often overlooked.

In the study Redmond and Valentine, carried out environmental 16s rRNA sequencing as well as stable isotope probing (SIP), allowing them to survey the diversity of organisms present and map the links between bacterial taxa and the respective hydrocarbon they degrade.

Deep sea samples were collected on three cruises between May to Sepetember 2010, sampling locations between 2 – 385km away from the wellhead. After the initial accident in April, the first deep sea samples were collected in May. These revealed that Oceanospirillales were dominant with 16 other groups of Gammaproteobacteria enriched in the samples. This diversity changed in June, with Oceanospirillales being replace with Gammaproteobacteria, Colwellia and Cycloclasticus as the dominant taxa. The well was capped in mid-July and samples collected two months after, had methylotrophs taking over while both Colwellia and Cycloclasticus were less abundant. Aswell as the deep sea samples, surface slicks were also analysised revealing that none of the dominant deep sea bacteria were present, indicating a possible link to temperature as both Oceanospirillales and Colwellia are psychrophiles, organisms that growth and reproduction in cold temperatures (−20 °C to +10 °C). This was followed up in lab tests which showed Colwellia was far more abundant in samples incubated at 4co, rather than room temperature, reinforcing the idea of temperature controlled proliferation.

Stable Isotope Probing was conducted which identified Colwellia as a key oxidiser of ethane, propane, and benzene while methylotrophs prefer methane. This indicates that Colwellia was a key component in the breakdown of ethane and propane, both key components of natural gas, however there is some indication of the possible degradation of higher molecular weight hydrocarbons. DWH was characterized by the release of large quantities of natural gas which likely helped the proliferation of Colwellia and Oceanospirillales compared to normal oil spills with less natural gas.

The conducted study was a success, revealing the direct links between oil-degrading microbes  and their associated hydrocarbons, these initial steps into mapping bacteria - hydrocarbon interactions could be vitally important in future bio-remediation techniques. The study of microbial responses to mass influxes of hydrocarbons is challenging due to the rarity and the unpredictable nature of such events, making preliminary work difficult, meaning measurements before an event are often missing leading teams to work without an accurate control.

In regards to further research , the mapping of microbe and hydrocarbon interactions should be heavily perused as well as further study analysing oil in the pelagic environment. This is more relevant than ever due to the practice of pumping dispersant into the broken well-head, roughly 5,000 feet below the surface. Resulting in the oil becoming suspended rather than floating. Suspended oil will have completely different microbial interactions that are critically under explored.

Redmond, M. and Valentine, D. (2011). Natural gas and temperature structured a microbial community response to the Deepwater Horizon oil spill. Proceedings of the National Academy of Sciences, 109(50), pp.20292-20297.