October 4, 2011
How Gas And Temperature Controlled Bacterial Response To Gulf Oil Spill
A new study explains how DNA was used to identify microbes present in the Gulf of Mexico following the Deepwater Horizon oil spill that devastated the region last year -- and the particular microbes responsible for consuming natural gas immediately after the spill.
The researchers, from UC Santa Barbara (UCSB), also explained how water temperature played a key role in the way bacteria reacted to the spill. Results of the study were published in this week´s Proceedings of the National Academy of Sciences (PNAS) journal.
The study was led by David Valentine, a geochemist and professor of earth science at UCSB, and Molly Redmond, a postdoctoral scholar in Valentine's laboratory and was supported by the National Science Foundation (NSF) and the Department of Energy (DOE).
The researchers said the oil spill was unique because it happened at such a great depth and contained so much natural gas -- mostly methane, ethane and propane. Those factors influenced the way bacteria responded to the spill.
Valentine, Redmond and colleagues have previously shown that ethane and propane were the key hydrocarbon compounds consumed in June 2010, two months after the oil spill. But, by September 2010, the researchers found that these gases, along with methane, had been consumed.
The team discovered that bacterial communities in the submerged plume in May and June 2010 were dominated by Oceanospirillales, Colwellia and Cycloclasticus bacteria, and were very different from control samples without large concentrations of oil and gas. The bacteria were also very different from the microbial communities in surface oil slicks collected at the same time.
“It's much warmer at the surface than in the deep water--around 80 degrees Fahrenheit versus 40 F, which is pretty close to the temperature in your refrigerator,” said Redmond. “There was very little natural gas in the surface samples, suggesting that both temperature and natural gas could be important in determining which bacteria bloomed after the spill,” she added.
The bacteria that the researchers observed in the deep-water samples in May and June were related to types of cold-loving (psychrophilic) bacteria.
“Most bacteria grow more slowly at cooler temperatures--that's why we keep our food in the refrigerator,” Redmond said. “But psychrophilic bacteria actually grow faster at cold temperatures than they would at room temperature.”
In determining how the temperature played a key role, the team conducted an experiment by adding oil to water from the Gulf, incubating one sample at 40 degrees Fahrenheit and another at room temperature (70F), to see which bacteria grew at different temperatures.
They found that in the 40F samples, Colwellia were most abundant, but not in the room temperature samples, suggesting that the bacteria have an advantage in colder water.
“To figure out which bacteria were consuming methane, ethane, and propane, we used a technique called stable isotope probing, in which we incubated fresh seawater samples from the Gulf with isotopically labeled methane, ethane, or propane,” said Redmond.
Redmond and Valentine found that the bacteria that grew as they consumed the hydrocarbon compounds, converted those gases into biomass, including their DNA. The team was able to identify the bacteria through DNA sequencing. And what they found was the bacteria were the same Colwellia found in the samples from May and June, when ethane and propane consumption rates were high. They were abundant when the team incubated oil at 40F, but not at room temperature.
This evidence suggests that Colwellia grow well at low temperatures, and can consume ethane and propane, the team said.
“The ability of oil-eating bacteria to grow with natural gas as their ℠foodstuff´ is important,” said Valentine, “because these bacteria may have reached high numbers by eating the more-abundant gas, then turned their attention to other components of the oil.”
“We´ve uncovered some of the relationships between hydrocarbons released from Deepwater Horizon and the bacteria that responded,” he said.
“This work continues to remind us that the ocean, its microbes, and petroleum hydrocarbons share an ecological history that extends far into the geological past,” said Don Rice, director of NSF´s chemical oceanography program, which funded the research. “Our ability to respond to marine oil spills is enormously advanced by this kind of basic research.”
Image 1: Flares of captured gas (left) and oil (right) at the Deepwater Horizon spill site in June 2010. Credit: David Valentine
Image 2: Scientists studied the interaction of the oil spill and microbes in Gulf of Mexico waters. Credit: Luke McKay, University of Georgia
Image 3: An oil slick at the sea's surface contains orange-colored 'older' oil and a fresher oil sheen. Credit: David Valentine
On the Net:
- University of California - Santa Barbara
- Proceedings of the National Academy of Sciences (PNAS)
- National Science Foundation (NSF)
- Department of Energy (DOE)