Wednesday, June 28, 2017

BACTERIA HAVE CONSUMED MOST OF THE DEEPWATER HORIZON OIL SPILL: Simulation of Deepwater Horizon oil plume reveals substrate specialization within a complex community of hydrocarbon-degraders

The Deepwater Horizon oil spill in the Gulf of Mexico in 2010 is one of the most studied spills in history, yet scientists haven't agreed on the role of microbes in eating up the oil. Now a research team at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has identified all of the principal oil-degrading bacteria as well as their mechanisms for chewing up the many different components that make up the released crude oil.

The team, led by Berkeley Lab microbial ecologist Gary Andersen, is the first to simulate the conditions that occurred in the aftermath of the spill. Their study, "Simulation of Deepwater Horizon oil plume reveals substrate specialization within a complex community of hydrocarbon-degraders," was just published in the Proceedings of the National Academy of Sciences.

"This provides the most complete account yet of what was happening in the hydrocarbon plumes in the deep ocean during the event," said Andersen. Berkeley Lab's Ping Hu, the lead author of the study, added: "We simulated the conditions of the Gulf of Mexico oil spill in the lab and were able to understand the mechanisms for oil degradation from all of the principal oil-degrading bacteria that were observed in the original oil spill."

This oil spill was the largest in history, with the release of 4.1 million barrels of crude oil as well as large amounts of natural gas from a mile below the surface of the ocean. After the initial explosion and uncontained release of oil, researchers observed a phenomenon that had not been seen before: More than 40 percent of the oil, combined with an introduced chemical dispersant, was retained in a plume nearly 100 miles long at this great depth.

Yet because of the difficulty in collecting samples from so far below the ocean surface, and because of the large area that was impacted by the spill, a number of gaps in understanding the fate of the oil over time remained.

Discovery of a new bacterium

Andersen and his team returned to the spill location four years later to collect water at depth. With the assistance of co-authors Piero Gardinali of Florida International University and Ron Atlas of the University of Louisville, a suspension of small, insoluble oil droplets was evenly distributed in bottles, along with the more soluble oil fractions and chemical dispersant to mimic the conditions of the oil plume. Over the next 64 days the composition of the microbes and the crude oil were intensively studied.

The researchers witnessed an initial rapid growth of a microbe that had been previously observed to be the dominant bacterium in the early stages of the oil release but which had eluded subsequent attempts by others to recreate the conditions of the Gulf of Mexico oil plume.

Through DNA sequencing of its genome they were able to identify its mechanism for degrading oil. They gave this newly discovered bacterium the tentative name of Bermanella macondoprimitus based on its relatedness to other deep-sea microbes and the location where it was discovered.

"Our study demonstrated the importance of using dispersants in producing neutrally buoyant, tiny oil droplets, which kept much of the oil from reaching the ocean surface," Andersen said. "Naturally occurring microbes at this depth are highly specialized in growing by using specific components of the oil for their food source. So the oil droplets provided a large surface area for the microbes to chew up the oil."

Working with Berkeley Lab scientist Jill Banfield, a study co-author and also a professor in UC Berkeley's Department of Earth and Planetary Sciences, the team used newly developed DNA-based methods to identify all of the genomes of the microbes that used the introduced oil for growth along with their specific genes that were responsible for oil degradation. Many of the bacteria that were identified were similar to oil-degrading bacteria found on the ocean surface but had considerably streamlined sets of genes for oil degradation.

Filling in the gaps

Early work on microbial activity after the oil spill was led by Berkeley Lab's Terry Hazen (now primarily associated with the University of Tennessee), which provided the first data ever on microbial activity from a deepwater dispersed oil plume.

While Hazen's work revealed a variety of hydrocarbon degraders, this latest study identified the mechanisms the bacteria used to degrade oil and the relationship of these organisms involved in the spill to previously characterized hydrocarbon-degrading organisms.

"We now have the capability to identify the specific organisms that would naturally degrade the oil if spills occurred in other regions and to calculate the rates of the oil degradation to figure out how long it would take to consume the spilled oil at depth," Andersen said.

Implications for future spills

Andersen noted that it is not clear if the degradation of oil at these depths would have occurred in other offshore oil-producing regions. "The Gulf of Mexico is home to one of the largest concentrations of underwater hydrocarbon seeps, and it has been speculated that this helped in the selection of oil-degrading microbes that were observed in the underwater plumes," he said.

Although the well drilled by the Deepwater Horizon rig was one of the deepest of its time, new oil exploration offshore of Brazil, Uruguay, and India has now exceeded 2 miles below the ocean surface. By capturing water from these areas and subjecting them to the same test, it may be possible in the future to understand the consequences of an uncontrolled release of oil in these areas in greater detail.

"Our greatest hope would be that there were no oil spills in the future," Andersen said. "But having the ability to manipulate conditions in the laboratory could potentially allow us develop new insights for mitigating their impact." 

More information: Ping Hu el al., "Simulation of Deepwater Horizon oil plume reveals substrate specialization within a complex community of hydrocarbon degraders," PNAS (2017).

Researchers from several leading institutions in the United States say they have discovered a new kind of naturally occurring underwater bacteria that has eaten a considerable portion of the oil that spilled out of the Deepwater Horizon drilling platform.

The microbe is one of many species that helped mitigate damage from one of the worst environmental disasters in history. The scientists also say their work gives insights into how future oil spills could be mitigated.

In April 2010, an explosion on the Deepwater Horizon oil platform in the Gulf of Mexico killed 11 people and caused more than 4 million barrels of oil to shoot out of the well one mile below the surface of the ocean.

Recovery teams poured millions of barrels of chemicals into the ocean to disperse the oil.

Previous research has suggested the dispersants actually slowed microbes' ability to degrade the oil. But this new paper, published Monday in the Proceedings of the National Academy of Sciences, suggests that the dispersants broke up the oil into tiny droplets, which made them less buoyant and unable to float to the surface. Thus, the chemicals kept the oil in a kind of three dimensional cloud below the surface, making it more available to the microbes that live in the deeper portions of the ocean.

In particular the fact that the oil formed a cloud of droplets meant more of the oil's surface area was exposed to the microbes, making it easier for the bacteria to degrade it than the portions of the oil that floated to the surface or spread out on the ocean floor.

The exact amount the microbes have degraded is difficult to determine, said the study's senior author, Gary Andersen, a microbial ecologist at the University of California.

The team took water samples from the area around the spill, and recreated the conditions of the spill in a computer simulation to mimic what would have happened in the ocean following the accident.

In their examinations of the water samples and their simulation, they found one particularly dominant bacterium — called Candidatus Bermanella macondoprimitus — that previous research teams had not seen. The team sequenced the bacterium's genome and were even able to identify the genes responsible for degrading various components of oil.

"Laboratory experimental studies by other groups have not replicated the suspension of dispersed oil droplet conditions observed in deep-water plumes," wrote the team in their paper, "which may explain why these studies have been unable to enrich the early responding Bermanella and recreate the succession of bacteria observed in the field."

In fact, Andersen said, researchers can identify which genes in potentially any such microbe can degrade components of oil, and which specific components they can degrade. The simulation he and his colleagues devised shows how those bacteria would behave in a spill.

These fast-growing, rapidly replicating bacteria that previous research had not seen led the team to conclude that the dispersants had not in fact prevented bacteria from degrading the oil.

This does not mean that this bacterium can be dropped into the middle of an oil spill anywhere on the planet, Andersen said. It cannot practically be removed from its native ecosystem. In addition, the bacterium cannot be cultured in a lab.

But scientists can travel to areas around oil rigs and use water sampling and genome sequencing to find bacteria that could degrade oil, should a spill ever occur in those regions. That could tell what the rates of oil degradation are likely to be, and how it would be best to manage or respond to an oil spill in that area, he said.

Scientists debate just how much oil from Deepwater Horizon is still out in the environment, Andersen said, but there are no visible signs of the oil in the deep waters of the Gulf of Mexico right now. The oil that made it to the surface is degrading far more slowly, he said, and it is hard to tell how much oil from that spill is on the floor, because other chemicals leave similar signatures in the environment.

"But the actual marine life has recovered well from that spill, and fishing has resumed, so it has improved," Andersen told CNBC in an interview.

In the future, it would be best to put a system in place to understand the ecology of any area where prospectors drill for oil, Andersen said. Understanding the area would allow drillers to optimize their strategy so that if a spill does occur, naturally occurring bacteria would be able to best respond to it.

"They should be looking everywhere they are drilling for oil, and doing these types of simulations to see what the natural oil-degrading organisms would be and how quickly they would degrade oil," Andersen said.