The Gulf of Mexico's undersea oil plume is no more.
For nearly a month, scientists sampling the site of a deepwater plume stretching southwest from BP PLC's failed well in the Gulf have been foiled. Their sensors have gone silent. Where once a vibrant -- if diffuse -- cloud of oil stretched for miles, 3,600 feet below the surface, there is now only ocean, and what seems to be the debris of a bacterial feeding frenzy.
"For the last three weeks, we haven't been able to detect the deepwater plume at all," said Terry Hazen, a microbiologist and oil spill expert at Lawrence Berkeley National Laboratory who has had a clutch of researchers monitoring the Gulf since late May. The disappearance is backed up by government sampling data. The plume is simply gone. And Hazen knows why.
"This all fits with the fact that the bugs have degraded the oil," he said.
Despite press accounts to the contrary, the disappearance of this deepwater oil plume, whose midsummer existence was detailed last week by the Woods Hole Oceanographic Institution, is far from a shock, at least to scientists. Undersea bacteria -- the single-cell janitors of the marine world -- along with currents and diffusion likely combined to degrade or isolate the dispersed oil to undetectable levels, Hazen said.
Indeed, once the spill was plugged, "eventually you get to this point where the signal-to-noise ratio of your sensor cannot detect the oil," said Richard Camilli, the lead author of last week's Woods Hole report. The plume persisted while the oil flowed, but it was only a matter of time before the oil would degrade or fall to such low levels as to avoid detection, he said.
Left in the plume's wake are flocks of cellular debris, likely the remains of a mass die-off of bacteria that followed the purging of the plume's oil, though that needs to be confirmed, Hazen said. In the Gulf's cold, deep waters, the debris looks like marine snow, he said, and oxygen levels have dipped, indicating that the microscopic life has begun to feed on itself.
The likely source of that debris is a previously undiscovered, cold-loving microbe that surged in response to the plume, a development Hazen details in a new study to be published later this week in the journal Science. It is the first peer-reviewed report to provide direct evidence of how undersea microbes responded to hydrocarbons in the Gulf's deep waters.
"This enrichment of [cold-loving] petroleum degraders, with their rapid oil biodegradation rates, appears to be one of the major mechanisms behind the rapid disappearance of the deepwater dispersed oil plume," Hazen said.
The bugs' success in degrading one plume does not invalidate fears of how the ecosystem may have reacted to the multiple, invisible mists of oil that stretched out from the Macondo well while it was flowing, or the lingering taint of diffused oil and methane or hard-to-degrade, if nontoxic, petroleum components like asphalt. But Hazen's report should go a long way toward assuaging concerns that microbes in these depths were not up to the task of breaking down oil's complex cocktail of chemicals, scientists said.
"It's comforting that these organisms can degrade, quite rapidly, hydrocarbons at that depth," said Ken Timmis, a microbiologist at Germany's Helmholtz Centre for Infection Research who helped discover, a decade ago, an oil-loving bacteria closely related to Hazen's microbe.
"The numbers are fairly typical with what people have measured so far [in shallower waters], which is comforting," Timmis said. "It might not have been that way. It might have been that the degradation rates down there were significantly lower."
Previous reports of bacteria activity in deep waters, including the Woods Hole study, have relied on the amount of dissolved oxygen in the plume to serve as a indirect proxy for microbial life. (Like humans, many of the most efficient oil-degrading bacteria use oxygen for respiration.) While most scientists have reported marked, but not stark, drops in oxygen use -- a result supported in Hazen's study -- these reports have given only the vaguest outlines on the actual biology ongoing in the plume.
Hazen's study amounts to a first draft of that biological story.
Comparing samples from the plume and similar, non-plume sites taken more than 20 miles away from the Macondo well in late May and early June, the researchers found a startling increase in one long, rod-shaped bacteria, closely related to the Oceanospirillales family. Under normal conditions, the microbe's DNA constituted 5 percent of the sequences analyzed, but at multiple sites in the plume, the bug made up more than 90 percent of all the detected DNA.
The unknown bug -- which, in true scientific fashion, Hazen has not even nicknamed yet -- was not the only microbe to be significantly enriched by the plume, though it showed by far the largest increase. Sixteen species increased, nearly all of which were "known to degrade hydrocarbons or are stimulated by the presence of oil in cold environments," the authors wrote.
Field trials and lab tests found that oil in the plume had a half-life between 1.2 and 6.1 days, the researchers found. This half-life, which measures the time needed for oil to reduce in size by half, includes mixing and dilution, but biodegradation is likely playing a large role in reducing the oil's alkanes, a principal component, the paper says. There are strong correlations between complex alkanes, which resist dissolution, and cell density, it notes.
The overall behavior of the unknown bacteria closely resembles that of several other oil-loving species that have been identified in the past decade, all of which play a significant role in removing petroleum from marine waters. Typically, these bacteria subsist off very low nutrient levels, and then surge once their environment is saturated with their preferred, oil-based diet.
"There is a surge in number, but the total number is not very dramatic," Timmis said. "The nice thing is that these organisms seem to be very, very active."
The particular bacteria identified by Hazen are perfectly adapted for the Gulf's deep waters, which sit under high pressure and remain cold, hovering around 5 degrees Celsius, despite their near-tropical locale.
"They actually degrade oil faster at 5 degrees than they do at 20 degrees," Hazen said.
How the bacterium was identified
Even five years ago, it would have been difficult for Hazen's team to identify the microbe with such speed. The team relied on a recently developed microarray designed at the Lawrence Berkeley National Laboratory, called the PhyloChip, which allows rapid testing for more than 8,000 bacteria species. The array quickly revealed a microbial community that was significantly altered by the plume.
After the array tests, Hazen's team puts its samples through several other hurdles. They found that the dominant fatty acids in the plume have also been reported as vital byproducts in a consortium of oil-degrading bacteria. Microscopic analysis of the unknown microbe found it hewed closely to the distinctive look of Oceanospirillales. And another chip-based analysis of the 5,000 mixed-up genes found in the samples flagged more than 1,600 related to oil degradation, many of which significantly increased in the plume.
Much work remains to be done, Hazen stressed. His team is currently sequencing the genome of their primary degrader, and further tests are likely that can directly tie the microbe to oil degradation, rather than relying on strong correlations. Sediment sampling is beginning this week, and his researchers are releasing oil-soaked traps deep underwater to test how microbes colonize the oil.
It remains too early to say if the bacterial activity validates BP's decision to spray large amounts of dispersants at the wellhead, which was intended to increase the surface area of oil available for biodegradation. Marine bacteria are far more efficient degraders than soil bacteria, Hazen said, but there are too many data gaps to say this efficiency outstrips toxicity concerns to deepwater ecology.
However, "it certainly looks like that [dispersants] helped," he added.
While it can be difficult to accept, if there is one disaster the Gulf is poised to handle, it is a leak of its own light crude, Hazen added. The bacteria have had millions of years to adapt to the oil, the petroleum itself is light and readily degraded and, in the plumes at least, the oil was already in low concentrations.
Hazen's study may finally raise public awareness that oil spills nearly always trigger substantial microbial hydrocarbon degradation, a fact that is too frequently ignored in initial responses, Timmis said. Future strategies to deal with oil spills must fully integrate measures to harness the microbial capacity to remove hydrocarbons, he said.
"It has to be part and parcel of the strategy," he said. "And that I think that will become very clear when this is published."