Undersea Oil Plume Vanishes in Gulf, Degraded by
Previously Unknown Bug
Published: August 24, 2010
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."
Biological account
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."
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