By Deborah Zabarenko, Environment Correspondent
WASHINGTON (Reuters) - A Manhattan-sized plume of oil spewed deep into the Gulf of Mexico by BP's broken Macondo well has been consumed by a newly discovered fast-eating species of microbes, scientists reported on Tuesday.
The micro-organisms were apparently stimulated by the massive oil spill that began in April, and they degraded the hydrocarbons so efficiently that the plume is now undetectable, said Terry Hazen of Lawrence Berkeley National Laboratory.
These so-called proteobacteria -- Hazen calls them "bugs" -- have adapted to the cold deep water where the big BP plume was observed and are able to biodegrade hydrocarbons much more quickly than expected, without significantly depleting oxygen as most known oil-depleting bacteria do.
Oxygen is essential to the survival of commercially important fish and shellfish; a seasonal low-oxygen "dead zone" forms most summers in the Gulf of Mexico, caused by farm chemical run-off that flows down the Mississippi River.
Hydrocarbons in the crude oil from the BP spill actually stimulated the new microbes' ability to degrade them in cold water, Hazen and his colleagues wrote in research published on Tuesday in the journal Science.
In part, Hazen said, this is because these new "bugs" have adjusted over millions of years to seek out any petroleum they can find at the depths where they live, which coincides with the depth of the previously observed plume, roughly 3000 feet. At that depth, water temperature is approximately 41 degrees F (5 degrees C).
FEASTING ON HYDROCARBONS
Long before humans drilled for oil, natural oil seeps in the Gulf of Mexico have put out the equivalent of an Exxon Valdez spill each year, Hazen said.
Another factor was the consistency of the oil that came from the Macondo wellhead: light sweet Louisiana crude, an easily digestible substance for bacteria, and it was dispersed into tiny droplets, which also makes it more biodegradable.
These latest findings may initially seem to be at odds with a study published last Thursday in Science by researchers from Woods Hole Oceanographic Institution, which confirmed the existence of the oil plume and said micro-organisms did not seem to be biodegrading it very quickly.
However, Hazen and Rich Camilli of Woods Hole both said on Tuesday that the studies complement each other.
The Woods Hole team used an autonomous robot submarine and a mass spectrometer to detect the plume, but were forced to leave the area in late June, when Hurricane Alex threatened. At that time, they figured the plume was likely to remain for some time.
But that was before the well was capped in mid-July. Hazen said that within two weeks of the capping, the plume could not be detected, but there was a phenomenon called marine snow that indicated microbes had been feasting on hydrocarbons.
As of Tuesday, there was no sign of the plume, Hazen said.
That doesn't mean there is no oil left from the 4.9 million barrels of crude that spilled into the Gulf after the April 20 blowout at BP's Deepwater Horizon rig. The U.S. government estimated on August 4 that 50 percent of the BP oil is gone from the Gulf and the rest is rapidly degrading.
(Editing by .....)