Perhaps it should have been called the Gulf of Mexico gas spill.
A vast majority of the natural gas that billowed out of BP PLC's failed well in the Gulf this summer did not escape to the surface and atmosphere. Instead, the gas -- including its main component, methane -- remained trapped deep underwater, priming the bacterial response to the spill, according to research published online yesterday in Science.
Oil has long been the most visible component of the hydrocarbon rush that gripped the Gulf this summer, even when invisible, in the form of underwater mists of oiled water. Natural gas, billowing out from the Macondo well alongside oil at double the amount, often received scant attention from the public, press and government.
Fortunately, scientists were not so easily fixated on the crude.
In June, while the well resisted control, scientists led by David Valentine, a microbial geochemist at the University of California, Santa Barbara, took several hundred samples of natural gas at 31 sites in a large circle around Macondo, extending to a maximum of 8 miles from the spill's epicenter. Data from their cruise, sponsored by a grant from the National Science Foundation, amount to the first independent snapshot of the immediate, short-term response triggered underwater by an onslaught of gas.
The majority of the methane, they found, remained dissolved more than 2,600 feet underwater, and the gas likely accounted for two-thirds of all the microbial activity in the undersea plumes. Based on government and BP data, some 206,000 metric tons of methane, 35,700 tons of ethane and 28,400 tons of propane snaked out into the subsurface, they estimated.
"The concentrations in the deep plumes were typically more than 500 times the concentrations in the shallower water," Valentine said. "This is good evidence that the majority of methane was trapped and is consistent with most other works on the matter."
Scientists have long predicted that gas leaked from deepwater wells would fail to reach the atmosphere. (One prominent researcher, Samantha Joye at the University of Georgia, has stressed the importance of subsea methane for months, though her research has not yet been published.) The long, dark ascent gives gas plenty of time to dissolve into the ocean's cold stretches, even when it does not form into ice-like hydrate formations, Valentine said. And while this behavior had been seen in natural seeps, the BP spill provided the perfect laboratory to test these assumptions on a grand scale.
At several locations near the well, methane concentrations were so high that they equaled the amount of oxygen in the water, Valentine said, a disturbing balance. At the same time, however, the more volatile gas components were disappearing, their chemistry enticing to a host of deepwater bacteria, many closely related to known hydrocarbon degraders.
The study also highlights uncertainty as to which hydrocarbons, after propane and ethane, were next to be degraded in the undersea plumes, said Rich Camilli, the lead author of a report out of the Woods Hole Oceanographic Institution last month that first delineated the boundaries of large hydrocarbon plume that stretched southwest from the well.
"This suggests that most all of the microbial degradation that is happening there is just the natural gas being utilized, which suggests that there is proportionately less microbial degradation of the oil itself," Camilli said.
"This is where [this spill] is really unique," Camilli added. "For most oil spills, it's just oil, it's not natural gas, but there is so much natural gas that came out of this leak. It appears as though the microbes are just interested in the natural gas. So it suggests that the oil may persist longer than we would like."
Few bugs degrading methane
Unlike previous studies, which tracked the southwest plume and laid out the bacterial response to that mist, Valentine's survey provides a snapshot of a limited radius and does not speak to the plume as it grew more mature farther from the well. Like past studies, his report makes no claim on the current state of the Gulf's waters, nor does it attempt to quantify the ecological impact methane could have on the ocean's wildlife.
Simpler than crude oil's stew, natural gas is a mix of several small chemicals, each assembled out of hydrogen and carbon. Sturdy methane is the most abundant gas, accompanied by ethane, propane and butane, all more complex, larger and unstable hydrocarbons. Unlike methane, these gases can be eaten up by a wide variety of bacteria.
"Put something like propane or butane into a natural environment and it tends to be consumed very quickly," Valentine said.
Indeed, chemical analysis of the well's surrounding waters found that ethane and propane degraded rapidly after emerging into the ocean, causing a bloom in bacterial activity. This surge, however, may have come at the expense of methane, which initially resisted degradation. As ethane disappeared, though, methane loss began to expand at an exponential rate, leaving uncertainty about how quickly it would degrade in more mature plumes.
It is unknown whether the loss of ethane could directly influence methane degradation, but it "could indicate active inhibition," Valentine said. Either way, the methane loss did appear "to be increasing as these other gases were disappearing," he said.
Whatever its ultimate degradation rate, it is not surprising that methane, which is the simplest and least reactive hydrocarbon, would not be devoured by the mongrel group of bugs degrading propane. Those volatile gases can easily be shunted into standard metabolisms, while methane requires a breed apart, Valentine said.
"The bacteria that consume methane basically have to devote their entire lifestyle to degrading methane," he said.
Typical methane-degrading microbes have highly stacked membranes surrounding their cells, which they pump full of enzymes that only target their preferred snack. Essentially, they need a "sledgehammer" to crack the methane, Valentine said, while propane only requires a "plastic knife." Even then, the methane eaters must evolve a unique metabolism to consume the gas.
Biological sampling from Valentine's group revealed two dominant microbe strains related to the Cycloclasticus and Colwellia bacteria groups. These bugs bloomed thanks to their ability to degrade propane and ethane, the researchers suspect, though Cycloclasticus is also known for its ability to chew through aromatic hydrocarbons, circle-shaped molecules that are typically found in crude oil in low amounts, and carry high toxicity concerns.
The strains found by Valentine contrast with the Oceanospirillaceae relatives identified by Terry Hazen, a microbiologist at Lawrence Berkeley National Laboratory, in a study published late last month. (Valentine, though, also detected Oceanospirillaceae in some of his unpublished data.) Hazen marshaled a wealth of genetic evidence to support conclusions that his bacteria were decaying common oil components, known as alkanes, but, like Valentine's report, Hazen did not identify a direct causal link.
Indeed, it is possible that Hazen's bacteria fed on methane, with oil concentrations dropping due to ocean mixing, Valentine said. Or not. "We don't really know what the metabolism is for any of these organisms," he said, though all are related to known hydrocarbon degraders.
There remains the possibility that ecology could control the bacterial surges even more than the choice of food source, Valentine added. Microbial blooms are unstable formations, subject to grazing from larger organisms, or massive viral attacks.
"When one species dominates, that's not a stable situation," he said.
Hazen, for one, was not surprised by Valentine's results. "I think it's just a different spot," Hazen said. "He was sampling areas a little bit different from where we sampled."
All three of the undersea studies published in Science over the past month remain complementary pieces in a larger puzzle, he added.
While Hazen suspected methane would linger underwater in ice-like hydrates, Valentine's data indicate that the gas has remained in a dissolved, gaseous state. Even at high concentrations near the well, the methane was dilute enough that it could not form hydrates, Valentine said. His team closely examined the underwater regions where the hydrates would have formed, he added, and found no evidence of hydrates.
The initial surge of ethane and propane degraders, and then, once their food had vanished, their subsequent deaths, could account for initial drastic drops in dissolved oxygen seen early in the spill, and the more stabilized levels seen by Woods Hole and Hazen in their studies of the mature plume. Persistent mixing with normal waters outside the plume also likely kept oxygen levels stable, Valentine added.
Much more research is set to be published on the spill in the coming months, and Valentine himself was recently back at sea, tracing the retreating edges of the oil and methane mists. Hazen, meanwhile, has had specialized microbe oil traps sitting underwater at the spill's center for weeks, and early this week launched a cruise to take soil samples at 48 underwater sites.
While his findings may surprise the layperson, Valentine's shock was that the hypothesis proposed by his team -- several of his researchers had already been studying methane seeps -- proved so accurate. Science often does not work that way, he said.
"We were surprised," Valentine said, "that nature was working the way we predicted it to work."
Reporter Allison Winter contributed.