May 27, 2011

Did Bacteria Really Consume Methane From BP Spill?

Marine scientists cast doubt on Thursday about an earlier study that found bacteria had consumed nearly all of the methane that leaked into the Gulf of Mexico amid last year's BP oil spill.

Calling the widely-publicized study's findings "ambiguous" and "unconvincing", they said the methods used were flawed and warranted further review.

"We find the complete methane oxidation hypothesis unconvincing," wrote Samantha Joye and colleagues from 12 other institutions in a technical comment published in the May 27 edition of the journal Science.

The initial study was published in January 21, 2011 edition of Science, and found that based on methane and oxygen distributions measured at 207 stations in the Gulf of Mexico, "nearly all" of the methane released from the well was consumed in the water column within approximately 120 days of the release.

In the current Science report, Joye and colleagues make the case that uncertainties in the hydrocarbon discharge from the blowout, oxygen depletion fueled by processes other than methane consumption, a problematic interpretation of genetic data and shortcomings of the model used by the authors of the January study challenge the attribution of low oxygen zones to the oxidation of methane gas.

"Our goal is to understand what happened to the methane released from the Macondo discharge and in the larger framework, to better understand the factors that regulate microbial methane consumption following large-scale gas releases," said Joye, a professor in the University of Georgia Franklin College of Arts and Sciences.

"I believe there is still a lot to learn about the environmental factors that regulate methane consumption in the Gulf's waters and elsewhere."

Joye and her co-authors note that low levels of oxygen are known to occur in the Gulf of Mexico due to bacterial consumption of carbon inputs from the Mississippi River, as well as the bacterial consumption of hydrocarbons that naturally seep from the seafloor.

The researchers emphasize that given the uncertainty in oxygen and methane budgets, strong supporting evidence is required to attribute oxygen depletion to methane removal.

However, a study published in the October 8, 2010 edition of Science showed low measured rates of methane consumption by bacteria.  Joye and her co-authors note that samples from the control stations and the low-oxygen stations that were analyzed for unique genetic markers in the January 2011 study showed no significant difference in the abundance of methane consuming bacteria. They also argue that the model the study used neglected important factors that affect the transport and biodegradation of methane, and that it only provided a tentative match of the observational data.

The scientists add that several other studies have found that microbes do not consume considerable amounts of methane released from natural deep-sea vents. 

The most vulnerable store of methane hydrates is not in the Gulf of Mexico, but in the deposits that underlie the shallow waters of the Arctic, they said.

"A range of data exists that shows a significant release of methane seeping out at the seafloor to the atmosphere, indicating that the microbial biofilter is not as effective," Joye said.

"Importantly for the future of the planet, there is even less evidence for a strong biofilter of methane hydrate destabilized in the shallow Arctic settings."


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