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Gulf methane feast was short-lived

Posted on behalf of Amanda Mascarelli

Gas-munching microbes had no trouble gobbling up the massive amount of methane released into the Gulf of Mexico during the Deepwater Horizon oil spill last year.

In addition to the three-quarters of a billion litres of oil that gushed into the Gulf of Mexico between April and July 2010, the broken wellhead also spewed roughly another 200,000 tonnes of methane into the ocean. Researchers report today in Science that bacteria that dined on the methane had entirely consumed the gaseous feast within a matter of months.

This finding came as a surprise to the researchers; when they initially surveyed the area surrounding the wellhead in June, the methane appeared to be breaking down quite slowly. And methane oxidation in the ocean is generally thought to occur over long timescales. Therefore the researchers initially speculated that the methane could hang around for a year or more, says David Valentine, a geomicrobiologist from the University of California in Santa Barbara and one of the lead authors of the study.

Valentine and his colleagues had planned to track the methane and study its fate when they returned to the spill site a couple of months later. But during research cruises from late August to early October, the team found that the methane had completely vanished, leaving an “oxygen sag” in its wake. This indicated that a bloom of methane-loving bacteria had been taking up oxygen in the surrounding water. They also found remnants of the bacterial community that had feasted on the methane.

In September, Valentine and his team reported in Science (based on data collected in mid-June) that bacteria were primarily gobbling up gases, rather than oil, and that the most easily degradable gases – propane and ethane – were being consumed first.

“After those were completely consumed the methane degraders were able to really get going,” says Valentine. “It seems like within fairly short order they grew and kept growing until there was no methane left.”

The team did not directly measure oil or the breakdown of oil. “There’s still a lot that we don’t know about the state of the oil,” says Valentine. “That’s going to be an ongoing process for years.”

The fact that bacteria quickly slurped up methane from the oil spill also has implications for the breakdown of methane from natural events. The work suggests that methane released from seeps, vents, or even from large-scale disruption of methane ‘clathrates’ – crystalline cages of bound methane gas – would also be quickly devoured by microbes.

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