Sep 17, 2010
Microbes munch up propane from Deepwater Horizon
While it's the oil released by the Deepwater Horizon accident that hit the headlines, a large amount of natural gas also escaped into the ocean. And because of the depth of the release, an unprecedented 1.5 km underwater, the hydrocarbon gases dissolved rather than escaping into the atmosphere.
"Early in the spill it became apparent to us that gas was an important component of emission at the sea floor," Dave Valentine of the University of California, Santa Barbara, US, told environmentalresearchweb. "We hypothesized the gas would drive much of the biology and respiration and set out to test our hypothesis."
Valentine estimates that 1.5 x 1010 moles of natural gas may have escaped to the ocean depths during the spill. Between 11 and 21 June 2010, he and colleagues from the University of California, Santa Barbara and Texas A&M University, US, found dissolved hydrocarbon gases deep underwater at 29 out of 31 survey locations between 1 and 12.5 km from the spill site.
The researchers believe the four plumes of dissolved gas they discovered originated at different times, and were distributed around the oil well by complex current patterns. Propane, ethane and methane were most abundant below 799 metres; at shallow depths concentrations were much lower, indicating that most of the gas dissolved, or was partitioned as gas hydrate, at depth.
Tests revealed that in fresh plumes, propane was respired by microbes in preference to ethane and both dissolved gases were removed prior to methane. Together, propane and ethane accounted for up to 70% of the oxygen demand of fresh plumes. According to DNA analysis, the bacteria involved in this respiration were mainly from the Cycloclasticus and Colwellia groups.
What's more, sites with a greater concentration of propane tended to have a higher potential for oxidizing the gas. A lab experiment showed that the rate of propane and ethane respiration reached a maximum level before a marked increase after 24 hours – the team interpreted this as a response by the microbial community to the change in conditions. And the plume closest to the wellhead had the highest levels of hydrocarbons and a microbial community consisting of only about 50% hydrocarbon degraders; the team believes that this was an early stage in the bloom of hydrocarbon-oxidizing bacteria.
"Propane and ethane trapped in the deep water may therefore promote rapid hydrocarbon respiration by low-diversity bacterial blooms, priming bacterial populations for degradation of other hydrocarbons in the aging plume," write the researchers in Sciencexpress.
As the plumes mature it's likely that methane and other alkanes may start to feed microbial respiration, but the team expects the respiration rate to drop once ethane, propane and butane levels fall. Once the gases are depleted, the bacteria may also start to degrade oil.
Valentine says there are several implications of the findings. "Natural gas is an important driver for respiration in deep oil spills, which have a strong gas-trapping tendency," he explained. "The bacterial blooms and respiration in the deep gulf plumes are strongly influenced by the gas, and cannot be attributed to oil without independent evidence of oil degradation."
Methane, ethane and propane released by the spill will exert a biological oxygen demand of up to 8.3 x 1011 g O2, 1.3 x 1011 g O2 and 1.0 x 1011 g O2, respectively, estimate the researchers. The oxygen demand for the oil that spilled from the Deepwater Horizon rig is 4.4 x 1011 g O2. The team believes that roughly two-thirds of the microbial productivity in deep plumes will arise as a result of the metabolism of natural gas.
"We are currently in the Gulf on a cruise aboard the NOAA ship Pisces, studying the aged plumes in the deep water," said Valentine. "We hope to continue to track the far-field fate of oil and gas in the deep plumes, as well as the fate of dispersants and the ecology of the related bacteria."
• Deepwater Horizon Oil Spill: ERL Call for Papers
Environmental Research Letters (ERL) – environmentalresearchweb's sister product – would like to invite you to submit your research into the effects of the Deepwater Gulf oil spill to a dedicated focus issue. This issue will build a collection of research on the initial impacts of the disaster and is the perfect forum to present your work to a massive global audience.
About the author
Liz Kalaugher is editor of environmentalresearchweb.