Published by the American Geological Institute
and Trends in the Geosciences
Take a stroll past a marshy bog and breathe deeply. Recognize that smell? Beneath the muck live many communities of bacteria. Some are busy reducing sulphate and producing that distinctive marshy sulphur smell. Others break down relatively simple hydrocarbons into their most basic form, methane gas. You don’t smell it, but it’s there.
Nobody thought that deep beneath Earth’s surface, below the zone of sulphate reduction — where there is nary a molecule of oxygen, sulphate, nitrate or any other easily degradable biomolecule — any individual type of anaerobic bacterium could break down oil (a complex, saturated hydrocarbon form) into methane gas. Recently, this idea has been challenged by some that believe that the deep environment that is home to Earth’s oil reservoirs is also home to communities of bacteria that can convert the saturated hydrocarbons into methane. Because methane pockets are often found in association with oil reservoirs, there must be a process by which oil can be biodegraded to explain the existence of deep microbial communities and methane gas in the old sediments, some researchers say.
Derek Lovley and Robert Anderson of the University of Massachusetts, Amherst, are in the forefront of discrediting the commonly held belief that saturated hydrocarbons resist anaerobic bacterial decay — an idea held in part because oil can exist in large reservoirs over long periods of geologic time. They investigated the biological decay of hexadecane, a saturated hydrocarbon, into methane gas and published their report in the April 13 Nature. They found that a naturally occurring community of anaerobic bacteria could convert oil into methane almost immediately and without the presence of sulphate.
Some are raising their eyebrows at the claims of Lovley and Anderson. John Parkes of the University of Bristol, UK, says that in order for the conversion to occur a minimal amount of sulphate must exist. Because oil reservoirs often exist in the absence of sulphate, he says such conditions are very rare and their effect on methane accumulation over relatively short periods of time is negligible. But he adds that the findings of Lovley and Anderson are the first steps in an area of research that could have major implications for the future of fossil fuel recovery — if it is shown that the conversion process can occur rapidly.
Last year a similar experiment showed that anaerobic bacteria could convert hexadecane into methane, but it took two years for enough methane to be collected for proper analyses. The researchers, led by Karsten Zengler of the Max Planck Institute for Marine Microbiology in Germany, published their report in the Sept. 16, 1999 Nature. Through laboratory experiments Zengler’s team found that after a strictly anoxic incubation period of four months the bacteria began producing a small amount of methane.
Lovley and Anderson, however, sampled an oil-bearing sediment, along with its naturally occurring bacterial community, and incubated them under strictly anoxic conditions similar to those used in Zengler's experiments. They applied a radiotracer to the samples, and to their surprise, found that almost immediately after the radiotracer was added the bacteria were producing radiolabeled methane. The original samples came from an aquifer in Bemidji, Minn., that was contaminated with oil from an oil spill that occurred more than 20 years ago. The samples were depleted in sulphate and nitrate. Since that time, the oil has saturated the sediments and communities of benthic bacteria have moved in and set up shop.
The sluggishness of Zengler's incubated samples in part explains why sulphate-free, anaerobic methanogenesis of saturated hydrocarbons has been overlooked in the past, according to Parkes. But he questions the conditions under which such decay could occur in a natural environment.
Zengler’s results showed that methanogenesis was stimulated in the presence of low levels of sulphate. As a result, Parkes reasons, only a very specific set of environmental conditions that are unusual in deep, oil bearing sediments will produce significant amounts of methane. It is hard to find sulphate in deep sediment because commonly it has been consumed by sulphate reducing bacteria. “I am sure the authors know that even very low sulphate concentrations can allow some sulphate reduction to occur and groundwater flow could replenish the sulphate,” he says.
Lovely makes clear that his samples were strictly sulphate free, and laboratory experiments were done using such samples in hopes of mimicking the conditions in an oil reservoir where samples could not easily be collected. “Our results show that sulphate is not required and there is no special environment, because there was no sulphate there [in our samples],” Lovley says. “It makes you wonder if this is a naturally occurring process. There was no adaptation lag time. Zengler saw a lag of literally hundreds of days — we saw immediate conversion.”