Gassy Bugs: Microbes may produce propane under the sea

For decades, scientists have been puzzled by periodic findings of ethane and propane in sediments that they’ve pulled from deep below the ocean floor. As far as they knew, these gases could be produced only as petroleum is—by great heat applied to ancient, buried organic matter. But sometimes, ethane and propane turn up in areas where that process seems unlikely.

A new report suggests a different source: microbes. Bacteria and archaea within underwater sediments could chew up buried organic material and spew out ethane and propane as waste products, assert Kai-Uwe Hinrichs of the University of Bremen in Germany, and his colleagues.

Heat can produce propane and ethane only at spots along cracks in Earth’s crust where the planet’s internal heat escapes but is then trapped by thick layers of sediment overlying the crust. Hinrichs’ team drew sediment samples from six sites off Peru that don’t meet these conditions. All had thin layers of sediment, and two were far from any cracks in the crust and therefore insulated from Earth’s internal heat.

Nevertheless, the researchers found ethane and propane locked in the sediments at all six sites. Adding to the mystery, gases at all the sites were in higher concentrations in pockets at shallow and middle depths in the sediments than in deeper locations. If the gases had been produced by heat, they would have been more abundant farther down, Hinrichs notes.

The researchers conclude that the gases at the sites must have been produced by microbes. “When you can’t come up with any geologic source, then biology is an obvious candidate,” Hinrichs says. The researchers report in the Oct. 3 Proceedings of the National Academy of Sciences that the isotopes in the ethane and propane within the sediment are characteristic of biologically produced materials.

Microbes under the seafloor commonly break down organic matter to produce methane, a gas similar to ethane and propane. Although the researchers haven’t isolated microbes that produce these two gasses, they point to chemical reactions that could produce them from materials available in undersea sediment.

The concentration of propane in the sediments is too low for commercial use as fuel. However, Hinrichs says that if the set of reactions producing the propane were better understood, scientists might fine-tune it to turn organic matter directly into propane.

The problem of the source of ethane and propane in ocean sediments had “been brushed under the carpet,” says John Parkes of Cardiff University in Wales. The new research “is like a breath of fresh air,” he says. The suggestion of a biological source of the gases is reasonable but still unproved, he adds. In particular, researchers must demonstrate that the reaction that they propose takes place in undersea microbes.

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