Microbes put ancient carbon on the menu

Scientists have found microorganisms within Kentucky shale that are eating the ancient carbon locked within the rock. This previously unrecognized dietary habit could have a prevalent role in the weathering and erosion of similar sedimentary rock at many other locations, say the researchers.

The as-yet-unidentified microbes are munching on kerogen, a mixture of large organic molecules formed from the remains of bacteria and algae that were incorporated into ocean-floor sediments millions of years ago. More than 95 percent of the organic matter in carbon-containing sediments is kerogen, which doesn’t dissolve out of the rocks and decomposes very slowly, says Steven T. Petsch, a geochemist at Woods Hole Oceanographic Institution in Woods Hole, Mass.

Petsch and his colleagues analyzed samples of shale collected near Clay City, Ky. The researchers found microbes even within the dark, oily-smelling sediments lying 60 centimeters below the surface of the ground, says Petsch. His team reported its findings in Sciencexpress, the online supplement to the March 30 Science.

Some of the rockbound microorganisms were spherical and 1 micrometer in diameter, but others were rod-shaped and ranged up to 3 m long. Genetic analysis should help determine whether these are separate organisms. The researchers proved the microbes were eating the ancient kerogen by growing them in isolation and analyzing the carbon isotopes in their cell membranes.

Any organic matter more than 60,000 years old contains almost no carbon-14, because half of any mass of this radioactive form of carbon, which creatures ingest only while alive, decays every 5,730 years, Petsch notes. Analysis showed that up to 94 percent of the carbon in the microbes’ cell membranes didn’t derive from living, carbon-14-rich food sources. Instead, it came from the decidedly dead kerogen in the Kentucky shale, which formed more than 365 million years ago.

“Microbes can do phenomenal things,” says David J. Hollander, an organic geochemist at the University of South Florida in St. Petersburg. Petsch’s finding adds fuel to the debate about a thriving biosphere deep within Earth’s crust that can use sedimentary carbon as a food source, Hollander adds.

The weathering of this type of rock is an important part of Earth’s carbon cycle, Petsch adds. It’s possible that kerogen-munching microbes accelerate erosion of exposed shales and similar sediments. Such feasting would remove oxygen from the atmosphere and generate carbon dioxide, an Earth-warming greenhouse gas. If these organisms reproduce more quickly at higher temperatures, it’s likely they could exacerbate global warming.

Scientists might be able to exploit these microbes’ ability to eat what previously was thought inedible, Petsch notes. Researchers might genetically engineer these organisms or their kin to consume other slow-to-decompose organic items such as rubber tires, polymer plastics, or even asphalt.