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Researchers sample natural springs near Irazú volcano in Costa Rica.

Tom Owens

Microbes are siphoning massive amounts of carbon from Earth’s tectonic plates

A few kilometers below our feet lies a hidden world of microbes whose chemical reactions are shaping the long-term habitability of the planet. A new study suggests some of these microbes are siphoning off massive amounts of carbon as it enters Earth, using it to fuel their own sunless ecosystems. The carbon, prevented from being buried even deeper in Earth, will eventually escape back into the atmosphere—where it could help warm the planet. Researchers say the microbes represent an overlooked factor in efforts to balance Earth’s deep carbon cycle.

“It’s a really big leap forward,” says Georgia Institute of Technology biogeochemist Jennifer Glass, who was not involved in the study. She adds that this is one of the first papers to show how subterranean microbes can trap globally significant amounts of carbon.

Warming from human emissions of carbon dioxide (CO2) will be the deciding factor for surface temperatures in the coming centuries. But there is also a deeper carbon cycle, one that plays out over hundreds of millions of years. Slabs of ocean crust dive into Earth’s mantle at subduction zones, taking carbon down with them for long-term storage in the mantle. Some of this carbon, dissolved in rising blobs of magma and gasses, gets re-emitted at volcanoes. But much of what goes down doesn’t come back up, and researchers still don’t fully understand why.

Scientists found some of that missing carbon in 2017, when they examined gasses and fluids bubbling up from more than 20 hot springs in Costa Rica. The springs were 40 to 120 kilometers above the subduction zone where the Cocos Plate dives beneath Central America. Scientists found that a portion of the CO2 that goes down with the descending plate is turned into rock and never reaches the deep mantle or the atmosphere. But they also saw hints that more CO2 was being siphoned off the plate than rock formation alone could explain.

Now, by performing additional analyses on the hot spring samples, the same research team has found signs of chemical reactions that could only be facilitated by living things. The ratio of carbon isotopes in the samples suggests microbes are trapping the CO2 from the descending plate and turning it into organic carbon to “feed” and grow their own community. Indeed, the scientists found many bacteria in their hot spring samples that had the genes necessary for this chemical reaction. If this conversion is in fact happening—and the team’s calculations are right—microbes under this small swath of Costa Rica could be sequestering enough carbon each year to total the mass of 650 to 6500 blue whales.

“Small things add up,” says Katherine Fullerton, a microbiologist at Pellissippi State Community College who helped lead the study.

These microbes could be sequestering 2% to 22% of the carbon previously thought to reach the deep mantle, the researchers report today in Nature Geoscience. By keeping carbon close to the surface, where it is likely to eventually percolate up and re-enter the atmosphere, the microbes may be helping warm the planet over the long term, although this would require additional research to confirm.

Two percent may not have much of an effect on the deep carbon cycle, says Oliver Plümper, an expert in rock-fluid interactions at Utrecht University who was not involved in the study. But 22% would be “quite exciting.” He says this calculation constitutes an important piece to the puzzle of the deep carbon cycle, and it could impact projections of how stable Earth’s climate will remain in the long term—and how long the planet is likely to be habitable.

The researchers also found evidence for a second group of microbes that live off the organic leftovers of the carbon-sequestering bacteria. “There’s a whole world happening underneath Costa Rica,” says Karen Lloyd, co-author and microbiologist at the University of Tennessee, Knoxville. The researchers suspect similar activity is taking place in other subduction zones all over the world.

Glass says the research is off to a good start. But the next step—proving the bacteria express the genes to make proteins that modify and trap carbon—will require a lot of work, including isotope tracers to track the chemical reactions and drilling projects to glean direct samples from within Earth. And although it’s clear that these microbes are having an impact on us, they seem to function relatively independently from the surface world, Lloyd says. If the Sun were snuffed out today, these residents of the underworld likely wouldn’t know the difference.