Microbes may act as gatekeepers of Earth's deep carbon

  • May 20, 2019
  • climate change, carbon cycle, microbes, Faculty, Research
  • Homepage News, Faculty & Staff, Research, College of Natural Science, EES
Image of scientists in the Costa Rican subduction zone
Peter Barry (front left) sets up gas sampling apparatus with colleagues Maarten de Moor (front right), Giulio Bini (rear left) and Angelo Battaglia (rear right). Photo: Tom Owens

Two years ago, a team of scientists, including Michigan State University geomicrobiologist Matt Schrenk, visited Costa Rica’s subduction zone, where the ocean floor sinks beneath the continent and volcanoes tower above the surface. They wanted to find out if microbes can affect the cycle of carbon moving from Earth’s surface into the deep interior.

According to their new study, which was recently published in Nature, the answer is yes, they can.

This groundbreaking research shows that microbes consume and—crucially—help trap some of the sinking carbon in this zone. This finding has important implications for understanding Earth’s fundamental processes and for revealing how nature can potentially help mitigate climate change.

Graphic showing subduction zone ans associated subduction arc showing release of carbon.
This graphic shows cross-section of a subduction zone and associated volcanic arc showing the release of carbon and other volatiles by the downgoing oceanic plate. A substantial amount of carbon was shown to be trapped in the forearc region between the ocean and the volcanoes, rather than being returned to the deep Earth as was previously thought. At a global scale, these processes impact the amount of carbon dioxide in the atmosphere, which serves to regulate Earth's climate. Courtesy image.

At a subduction zone there is communication between Earth’s surface and interior. Two plates collide and the denser plate sinks, transporting material from the surface into Earth’s interior. Showing that the microbes at the near surface are playing a fundamental role in how carbon and other elements are being locked up into the crust provides a profound new understanding of Earth processes and helps researchers model how Earth’s interior may develop over time.

“The balance between carbon buried in the deep Earth and that released through volcanoes as gases plays an important role in regulating climate over geologic time scales,” explained Schrenk, an assistant professor in the Department of Earth and Environmental Sciences in the MSU College of Natural Science and study co-author. “This study shows that a substantial amount of subducted carbon is emitted in groundwater in the forearc region [the area between the subduction zone and the volcanic chain] is ‘filtered’ by geochemical and microbial processes before it enters the atmosphere. This insight was aided by the holistic, interdisciplinary study of the Costa Rican arc, and is one of the first studies to show the impact of subsurface microorganisms upon global-scale biogeochemical processes.”

Image of scientists Giovannelli and Karen Lloyd collecting samples from the crater lake in Poas volcano.
Microbiologists Donato Giovannelli and Karen Lloyd collect samples from the crater lake in Poás volcano. Photo: Katie Pratt

This is the first evidence that subterranean life plays a role in removing carbon from subduction zones. It has been well established that microbes are capable of taking carbon dissolved in water and converting it into a mineral within the rocks. This work showed that this happens on a large scale across a subduction zone. It is a natural CO2 sequestration process that can control the availability of carbon on Earth’s surface.

Lead author Peter Barry, who carried out the research while at Oxford University’s Department of Earth Sciences said the team found that a substantial amount of carbon is being trapped in non-volcanic areas instead of escaping through volcanoes or sinking into Earth’s interior.

“Until this point, scientists had assumed that life plays little to no role in whether this oceanic carbon is transported all the way into the mantle,” Barry said, “but we found that life and chemical processes work together to be the gatekeepers of carbon delivery to the mantle.”

Image of microbial sample collection in Costa Rica
Microbial samples being collected in a hot spring in Costa Rica.

During the 12-day expedition, the 25-person group of multi-disciplinary scientists collected water samples from thermal springs throughout Costa Rica. Scientists have long predicted that these thermal waters spit out ancient carbon molecules, subducted millions of years before. By comparing the relative amounts of two different kinds of carbon – called isotopes – the scientists showed that the predictions were true and that previously unrecognized processes were at work in the crust above the subduction zone, acting to trap large amounts of carbon.

Following their analyses, the scientists estimated that about 94 percent of that carbon transforms into calcite minerals and microbial biomass.

“These microbes are literally sequestering carbon,” said Karen Lloyd, associate professor of microbiology at the University of Tennessee, Knoxville, and study senior author. “Scientists are actively working on carbon sequestration to mitigate climate change and carbon capture and storage as a means to bury greenhouse gases over long time periods. Our study is a good example of where this is happening naturally; it was previously unrecognised. The findings show that this happens on a big, reservoir scale.”

The researchers now plan to investigate other subduction zones to see if this trend is widespread. If these biological and geochemical processes occur worldwide, they would translate to 19 percent less carbon entering the deep mantle than previously estimated.

The research is part of the Deep Carbon Observatory’s Biology Meets Subduction project. The interdisciplinary team included 25 researchers from six nations belonging to each of the Deep Carbon Observatory Science Communities: Deep Life, Extreme Chemistry and Physics, Reservoirs and Fluxes, and Deep Energy.

 

Banner image: Drone view above Poás volcano located in Costa Rica's subduction zone, where scientists are investigating whether microbes can affect the cycle of carbon moving from Earth’s surface into the deep interior. This finding has important implications for understanding Earth’s fundamental processes and for revealing how nature can potentially help mitigate climate change. Photo: Peter Barry.