Study finds missing piece of biogeochemical puzzle in aquifers

May 01, 2014
Deep underground, microbes have to breathe iron and sulfur to get energy. Argonne scientists announced they have found what appears to be a missing step in the iron-sulfur cycle in underground aquifers. It turns out that sulfur (white-yellow power, on top) may be far more essential than previously thought in helping microbes harvest energy from iron minerals (from top to bottom: yellow goethite, red hematite, orange lepidocrocite) and produce sulfur-iron minerals, like mackinawhite (black). Understanding these cycles is important for carbon sequestration and for predicting the fate of ground pollution. Credit: Mark Lopez/Argonne National Laboratory.

(Phys.org) —A study published today in Science by researchers from the U.S. Department of Energy's Argonne National Laboratory may dramatically shift our understanding of the complex dance of microbes and minerals that takes place in aquifers deep underground. This dance affects groundwater quality, the fate of contaminants in the ground and the emerging science of carbon sequestration.

Deep underground, microbes don't have much access to oxygen. So they have evolved ways to breathe other elements, including solid minerals like and sulfur.

The part that interests scientists is that when the microbes breathe solid iron and sulfur, they transform them into highly reactive dissolved ions that are then much more likely to interact with other minerals and dissolved materials in the aquifer. This process can slowly but steadily make dramatic changes to the makeup of the rock, soil and water.

"That means that how these microbes breathe affects what happens to pollutants—whether they travel or stay put—as well as groundwater quality," said Ted Flynn, a scientist from Argonne and the Computation Institute at the University of Chicago and the lead author of the study.

About a fifth of the world's population relies on groundwater from aquifers for their drinking water supply, and many more depend on the crops watered by aquifers.

For decades, scientists thought that when iron was present in these types of deep aquifers, microbes who can breathe it would out-compete those who cannot. There's an accepted hierarchy of what microbes prefer to breathe, according to how much energy each reaction can theoretically yield. (Oxygen is considered the best overall, but it is rarely found deep below the surface.)

According to these calculations, of the elements that do show up in these aquifers, breathing iron theoretically provides the most energy to microbes. And iron is frequently among the most abundant minerals in many aquifers, while solid sulfur is almost always absent.

This illustration shows the new sulfur and iron cycle proposed by Argonne researchers in a new paper out today in Science magazine. In very alkaline environments, microbes that reduce sulfur and iron co-exist. Credit: Ted Flynn/Science.

But something didn't add up right. A lot of the microorganisms had equipment to breathe both iron and sulfur. This requires two completely different enzymatic mechanisms, and it's evolutionarily expensive for microbes to keep the genes necessary to carry out both processes. Why would they bother, if sulfur was so rarely involved?

The team decided to redo the energy calculations assuming an alkaline environment—"Older and deeper aquifers tend to be more alkaline than pH-neutral surface waters," said Argonne coauthor Ken Kemner—and found that in alkaline environments, it gets harder and harder to get energy out of iron.

"Breathing sulfur, on the other hand, becomes even more favorable in alkaline conditions," Flynn said.

The team reinforced this hypothesis in the lab with bacteria under simulated aquifer conditions. The bacteria, Shewanella oneidensis, can normally breathe both iron and sulfur. When the pH got as high as 9, however, it could breathe sulfur, but not iron.

There was still the question of where microorganisms like Shewanella could find sulfur in their native habitat, where it appeared to be scarce.

The answer came from another group of microorganisms that breathe a different, soluble form of sulfur called sulfate, which is commonly found in alongside iron minerals. These microbes exhale sulfide, which reacts with iron minerals to form solid sulfur and reactive iron. The team believes this sulfur is used up almost immediately by Shewanella and its relatives.

(From left) Red hematite, white-yellow elemental sulfur, black mackinawhite, dark yellow goethite, and orange lepidocrocite. Hematite, goethite, and lepidocrocite are all common soil minerals — in fact, goethite makes up paint pigments in early cave paintings, such as the famous drawings in Lascaux. Mackinawhite is a mineral containing both iron and sulfur. Argonne scientists announced they have found what appears to be a missing step in the iron-sulfur cycle in underground aquifers; understanding these cycles is important for carbon sequestration and for predicting the fate of ground pollution. Credit: Mark Lopez/Argonne National Laboratory.

"This explains why we don't see much sulfur at any fixed point in time, but the amount of energy cycling through it could be huge," Kemner said.

Indeed, when the team put iron-breathing bacteria in a highly alkaline lab environment without any sulfur, the bacteria did not produce any reduced iron.

"This hypothesis runs counter to the prevailing theory, in which microorganisms compete, survival-of-the-fittest style, and one type of organism comes out dominant," Flynn said. Rather, the iron-breathing and the sulfate-breathing depend on each other to survive.

Understanding this complex interplay is particularly important for sequestering carbon. The idea is that in order to keep harmful carbon dioxide out of the atmosphere, we would compress and inject it into deep underground aquifers. In theory, the carbon would react with iron and other compounds, locking it into solid minerals that wouldn't seep to the surface.

Iron is one of the major players in this scenario, and it must be in its reactive state for carbon to interact with it to form a solid mineral. Microorganisms are essential in making all that reactive iron. Therefore, understanding that —and the microbe junkies who depend on it—plays a role in this process is a significant chunk of the puzzle that has been missing until now.

Explore further: A shocking diet: Researchers describe microbe that 'eats' electricity

More information: The study, "Sulfur-Mediated Electron Shuttling During Bacterial Iron Reduction," appears online today in the May 1 edition of Science Express and will be published in Science at the end of the month. Other authors on the study were Argonne scientists Bhoopesh Mishra (also of the Illinois Institute of Technology) and Edward O'Loughlin and Georgia Tech scientist Thomas DiChristina.

add to favorites email to friend print save as pdf

Related Stories

Oxygen 'sensor' may shut down DNA transcription

Jun 19, 2012

(Phys.org) -- A key component found in an ancient anaerobic microorganism may serve as a sensor to detect potentially fatal oxygen, a University of Arkansas researcher and his colleagues have found. This helps ...

Fools' gold found to regulate oxygen

Jul 23, 2012

As sulfur cycles through Earth's atmosphere, oceans and land, it undergoes chemical changes that are often coupled to changes in other such elements as carbon and oxygen. Although this affects the concentration of free oxygen, ...

Iron in primeval seas rusted by bacteria

Apr 25, 2013

(Phys.org) —Researchers from the University of Tübingen have been able to show for the first time how microorganisms contributed to the formation of the world's biggest iron ore deposits. The biggest known ...

Recommended for you

Tropical Storm Dolly forms, threatens Mexico

7 hours ago

Tropical Storm Dolly formed off Mexico's northeastern coast on Tuesday and headed toward landfall in Tamaulipas state, threatening to spark floods and mudslides, forecasters said.

Giant garbage patches help redefine ocean boundaries

10 hours ago

The Great Pacific Garbage Patch is an area of environmental concern between Hawaii and California where the ocean surface is marred by scattered pieces of plastic, which outweigh plankton in that part of ...

New satellite maps out Napa Valley earthquake

11 hours ago

Scientists have used a new Earth-observation satellite called Sentinel-1A to map the ground movements caused by the earthquake that shook up California's wine-producing Napa Valley on 24 August 2014.

User comments : 0