A life less obvious: Study sheds light on the evolution of underground microbes

By Daniel Stolte, University Communication


Calcite crystals

Calcite, a mineral related to the presence of microorganisms, was extracted from a deep quarry in Swedish granite. Reiners and Drake used mineral-related biosignatures like these to look for ancient habitats deep inside the Earth.
Henrik Drake / Linnaeus University

Deep, dark fractures that reach deep into the oldest rocks on Earth may seem about as hospitable to life as outer space, but some estimates suggest that microbes that live deep in the earth’s crust account for the majority of Microbial life. These subterranean life forms, which make up what is known as the deep biosphere, can account for as much as 20% of all biomass on Earth.

These ecosystems host microbial lineages that are of interest in understanding the origin and evolution of life on our planet, but remain the least explored and understood ecosystems on Earth, according to the authors of a new study that looks at how deep habitats changed during the Earth’s tumultuous past.

“Understanding the history of the deep biosphere can provide insight into the evolution of life on Earth,” said Peter Reiners, professor of geoscience and associate dean at the University of Arizona College of Science, who co-authored the paper with Henrik Drake, a Associate Professor at Linnaeus University in Sweden. “This requires an understanding of the complex evolution of habitable conditions in these underground environments, but such an assessment had not been presented until now.”

While microbes have been known to live as deep as 3 miles below the Earth’s surface, and possibly longer, very little is known about how the deep biosphere has evolved through geological history and how modern microbes are related to their ancient ancestors underground.

Reiners and Drake focused on Precambrian cratons, which are some of the oldest rocks still found today, to find out where and when underground microbes should have been active on Earth hundreds of millions to billions of years ago. The results of their study, published this week in the Proceedings of the National Academy of Sciences, reveal that many cratons were uninhabitable for microbes for much of their existence, with the longest period of habitability not much beyond a billion years, and many cratons have only been habitable in the last 50 million to 300 million years.

Cratonic rocks in the Granite Mountains of Wyoming

These rocks at Sweetwater Gap in the Granite Mountains of Wyoming are part of the Wyoming Craton, a landscape dominated by 2.8 billion year old granite.
Peter Reiners

“We showed that because microbial habitat generally requires temperatures less than about 100 degrees Celsius (212 degrees Fahrenheit), we expect only a few places to find evidence of underground microbial life older than about a billion years,” Reiners said. “Just because these rocks are really old, and the liquids in them can also be old, does not mean that they could have supported life until relatively recently, when they came very close to the surface of erosion.”

Precambrian cratons are home to microorganisms that derive their energy from the consumption of nutrients, including sparingly available organic carbon, but also from chemical reactions between liquids and rocks. Drake and Reiners estimate that subterranean bacteria and archea (single-celled prokaryotes resembling bacteria), which now make up up to 90% of all microbial life on Earth, probably accounted for an even larger share of total life for hundreds of millions to billions of years ago .

“Their evolution, especially the evolution of their metabolism – how they get energy, and what chemical elements they ‘eat’ and ‘poop’ – provides key insights into the evolution of all other beings,” Reiners said, adding that some researchers believes that life may have first evolved beneath the Earth’s surface.

The researchers used a combination of records of deep ancient life found in craton fractures and recent advances in medium and low temperature thermochronology, a technique that allows researchers to reconstruct the temperature histories of rocks. Rocks may have endured higher temperatures and pressures during periods when sediments accumulated on top of them, only to be brought closer to the surface and into more habitable conditions when these sedimentary layers are eroded away.

“By combining thermochronological results from several different radioisotopic dating systems, we can reconstruct their thermal histories through ups and downs of burial and erosion over time,” Reiners said. “This approach gives us context to prospect and interpret the little-explored geological record of the deep biosphere of the Earth’s cratons.”

By assessing when these rocky environments became habitable, and in some cases, when they may have been buried and sterilized again, the study provides new insights into the evolutionary aspect of the deep biosphere.

“Cratonic rocks were formed billions of years ago, often deep in the crust, at temperatures too high for any life,” Reiners said. “It was only much later, after erosion, that the currently exposed rocks reached levels in the crust where temperatures were habitable.”

Drake said thermochronology could help identify areas where scientists could look for the oldest records of subterranean microorganisms on Earth.

“Eastern Finland, Greenland and perhaps parts of the Canadian shield look particularly interesting, with habitable conditions stretching back a billion years or even more,” he said. “These cratons are good targets for further studies of deep microbial evolution.”

The article “Thermochronological Perspectives on the Deep Time Evolution of the Deep Biosphere” is published online under DOI number 10.1073 / pnas.2109609118.

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