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

Date:
November 2, 2021
Source:
University of Arizona
Summary:
Precambrian cratons -- some of Earth's oldest rocks -- were
uninhabitable for microbes for much of their existence, with the
longest period of habitability not much beyond a billion years, and
many only for the past 50 million to 300 million years, according
to a paper correlating Earth's deep biosphere with geologic history.



FULL STORY ========================================================================== Deep, dark fractures reaching far down into the oldest rocks on Earth
may seem about as hospitable to life as outer space, but some estimates
suggest that microbes dwelling deep in the Earth's crust account for
the majority of microbial life. These underground lifeforms, which make
up what's known as the deep biosphere, could account for as much as 20%
of all biomass on Earth.


========================================================================== These ecosystems host microbial lineages that are of interest for
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 takes a closer look at how deep habitats
changed during Earth's tumultuous past.

"Understanding the history of the deep biosphere can provide insight
into the evolution of life on Earth," said Peter Reiners, a professor
of geosciences and associate dean of the University of Arizona College
of Science, who co-authored the paper with Henrik Drake, an associate
professor at the Linnaeus University in Sweden. "This requires
understanding the complex evolution of habitable conditions in these underground environments, but such assessment had not been presented
until now." While microbes have been known to eke out a living as deep as
3 miles below Earth's surface, and possibly beyond, very little is known
about how the deep biosphere has evolved over geologic history, and how
modern microbes are related to their ancient ancestors in the subsurface.

Reiners and Drake focused on Precambrian cratons, which are some of the
oldest rocks still present today, to find out where and when subsurface microbes should have been active on Earth hundreds of millions to billions
of years ago.

The results of their study, published this week in theProceedings 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 for the past 50 million to 300 million years.

"We showed that because microbial habitability generally requires
temperatures less than about 100 degrees Celsius (212 degrees Fahrenheit),
in only a few places do we expect to find evidence of subsurface microbial
life older than about a billion years," Reiners said. "Just because these
rocks are really old, and the fluids in them may be old, too, doesn't
mean that they could've supported life until relatively recently, when
they got very close to the surface by erosion." Precambrian cratons
are home to microorganisms that get their energy from consumption of
nutrients including sparsely available organic carbon but also from
chemical reactions between fluids and rocks. Drake and Reiners estimate
that subsurface bacteria and archea (single-celled prokaryotes similar to bacteria), which now compose up to 90% of all microbial life on Earth,
probably composed an even larger fraction of total life hundreds of
millions to billions of years ago.



========================================================================== "Their evolution, particularly the evolution of their metabolisms --
how they get energy and what chemical elements they 'eat' and 'poop' --
provide key insights into the evolution of all other critters," Reiners
said, adding that some researchers think that life may have first evolved beneath Earth's surface.

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

"By combining thermochronologic results from several different
radioisotopic dating systems, we can reconstruct their thermal histories through the ups and downs of burial and erosion over time," Reiners
said. "This approach gives us context for prospecting and interpreting the little-explored geologic record of the deep biosphere of Earth's cratons."
By assessing when these rock 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 formed billions of years ago, often deep in the crust,
at temperatures too high for any life," Reiners said. "It was only much
later, following erosion, that the currently exposed rocks reached
levels in the crust where temperatures were habitable." Drake said thermochronology could help identify areas where researchers could look
for the oldest records of subsurface microorganisms on Earth.

"Eastern Finland, Greenland and perhaps parts of the Canadian
shield look particularly interesting, with habitable conditions
spanning back a billion years or even more," he said. "Those cratons
are good targets for further studies of deep microbial evolution." ========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Daniel Stolte.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Henrik Drake, Peter W. Reiners. Thermochronologic perspectives
on the
deep-time evolution of the deep biosphere. Proceedings of the
National Academy of Sciences, 2021; 118 (45): e2109609118 DOI:
10.1073/ pnas.2109609118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211102111213.htm

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