Deep underground ecosystems
Deep Carbon Observatory collaborators, exploring the
‘Galapagos of the deep,’ add to what’s known, unknown, and unknowable about
Earth’s most pristine ecosystem
Bacteria, archaea, and other microbes—some of them
zombies—exist even in deepest known subsurface, and they’re weirder than their
surface counterparts
~70% of Earth's bacteria and archaea live underground
Earth’s deep life suggests microbes might inhabit the
subsurface of other planets
Barely living “zombie” bacteria and other forms of life
constitute an immense amount of carbon deep within Earth’s subsurface—245 to
385 times greater than the carbon mass of all humans on the surface, according
to scientists nearing the end of a 10-year international collaboration to
reveal Earth’s innermost secrets.
On the eve of the American Geophysical Union’s annual
meeting, scientists with the Deep Carbon Observatory today reported several
transformational discoveries, including how much and what kinds of life exist
in the deep subsurface under the greatest extremes of pressure, temperature,
and low energy and nutrient availability.
Drilling
2.5 kilometers into the seafloor, and sampling microbes from continental mines
and boreholes more than 5 km deep, the team has used the results to construct
models of the ecosystem deep within the planet.
With insights from now hundreds of sites under the
continents and seas, they have approximated the size of the deep biosphere—2 to
2.3 billion cubic km (almost twice the volume of all oceans)—as well as the
carbon mass of deep life: 15 to 23[1] billion
tonnes (an average of at least 7.5 tonnes of carbon per cu km subsurface).
The work also helps determine types of extraterrestrial
environments that could support life.
Among many key discoveries and insights:
The deep biosphere constitutes a world that can be viewed
as a sort of “subterranean Galapagos” and includes members of all three domains
of life: bacteria and archaea (microbes with no membrane-bound nucleus), and
eukarya (microbes or multicellular organisms with cells that contain a nucleus
as well as membrane-bound organelles)
Two types of microbes—bacteria and archaea—dominate Deep
Earth. Among them are millions of distinct types, most yet to be discovered or
characterized. This so-called microbial “dark matter” dramatically expands our
perspective on the tree of life. Deep Life scientists say about 70% of Earth's
bacteria and archaea live in the subsurface
Deep microbes are often very different from their surface
cousins, with life cycles on near-geologic timescales, dining in some cases on
nothing more than energy from rocks
The genetic diversity of life below the surface is
comparable to or exceeds that above the surface
While subsurface microbial communities differ greatly
between environments, certain genera and higher taxonomic groups are ubiquitous
- they appear planet-wide
Microbial community richness relates to the age of marine
sediments where cells are found—suggesting that in older sediments, food energy
has declined over time, reducing the microbial community
The absolute limits of life on Earth in terms of
temperature, pressure, and energy availability have yet to be found. The
records continually get broken. A
frontrunner for Earth’s hottest organism in the natural world is Geogemma
barossii, a single-celled organism thriving in hydrothermal vents on the
seafloor. Its cells, tiny microscopic spheres, grow and replicate at 121
degrees Celsius (21 degrees hotter than the boiling point of water)
Microbial life can survive up to 122°C, the record achieved
in a lab culture (by comparison, the record-holding hottest place on Earth’s
surface, in an uninhabited Iranian desert, is about 71°C—the temperature of
well-done steak)
The record depth at which life has been found in the
continental subsurface is approximately 5 km; the record in marine waters is
10.5 km from the ocean surface, a depth of extreme pressure; at 4000 meters
depth, for example, the pressure is approximately 400 times greater than at sea
level
Scientists have a better understanding of the impact on
life in subsurface locations manipulated by humans (e.g., fracked shales,
carbon capture and storage)
Ever-increasing accuracy and the declining cost of DNA
sequencing, coupled with breakthroughs in deep ocean drilling technologies
(pioneered on the Japanese scientific vessel Chikyu, designed to
ultimately drill far beneath the seabed in some of the planet’s most
seismically-active regions) made it possible for researchers to take their
first detailed look at the composition of the deep biosphere.
There are comparable efforts to drill ever deeper beneath
continental environments, using sampling devices that maintain pressure to
preserve microbial life (none thought to pose any threat or benefit to human
health).
To estimate the total mass of Earth’s subcontinental deep
life, for example, the team compiled data on cell concentration and microbial
diversity from locations around the globe.
Led by Cara Magnabosco of the Flatiron Institute Center for
Computational Biology, New York, the scientists factored in a suite of
considerations, including global heat flow, surface temperature, depth and
lithology—the physical characteristics of rocks in each location—to estimate
that the continental subsurface hosts 2 to 6 × 1029 cells.
Combined with estimates of subsurface life under the
oceans, total global Deep Earth biomass is approximately 15 to 23 petagrams (15
to 23 billion tonnes) of carbon.
Says Mitch Sogin of the Marine Biological Laboratory Woods
Hole, USA, co-chair of DCO’s Deep Life community of more than 300 researchers
in 34 countries: “Exploring the deep subsurface is akin to exploring the Amazon
rainforest. There is life everywhere, and everywhere there’s an awe-inspiring
abundance of unexpected and unusual organisms.
“Molecular studies raise the likelihood that microbial dark
matter is much more diverse than what we currently know it to be, and the
deepest branching lineages challenge the three-domain concept introduced by
Carl Woese in 1977. Perhaps we are approaching a nexus where the earliest
possible branching patterns might be accessible through deep life
investigation.”
“Ten years ago, we knew far less about the physiologies of
the bacteria and microbes that dominate the subsurface biosphere,” says Karen
Lloyd, University of Tennessee at Knoxville, USA. “Today, we know that, in many
places, they invest most of their energy to simply maintaining their existence
and little into growth, which is a fascinating way to live.
“Today too, we know that subsurface life is common. Ten
years ago, we had sampled only a few sites—the kinds of places we'd expect to
find life. Now, thanks to ultra-deep sampling, we know we can find them pretty
much everywhere, albeit the sampling has obviously reached only an
infinitesimally tiny part of the deep biosphere.”
“Our studies of deep biosphere microbes have produced much
new knowledge, but also a realization and far greater appreciation of how much
we have yet to learn about subsurface life,” says Rick Colwell, Oregon State
University, USA. “For example, scientists do not yet know all the ways in which
deep subsurface life affects surface life and vice versa. And, for now, we can
only marvel at the nature of the metabolisms that allow life to survive
under the extremely impoverished and forbidding conditions for life in deep
Earth.”
Among the many remaining enigmas of deep life on Earth:
Movement: How does deep life spread—laterally through
cracks in rocks? Up, down? How can deep life be so similar in South Africa and
Seattle, Washington? Did they have similar origins and were separated by plate
tectonics, for example? Or do the communities themselves move? What roles do
big geological events (such as plate tectonics, earthquakes; creation of large
igneous provinces; meteoritic bombardments) play in deep life movements?
Origins: Did life start deep in Earth (either within
the crust, near hydrothermal vents, or in subduction zones) then migrate up,
toward the sun? Or did life start in a warm little surface pond and migrate
down? How do subsurface microbial zombies reproduce, or live without dividing
for millions to tens of millions of years?
Energy: Is methane, hydrogen, or natural radiation
(from uranium and other elements) the most important energy source for deep
life? Which sources of deep energy are most important in different settings?
How do the absence of nutrients, and extreme temperatures and pressure, impact
microbial distribution and diversity in the subsurface?
Reference:
No comments:
Post a Comment