[meteorite-list] Life's Building Blocks Form in Replicated Deep Sea Vents

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 31 Mar 2016 16:45:01 -0700 (PDT)
Message-ID: <201603312345.u2VNj1WE029161_at_zagami.jpl.nasa.gov>

http://nai.nasa.gov/articles/2016/3/9/lifes-building-blocks-form-in-replicated-deep-sea-vents/

Life's Building Blocks Form in Replicated Deep Sea Vents
Charles Q. Choi
NASA Astrobiology Institude
March 9, 2016
 
Chimney-like mineral structures on the seafloor could have helped create
the RNA molecules that gave rise to life on Earth and hold promise to
the emergence of life on distant planets.

Scientists think Earth was born roughly 4.54 billion years ago. Life on
Earth may be nearly that old with recent findings suggesting that life
might have emerged only about 440 million years after the planet formed.

However, it remains a mystery how life might have first arisen. A major
component of life now is DNA, a molecule that stores the genetic data
that codes for proteins, including enzymes that can speed up chemical
reactions. However, DNA requires proteins in order to form, and proteins
need DNA to form, raising the chicken-and-egg question of how protein
and DNA could have formed without each other.

To resolve this conundrum, scientists have suggested that life may have
first primarily depended on compounds known as RNA. These molecules can
store genetic data like DNA, serve as enzymes like proteins, and help
create both DNA and proteins. Later DNA and proteins replaced this "RNA
world" because they are more efficient at their respective functions,
although RNA still exists and serves vital roles in biology.

However, it remains uncertain how RNA might have arisen from simpler precursors
in the primordial soup that existed on Earth before life originated. Like
DNA, RNA is complex and made of helix-shaped chains of smaller molecules
known as nucleotides.

One way that RNA might have first formed is with the help of minerals,
such as metal hydrides. These minerals can serve as catalysts, helping
create small organic compounds from inorganic building blocks. Such minerals
are found at alkaline hydrothermal vents on the seafloor.

Alkaline hydrothermal vents are also home to large chimney-like structures
rich in iron and sulfur. Prior studies suggested that ancient counterparts
of these chimneys might have isolated and concentrated organic molecules
together, spurring the origin of life on Earth.

To see how well these chimneys support the formation of strings of RNA,
researchers synthesized chimneys by slowly injecting solutions containing
iron, sulfur and silicon into glass jars. Depending on the concentrations
of the different chemicals used to grow these structures, the chimneys
were either mounds with single hollow centers or, more often, spires and
"chemical gardens" with multiple hollow tubes.

"Being able to perform our experiments in chimney structures that looked
like something one might encounter in the darker regions of Tolkien's
Middle Earth gave these studies a geologic context that sparked the imagination,"
said study co-author Linda McGown, an analytical chemist and astrobiologist
at Rensselaer Polytechnic Institute in Troy, N.Y.

The chimneys were grown in liquids and gases resembling the oceans and
atmosphere of early Earth. The liquids were acidic and enriched with iron,
while the gases were rich in nitrogen and had no oxygen. The scientists
then poked syringes up the chimneys to pump alkaline solutions containing
a variety of chemicals into the model oceans. This simulated ancient vent
fluid seeping into primordial seas.

Sometimes the researchers added montmorillonite clay to their glass jars.
Clays are produced by interactions between water and rock, and would likely
have been common on the early Earth, McGown said.

The kind of nucleotides making up RNA are known as ribonucleotides, since
they are made with the sugar ribose. The scientists found that unmodified
ribonuclotides could form strings of two nucleotides. In addition, ribonucleotides
"activated" with a compound known as imidazole - a molecule created
during chemical reactions that synthesize nucleotides - could form RNA
strings or polymers up to four ribonucleotides long.

"In order to observe significant RNA polymerization on the time scale
of laboratory experiments, it is generally necessary to activate the nucleotides,"
McGown said. "Imidazole is commonly used for nucleotide activation in
these types of experiments."

The scientists found that not only was the chemical composition of the
chimneys important when it came to forming RNA, but the physical structure
of the chimneys was key too. When the researchers mixed iron, sulfur and
silicon solutions into their simulated oceans, instead of slowly injecting
them into the seawater to form chimneys, the resulting blend could not
trigger RNA formation.

"The chimneys, and not just their constituents, are responsible for
the polymerization," McGown said.

These experiments for the first time demonstrate that RNAs can form in
alkaline hydrothermal chimneys, albeit synthetic ones.

"Our goal from the start of our RNA polymerization research has been
to place the RNA polymerization experiments as closely as possible in
the context of the most likely early Earth environments," McGown said.
"Most previous RNA polymerization research has focused on surface environments,
and the exploration of deep-ocean hydrothermal vents could yield exciting
new possibilities for the emergence of an RNA world."

One concern about these findings is that the experiments were performed
at room temperature. Hydrothermal vents are much hotter, and such temperatures
could destroy RNA.

"Keep in mind, however, that hydrothermal vents are dynamic systems
with gradients of chemical and physical conditions, including temperature,"
McGown said.

In principle, cooler sections of hydrothermal vents might have nurtured
RNA and its precursor molecules, she said.

In the future, McGown and her colleagues will perform experiments investigating
what effects variables such as pressure, temperature and mineralogy might
have on the formation of RNA molecules, focusing primarily on conditions
mimicking deep-ocean environments on an early Earth and those that may
also have existed on Mars and elsewhere, McGown said.

The scientists detailed their findings in the July 22 issue of the journal
Astrobiology.

This research was supported in part by the NASA Astrobiology Institute
(NAI) element of the NASA Astrobiology Program through the New York Center
for Astrobiology at Rensselaer Polytechnic Institute (RPI) and the Icy
Worlds team at NASA's Jet Propulsion Laboratory. Researchers Bradley
T. Burcar and Laura M. Barge were recipients of Astrobiology Program Early
Career Collaboration Awards. Bradley also held RPI's James P. Ferris
Fellowship in Astrobiology. Barge was additionally supported by the Astrobiology
Program through the NASA Postdoctoral Program, administered by Oak Ridge
Associated Universities.
Received on Thu 31 Mar 2016 07:45:01 PM PDT