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

[Ron Baalke]

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.